Modern Techniques of Spectroscopy: Basics, Instrumentation, and Applications 9789813360846, 9813360844

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Progress in Optical Science and Photonics

Dheeraj Kumar Singh Manik Pradhan Arnulf Materny   Editors

Modern Techniques of Spectroscopy Basics, Instrumentation, and Applications

Progress in Optical Science and Photonics Volume 13

Series Editors Javid Atai, Sydney, NSW, Australia Rongguang Liang, College of Optical Sciences, University of Arizona, Tucson, AZ, USA U. S. Dinish, Singapore Bioimaging Consortium (SBIC), Biomedical Sciences Institutes, A*STAR, Singapore, Singapore

The purpose of the series Progress in Optical Science and Photonics is to provide a forum to disseminate the latest research findings in various areas of Optics and its applications. The intended audience are physicists, electrical and electronic engineers, applied mathematicians, biomedical engineers, and advanced graduate students.

More information about this series at http://www.springer.com/series/10091

Dheeraj Kumar Singh · Manik Pradhan · Arnulf Materny Editors

Modern Techniques of Spectroscopy Basics, Instrumentation, and Applications

Editors Dheeraj Kumar Singh Department of Physics Institute of Infrastructure Technology Research And Management (IITRAM) Ahmedabad, Gujarat, India

Manik Pradhan S. N. Bose National Centre for Basic Sciences Kolkata, West Bengal, India

Arnulf Materny Department of Physics and Earth Sciences Jacobs University Bremen Bremen, Germany

ISSN 2363-5096 ISSN 2363-510X (electronic) Progress in Optical Science and Photonics ISBN 978-981-33-6083-9 ISBN 978-981-33-6084-6 (eBook) https://doi.org/10.1007/978-981-33-6084-6 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Dedicated to my loving mother Pramila Singh

Preface

This book highlights the recent developments in the broad field of spectroscopy to provide readers with an updated overview giving insights into different modern fields by experts from different countries. The focus of this book is on the fundamental concepts of modern spectroscopic techniques, recent technological innovations, and latest examples of applications to molecules and materials relevant for academia and industry. The book will be beneficial to researchers from various branches of science and technology, and is intended to point them to modern techniques, which might be useful for their specific problems. The applications introduced in the different chapters demonstrate the usefulness of the spectroscopic techniques for the characterization of basic properties of molecules, e.g. in connection with environmental impact, bio-activity, or usefulness for pharmaceutical drugs, and materials important e.g. for nano-science, nuclear chemistry, or bio-applications. We believe that the present book can help to learn about techniques, which are useful for the characterization of molecular systems, which might also have social and economic relevance (environment, alternative energy, etc.). In the book, spectroscopic techniques are presented, which are useful for research, academia, and industry. It is divided into five parts: (I) “New Frontiers in Optical Absorption/Reflectance-Based Spectroscopic Techniques and Applications,” (II) “Recent Advances in Raman Spectroscopy and Applications,” (III) “Optical Cavity and Laser-Based High-Resolution Spectroscopic Techniques and Applications,” (IV) “Spectroscopic Techniques Important for Chemical Analytics and Material Science” and (V) “Other Spectroscopic Techniques for Characterization of Optical, Electrical, and Mechanical Properties of Molecules.” Part One is directed towards fundamentals and recent advances in optical absorbance/reflectance spectroscopy. In this part, Prof. Kailash Jena, from IIT Ropar, India, extensively describes the basics of ATR-IR spectroscopy and its applications in the Chapter “Fundamentals of ATR-FTIR Spectroscopy and Its Role for Probing In-Situ Molecular-Level Interactions”. Further, beyond the linear IR spectroscopy, Prof. Sayan Bagchi, from CSIR-NCL Pune, India, explains the principles and applications of non-linear 2D IR spectroscopy in the femtosecond time domain in the

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Chapter “Two Dimensional Infrared Spectroscopy: A Structure Sensitive Technique with Ultrafast Time Resolution”. Then, the group of Prof. Aloke Das, IISER Pune, India, introduces detailed information relevant for the fundamental understanding, mechanism, and applications of UV-Vis spectroscopy in the Chapter “Exploring Non-covalent Interactions by Jet-Cooled Electronic and Vibrational Spectroscopy”. To analyze surfaces and interfaces of materials, sum-frequency-generation vibrational spectroscopy where two laser beams overlap with spatial and temporal resolution is an important technique. This approach is introduced in Chapter “Classicaland Heterodyne-Detected Vibrational Sum Frequency Generation (VSFG) Spectroscopy and Its Application to Soft Interfaces” of the book by Dr. Jahur Mondal from BARC Mumbai, India. In Chapter “Broadband Terahertz Spectroscopy”, Prof. Pankaj Mandal from IISER Pune, India, explains terahertz spectroscopy, which is used for low-energy electronic excitations and the investigation of charge-carrier dynamics in semiconductor devices. Part Two is focused on Raman spectroscopy and its variants. Here, Chapter “Overview of Raman Spectroscopy: Fundamental to Applications” is contributed by Dr. Dheeraj Kumar Singh and his research group from IITRAM Ahmedabad, India. It presents an overview about Raman spectroscopy and its recent applications in various fields of science and technology. Prof. Ashish Mishra from IIT BHU Varanasi, India, presents fundamentals of surface-enhanced Raman spectroscopy (SERS) in Chapter “Fundamentals and Applications of Surface Enhanced Raman Spectroscopy”, which helps to obtain vibrational spectra of molecules at low concentrations making Raman spectroscopy an attractive tool for applications in e.g. environmental science, explosive detection, archeology, or food quality control where small amounts of substances have to be detected. The next chapter, “Tip-Enhanced Raman Spectroscopy”, contains a detailed discussion of tip-enhanced Raman scattering (TERS) used to drastically increase the spatial resolution, which in standard micro-Raman spectroscopy is determined by the diffraction limit. Prof. Prabhat Verma and Prof. Takayuki Umakoshi from Osaka University, Japan, give a detailed introduction into the basics as well as prominent applications of TERS. As an example for nonlinear Raman techniques, in Chapter “Coherent Anti-Stokes Raman Scattering: Basics, Theoretical Background, and Applications”, Prof. Arnulf Materny and co-workers from Jacobs University Bremen, Germany, introduce coherent anti-Stokes Raman spectroscopy. The interaction of three laser pulses for the generation of the anti-Stokes signal is ideally suited for studies of vibrational dynamics on a femtosecond time scale giving access to elementary processes in molecular systems. Part Three covers the essentials of spectroscopic techniques, which involve optical cavities, and demonstrates the advancement in resolution. Prof. Rajesh Kushawaha from PRL Ahmedabad, India, extensively discusses the fundamentals and applications of ultrafast spectroscopy in Chapters “Modern Experimental Techniques in Ultrafast Atomic and Molecular Physics” and “Improving the Signal Strength and Detection Limits of Laser-Induced Breakdown Spectroscopy”, he

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introduces laser-induced breakdown spectroscopy. In Chapter “Cavity Ring-Down Spectroscopy”, cavity ring-down spectroscopy (CRDS), and in Chapter “Quantum Cascade Laser Spectroscopy”, quantum cascade laser spectroscopy (QCL) are explained in detail by the research group of Prof. Manik Pradhan from SN Bose National Institute for Basic Sciences Kolkata, India. In Chapter “Wavelength Modulation Spectroscopy”, fundamentals and applications of wavelength modulation spectroscopy are presented by Prof. A. L. Chakraborty from IIT Gandhinagar, India, and Prof. S. K. Singh from IIT BHU Varanasi, India, explains photon upconversion spectroscopy in Chapter “Photon Upconversion Spectroscopy”. Further spectroscopic techniques important for chemical analytics and material science are introduced in Part Four. The use of X-ray photoelectron spectroscopy is discussed by Prof. Aditi Haldar from IIT Mandi, India, in Chapter “Application of X-Ray Photoelectron Spectroscopy in Materials for Energy Conversion and Environmental Remediation”. Further, in Chapter “Fluorescence Spectrometry”, Prof. Dharmendra P. Singh from Université du Littoral Côte d’Opale (ULCO), France, and Prof. Sandeep Kumar from Raman Research Institute (RRI) Bangalore, India, give an overview about the basics and applications of fluorescence spectrometry. Of course, not only optical spectroscopic techniques are of importance. Chapter “Nuclear Magnetic Resonance Spectroscopy: Theory and Applications” provides a contribution by Dr. Sachin Kumar Singh from Gorakhpur University and Prof. Chandan Singh from BHU Varanasi, India, about nuclear magnetic resonance (NMR) spectroscopy, which is widely used to identify organic compounds in complex molecular structures and belongs to the basic techniques in chemical analytics. Finally, Part Five addresses other spectroscopic techniques, which find broad application for the determination of mechanical and electrical properties of molecules. The part starts with Chapter “Impedance Spectroscopy”, in which Prof. Ravindra Dhar from University of Allahabad, India, discusses the importance of impedance spectroscopy, which is used to determine the capacitance and resistance of materials applying a sinusoidal potential. Ellipsometry spectroscopy, which yields information about the dielectric properties (complex refractive index) and thickness of thin layers, is introduced by Prof. Mukesh Ranjan from IPR Gandhinagar, India, in Chapter “Ellipsometry Techniques and Its Advanced Applications in Plasmonics”. In the following chapter “Atomic Force Microscopy-Based Force Spectroscopy and Its Various Applications”, Prof. Umesh Kumar from Central University of Gujarat, India, presents a detailed discussion of the principles and applications of force spectroscopy. An explanation of surface photovoltaic spectroscopy is given by Prof. Vipul Kheraj from SVNIT Surat, India, in Chapter “Surface Photo-Voltage Spectroscopy: A Versatile Technique to Probe Semiconductor Materials and Devices”. The part ends with the Chapter “Optical Imaging in Biology: Basics and Applications”, in which Profs. Surya P. Singh from IIT Dharwad, India, and Soumik Siddhanta from IIT Delhi, India, present optical imaging techniques. Even when of course not all spectroscopic methods can be included in this book, it gives an important overview about some prominent examples. Thus, it will also be helpful for students at different stages of education and young researchers. Scientists

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and engineers working in different fields of science will get further information about available spectroscopic techniques and their potential applications and will also be referred to other literature in the different chapters of the book. Additionally, ideas for new spectroscopic methods and applications might be triggered. Ahmedabad, India Kolkata, India Bremen, Germany

Dheeraj Kumar Singh Manik Pradhan Arnulf Materny

Acknowledgements

First and foremost, I would like to express my sincere gratitude to Almighty God Prabhu Shree Ram, Bajarang Balee and Maa Saraswati to give me strength for the successful collections of the quality book chapters. The book entitled as Modern Techniques of Spectroscopy: Basics, Instrumentation and Applications contains 23 chapters from various reputed institutions across India and abroad. The accomplishment of the book was possible with the unconditional support and help by many individuals, scientists, and academicians. I would like to express my sincere thanks to all of them. At first, I would like to show my heartfelt gratitude and thanks to all contributors for providing a quality chapter in the recent developments of spectroscopic techniques. I extend my heartfelt gratitude to Prof. S. Prasanna, Dean; Prof. A. U. Digraskar, Director; and Prof. Shiva Prasad, Director General of the IITRAM for their motivations and good infrastructure facilities at IITRAM. I owe my deepest gratitude towards Springer publisher team especially Loyola D’Silva, Prasanna Kumar Narayanasamy, Aninda Bose, books production team, Editorials team, etc for their guidance to complete the book. I do take the opportunity to thank my collaborators Prof. Arnulf Materny, Prof. Manik Pradhan and Prof. Sunil Singh for their enormous support and scientific discussions. I am grateful to the Science and Engineering Research Board (SERB), DST, Government of India for financial support to conduct research work at IITRAM, which make a basis to collaboration with the expert researchers and exchange our ideas. My heartfelt and special thanks go to my Ph.D. students Deepak, Hardik, Paridhi, and Amit for their valuable time to help me during editing the book. Last but not least, I would like to thank my family, my wife Dr. Shweta Singh and my daughter Drishti Singh, father Jamuna Prasad Singh and mother Pramila Singh for their valuable support and sacrifices during the preparations of book. Ahmedabad, India

Dr. Dheeraj Kumar Singh

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Contents

New Frontiers in Optical Absorption/Reflectance-Based Spectroscopic Techniques and Applications Fundamentals of ATR-FTIR Spectroscopy and Its Role for Probing In-Situ Molecular-Level Interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Harsharan Kaur, Bhawna Rana, Deepak Tomar, Sarabjeet Kaur, and Kailash C. Jena Two Dimensional Infrared Spectroscopy: A Structure Sensitive Technique with Ultrafast Time Resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . Deborin Ghosh, Samadhan Deshmukh, Srijan Chatterjee, Sushil Sakpal, Tapas Haldar, Ambuj Dhakad, Somnath Kashid, and Sayan Bagchi Exploring Non-covalent Interactions by Jet-Cooled Electronic and Vibrational Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Prakash Panwaria and Aloke Das Classical- and Heterodyne-Detected Vibrational Sum Frequency Generation (VSFG) Spectroscopy and Its Application to Soft Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Subhadip Roy, Subhamoy Saha, and Jahur Alam Mondal

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Broadband Terahertz Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 Sneha Banerjee, Gurivireddy Yettapu, Sohini Sarkar, and Pankaj Mandal Recent Advances in Raman Spectroscopy and Applications Overview of Raman Spectroscopy: Fundamental to Applications . . . . . . . 145 Deepak K. Pandey, Hardik L. Kagdada, Paridhi Sanchora, and Dheeraj K. Singh Fundamentals and Applications of Surface Enhanced Raman Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185 Bishnu Pada Majee and Ashish Kumar Mishra xiii

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Tip-Enhanced Raman Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209 Takayuki Umakoshi and Prabhat Verma Coherent Anti-Stokes Raman Scattering: Basics, Theoretical Background, and Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235 Alireza Mazaheri Tehrani, Faezeh Mohaghegh, and Arnulf Materny Optical Cavity and Laser-Based High-Resolution Spectroscopic Techniques and Applications Modern Experimental Techniques in Ultrafast Atomic and Molecular Physics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 P. Madhusudhan, Rituparna Das, Pranav Bharadwaj, Pooja Chandravanshi, Swetapuspa Soumyashree, Vinitha Nimma, and Rajesh K. Kushawaha Cavity Ring-Down Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287 Abhijit Maity, Mithun Pal, and Manik Pradhan Improving the Signal Strength and Detection Limits of Laser-Induced Breakdown Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . 307 Rituparna Das, K. M. Muhammed Shameem, Vinitha Nimma, Swetapuspa Soumyashree, Prashant Kumar, and Rajesh K. Kushawaha Wavelength Modulation Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321 Arup Lal Chakraborty and Anirban Roy Quantum Cascade Laser Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363 Mithun Pal and Manik Pradhan Photon Upconversion Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 389 Priyam Singh, Prabhakar Singh, and S. K. Singh Spectroscopic Techniques Important for Chemical Analytics and Material Science Application of X-Ray Photoelectron Spectroscopy in Materials for Energy Conversion and Environmental Remediation . . . . . . . . . . . . . . 411 Ankita Mathur, Ravinder Kaushik, and Aditi Halder Fluorescence Spectrometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 431 Dharmendra Pratap Singh, Sanjeev R. Inamdar, and Sandeep Kumar Nuclear Magnetic Resonance Spectroscopy: Theory and Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 469 Madeeha Rashid, Sachin Kumar Singh, and Chandan Singh

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Other Spectroscopic Techniques for Characterization of Optical, Electrical and Mechanical Properties of Molecules Impedance Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 515 Neelam Yadav and Ravindra Dhar Ellipsometry Techniques and Its Advanced Applications in Plasmonics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 541 Mahesh Saini, Sebin Augustine, K. P. Sooraj, and Mukesh Ranjan Atomic Force Microscopy-Based Force Spectroscopy and Its Various Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 585 Jyoti Jaisawal, Amit Kumar Srivastav, Dheeraj K. Singh, and Umesh Kumar Surface Photo-Voltage Spectroscopy: A Versatile Technique to Probe Semiconductor Materials and Devices . . . . . . . . . . . . . . . . . . . . . . . 605 Akshay Jariwala, Aditi Toshniwal, and Vipul Kheraj Optical Imaging in Biology: Basics and Applications . . . . . . . . . . . . . . . . . . 637 S. P. Singh and Soumik Siddhanta

About the Editors

Dr. Dheeraj Kumar Singh received his Ph.D. degree in Physics from Banaras Hindu University (BHU), Varanasi, India in 2010. Dr. Singh was subsequently a CSIR-Nehru Post-doc at NCL Pune, NRF post doc at Seoul, South Korea, and prestigious Alexander von Humboldt (AvH) post doc Fellow in Germany. Dr. Singh was also awarded the prestigious international award of JSPS fellowship, Japan, Dr. D. S. Kothari Post-doc fellowship, CSIR-SRF, UGC- Meritorious fellowship (RFSMS), etc. Currently, Dr. Singh is working as Assistant Professor of Physics at IITRAM Ahmedabad. His research group is mainly focused on Frequency and time domain Spectroscopic tencinques, their developments and applications on Ionic Liquids, Biomolecules, Functionalized Nanomaterials, Molecular Interactions, etc. Moreover, he has written many Book chapter and have the patent granted by the USA. He has organized many workshops and conference on Spectroscopy of Molecules and Materials. His recent Springer Nature book as proceeding in “Physics-236 on Advances in Spectroscopy: Molecules to Materials” was published as a leading Editor. Dr. Singh has been awarded the prestegious ECR project by SERB-DST Government of India and DST-Inspire project for his Ph.D. student. Dr. Singh has published more than 50 reserch papers in international journal of repute including the USA patent.

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About the Editors

Dr. Manik Pradhan received his Ph.D. in Chemical Physics from the group of Professor Andrew OrrEwing, FRS in 2008 from the University of Bristol, UK, having been awarded a prestigious Dorothy Hodgkin Fellowship. Subsequently, he moved to the University of Cambridge, UK for his postdoctoral work (2008– 2010) in the group of Professor Richard Lambert after receiving the Isaac Newton Trust Postdoctoral Fellowship from Trinity College, UK. Hereafter, he came to work as a Postdoctoral Research Associate (2010– 2011) in the group of Professor Richard Zare, FRS in Stanford University, USA. He also worked as a Visiting Research Assistant (2004–2005) at the Institute of Atomic and Molecular Sciences (IAMS), Academia Sinica, Taiwan. Currently he is leading many multidisciplinary research projects as a Principal Investigator (PI) as well as a Co-investigator. His research group focuses on the cutting-edge research in interdisciplinary areas involving experimental laser spectroscopy, biomedical and, environmental sciences along with developments of new-generation optical cavitybased laser spectroscopy techniques. Dr. Pradhan has published more than 50 research articles in international peer-reviewed journals and has several patents to his name. Prof. Dr. Arnulf Materny recieved his Ph.D. in 1992 with distinction under the supervision of pioneer spectroscopist Prof. Wolfgang Kiefer at the University of Würzburg, Germany. Further, he was a postdoc at Caltech, Pasadena, USA. There, he started to work in the field of “femtosecond spectroscopy” in the group of Nobel Laureate Prof. Ahmed H. Zewail. Currently, Prof. Materny is serving as a Full Professor of Chemical Physics at the Jacobs University, Bremen Germany. His reserach intrests cover the applications of freqencny and time domain spectroscopic techniques in various molecules and materials. Prof. Materny has received several awards, including Kekule Fellowship and the Hoechst Prize for his Ph.D. work, a Faculty Award by the JMU and the Heisenberg Fellowship by the DFG for his postdoc research. More than 16 Ph.D.s and 15 post doc reserachers including many Humboldt fellow succesfully completed his project under his guidance.

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He has completed many DFG projects at Jacobs University, Germany. He is the member of several Editorial boards including Journal of Raman Spectroscopy. Prof. Materny has published more than 250 articles in international journals of repute.

New Frontiers in Optical Absorption/Reflectance-Based Spectroscopic Techniques and Applications

Fundamentals of ATR-FTIR Spectroscopy and Its Role for Probing In-Situ Molecular-Level Interactions Harsharan Kaur, Bhawna Rana, Deepak Tomar, Sarabjeet Kaur, and Kailash C. Jena

Abstract Infrared (IR) vibrational spectroscopy is the most reliable technique to determine the molecular composition and structure of chemical compounds. In the past two decades, with the advancement in the IR instrumentation, attenuated total reflectance Fourier transform infrared (ATR-FTIR) vibrational spectroscopic tool have taken an invaluable position in the field of chemistry, physics, material and biological sciences. It readily probes the molecular structure, chemical interactions and dynamics of the molecular species. Owing to its higher sensitivity with the surrounding fluctuations, ATR-FTIR aids in extracting the information about the molecular configuration and the local interactive framework of different analytes existing in diverse morphological states. ATR-FTIR can be employed for interfacial studies at distinct levels by varying the incident angle and the material used for ATR crystal to obtain a required penetration depth of the IR beam in the sample medium at the ATR crystal/sample interface. The current chapter introduces the background, modern theoretical, and practical aspects of the ATR-FTIR spectroscopy descriptively. The chapter also previews a considerable amount of research currently pursued in different application domains including bio-nano model systems, biomolecular assembly, binary solvents, forensics and electrochemical devices, covering the molecular-scale perspectives using the ATR-FTIR vibrational spectroscopy. Keywords Vibrational spectroscopy · ATR-FTIR spectroscopy · Evanescent wave · Internal reflection elements · Molecular structure

H. Kaur · K. C. Jena (B) Center for Biomedical Engineering, Indian Institute of Technology Ropar, Rupnagar, Punjab 140001, India e-mail: [email protected] B. Rana · D. Tomar · S. Kaur · K. C. Jena Department of Physics, Indian Institute of Technology Ropar, Rupnagar, Punjab 140001, India © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 D. K. Singh et al. (eds.), Modern Techniques of Spectroscopy, Progress in Optical Science and Photonics 13, https://doi.org/10.1007/978-981-33-6084-6_1

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1 Introduction Infrared (IR) spectroscopy is a well-recognized fundamental spectroscopic tool that involves the analytical processing of atomic and molecular vibrations of the molecule upon resonantly associating with an incident IR radiation source. It is employed as one of the most convenient tools to selectively probe the molecular functional groups through their distinct IR absorption bands, which are greatly influenced by the molecular composition, conformation, and the condition of the surrounding medium [1–3]. These properties directly influence the associated vibrational transitions. Established by Coblentz in the early twentieth century [1], IR spectroscopic techniques have been revamped since then and revolutionized the modern molecular spectroscopic methods. Improvements at the instrumental level have led to the evolution of sensitivity and efficiency of probing by IR spectroscopic techniques that could be applied for exploring the properties of any sample with an effective polarity in any of its morphological states (liquid, powder, films, gases, etc.) [1–4]. The advancements have further provided precedence in the requirement of less sample amount for a cost-effective spectroscopic analysis. These include the designing of Fourier transform infrared (FTIR) spectrometer in the 1980s, which aimed to improve the performance of IR spectrometer by collecting the data at a faster pace with an enhanced signal to noise ratio [1, 2, 5]. Among the other IR based spectroscopic techniques, attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy is the most promising tool that works on the amalgamation of the biochemical and biophysical properties of the system [3–5]. The concept of the ATR technique was first suggested by N. J. Harrick in 1960 and J. Fahrenfort in 1961 [6, 7]. Both of them advocated the possible working model of the ATR technique with multiple and single reflection ATR geometries, respectively. The ATR mode of IR spectroscopy allows the evaluation of waterbased samples known to be strong absorber of IR radiation which makes experiments very difficult to conduct by conventional FTIR experiments. The ATR-FTIR tool addresses the issues of absorption by acquiring the molecular vibrations through reduced pathlength of the probing beam across the sample. With the ATR mode configuration, the molecular structural information in terms of vibrational modes of the molecules can be extracted from the probing depth of a few micrometers via evanescent wave generated at the interface between ATR crystal and the sample medium. The IR beam strikes the crystal-sample interface at an angle greater than the critical angle of incidence (θc ) then undergoes total internal reflection (TIR) as a result of refractive index differences, with the ATR crystal having a higher refractive index value than the sample. The ATR-FTIR configuration provides the right platform to investigate solid-aqueous interfaces effectively [1, 2, 5, 8, 9]. The ATR-FTIR spectroscopic technique has been further advanced to provide a detailed assessment of the molecular bonding, surface adsorption, interactions, molecular orientation, and kinetics, as well as the structural parameter of the sample [8–12].

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Several researchers have focused their attention towards utilizing the simplistic non-invasive approach of ATR-FTIR spectroscopy for a wide domain of applications in various fields of science. It has been combined with several theoretical algorithms for improving point-of-care diagnostics in the health care sector with sensitive detection efficiency for different bio-analytes and disease biomarkers [13– 15]. By tuning the IR incident angle and the subsequent penetration depth, the ATRFTIR spectroscopy can be used to probe specific components of the layered surfaces over the ATR crystal, for example, thin biofilms or peptide layers, and electrolyte interphases [9, 16]. The polarization (parallel or perpendicular) of the incident IR beam can be used in the ATR-FTIR spectrometer to extract the molecular orientation of the molecules [17, 18]. The concept of ATR-FTIR spectrometer has also been explored for various IR imaging modalities like histopathology, live cells and tissues, identifying material surface properties, etc. [19–21]. In this book chapter, we aim to address the fundamental aspects of the ATRFTIR vibrational spectroscopic technique. Primary sections include the working principles, the instrumental details, and the theoretical aspects of the ATR-FTIR spectroscopy. Some of the application areas of the ATR-FTIR technique are also been covered by the comprehensive review of the work done in this field by emphasizing the broader perspective of the ATR-FTIR spectroscopic tool for conducting in-situ molecular-level characterizations of various molecular systems.

2 Working Principle and Instrumental Details 2.1 FTIR Spectrometer Modern IR spectroscopic instruments are widely equipped with the FTIR design to accelerate the scanning and data collection process. The fundamental unit of the FTIR spectrometer constitutes the Michelson interferometer. The detailed optical layout of the ATR-FTIR spectrometer is depicted in Fig. 1 along with the path ways of generating the IR spectrum [1, 2, 22]. This device introduces a path difference to the IR beams (emanated from an IR light source) through the involvement of a beam splitter at the center, two optical mirrors (stationary and motorized), and an efficient detector system. IR source utilized most commonly for the mid-infrared beam generation in FTIR-based instruments is the silicon carbide rod, and it is also called globar source. The IR beam from the source is split by the beam splitter and projected towards both the stationary and the motorized mirrors. The beam reflected from the motorized moving mirror induces a path difference with respect to the second beam which is being reflected from the stationary mirror. The resultant IR beams with an effective path difference undergo interference process (constructive and destructive). This recombined IR radiation is then allowed to interact with the sample assembly and the resultant output is collected by a DTGS (deuterated triglycine sulfate) detector as a Fourier transform spectra of the sample response.

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Fig. 1 Schematic optical layout of the ATR-FTIR spectrometer with a variable angle horizontal ATR accessory mounted with a trapezoidal ATR crystal. The spectrometer is interfaced with OPUS software for data interpretation

2.2 Attenuated Total Reflection (ATR) and Allied Accessories Based on the reflectance phenomena, the ATR technique involves the important role of an internal reflection element (IRE) or ATR crystal (bearing a higher optical density) which is in direct contact with the sample at its surface. The IR beam from the source enters the ATR crystal and gets total internally reflected (TIR) at an incidence angle greater than the critical angle (θc ) from the crystal surface in direct contact with the sample. As a result of this TIR process, an evanescent wave is generated at the crystal boundary. The evanescent field at the interface interacts with the sample over a distance called depth of penetration (dp ) of the beam. Thus, the incoming IR beam gets attenuated or absorbed by the sample depending on its composition and morphology, and the final reflected output beam with a relatively weaker intensity is recorded by the detector [1, 2, 5, 22]. A complex optical layout has been utilized to focus and direct the light beam onto the sample. It also makes a correction for an increased path of travel due to the reflection of the light beam within the accessory by directing the IR beam towards the IRE boundary at the normal incidence. The IREs are usually mounted on ATR accessories with specified geometry that places the sample containing IREs in a particular orientation. These include vertical and horizontal geometry of the ATR accessory. However, the horizontal ATR accessory is a common choice for the sampling procedure as it could account for a variety of sample types with different morphological states [1, 5, 16, 22, 23]. A typical optical alignment of the horizontal ATR accessory with a variable angle geometry is presented in Fig. 1. The horizontal accessory could be easily connected and placed within the sample compartment. It could be readily sealed with the FTIR spectrometer boundaries interconnecting the

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source and the detector. In this way, a similar atmospheric condition that is maintained within the spectrometer is sensed by the sampling accessory. The ATR accessory can have different types of sampling assemblies mounted with the ATR crystal for facilitating the evaluation of the sample. The flat plate assembly is utilized to analyze solid samples like films, powders, etc., by simply pressing the sample over the face of IRE. Trough crystal assembly accommodates the crystal in a lower position to the accessory surface creating a trough, where both liquid and solid samples can be placed bearing a direct contact with the crystal surface. Another sampling assembly is a liquid flow cell that provides a sealed channel with the ATR crystal, and an external temperature and flow controller devices [1, 2, 5, 22]. These sampling assemblies can be mounted over the horizontal ATR accessory.

2.3 Internal Reflection Elements for ATR-FTIR Spectrometer The internal reflection elements (IRE), also called as ATR crystals, are widely used for undertaking the ATR-FTIR spectroscopic process. These elements must have a higher refractive index than the sample in contact for IR probing, which is the requirement for satisfying the TIR condition. Then the material has to be transparent in the specified wavelength region of scanning with IR incident beams. In order to achieve the above-mentioned requirements, different types of materials are used for fabricating the IREs. The most common materials being utilized for this purpose include Zinc selenide (ZnSe), Germanium (Ge), Silicon (Si), Diamond, and Thallium bromide/iodide (KRS-5). These IREs differ among themselves in their refractive index values (n1 ) and specific critical angle values (θc ). Moreover, the choice of the crystal or IRE depends on their overall dimensions, durability, material strength, working pH and sample sensitivity. For inference, ZnSe and Ge provide a costeffective and wider pH range stability for sample probing and are commonly used for spectrometry, while the diamond crystal is a highly robust material but its usage is limited by its high cost and limited dimensional area of probing [1, 5, 22, 24, 25]. Details about the other IRE or ATR crystal materials used in the mid-infrared range are elaborated in Table 1 [22, 24, 25]. Considering the different geometries of IREs for the ATR technique, the internal reflection could be conducted by either single or multiple reflection geometries. The optical layout of both the geometries are depicted in Fig. 2 (panel a and b). Both the schemes differ in terms of number of reflections of the incident IR beam that generates an equivalent number of evanescent waves and dictates the corresponding attenuation by the sample due to absorption. Multiple reflections provide a much intense signal in comparison to the single reflection mode of the IREs due to its multiscale provision of light-matter interaction option. Although a single reflection mode IRE provides a weaker signal, it could conveniently probe the samples having a smaller amount. The active area of the IRE for carrying out a single reflection process could be achieved either by the type of IRE geometry or by using the highly focusing optics inside the ATR accessory to maintain the beam diameter at the focal spot (Fig. 1).

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Table 1 Description of common IRE or ATR crystal materials used in the ATR accessory for obtaining the IR spectrum in the Mid-IR range [22, 24, 25] IRE material

Refractive index (n1 ) at 1000 cm−1

Working spectral range (cm−1 )

Penetration depth Effective (dp ) in μm at working pH range 1000 cm−1 , θ = 45°, n2 (sample) = 1.4

KRS-5 (Thallium bromide/Thallium iodide)

2.37

17,900–400

1.73

5–8

Zinc selenide (ZnSe)

2.40

20,000–630

1.66

5–9

Diamond

2.40

45,000–10

1.66

1–14

20,000–350

1.46

1–9

AMTIR (Se/As/Ge 2.50 glass) Silicon (Si)

3.40

8300–1500

0.84

1–12

Germanium (Ge)

4.00

5500–780

0.65

1–14

Fig. 2 Optical ray diagram of a single reflection and b multiple reflection internal reflection element (IRE) geometries of ATR-FTIR spectrometer

3 Theoretical Aspects of ATR-FTIR Spectroscopy IR or vibrational spectroscopy is an optical spectroscopic tool with sensitivity and specificity to measure the various vibrations of molecules in their native state by detecting the IR radiation of a selected wavelength after passing through the sample. The detected IR radiation in the IR spectrum provides the platform to get the information about the structural molecular composition and bonding environment present in the sample [1, 2, 5, 10, 22, 26, 27]. The absorbed IR radiation is an outcome of

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the change in the dipole moment corresponding to vibration and it results in the molecular transition between the ground and excited state vibrational levels. Thus, = 0, here dμ is the molecule which absorbs the IR radiation or IR active has dμ dx the change in dipole moment and d x is the change in the bond distance. The IR absorption intensity (I Abs ) can be given as [1, 2, 22]:  I Abs ∝

dμ dx

2 (1)

Usually, the IR quantification of the absorbance can be done by Beer-Lambert law [1, 2, 6, 26, 27]:  A = − log10

IT (ω I R ) I0 (ω I R )

 = − log10 (T ) = ε(ω I R )Cd

(2)

Here, A and T are the IR absorbance and transmittance of the sample, ω I R is the IR wavenumber and I 0 is the intensity of the incident IR radiation while I T is the intensity of the transmitted IR radiation through the sample; ε is the molar extinction coefficient, C is the molar concentration of the chemical species present in the sample, and d is the path length of the IR radiation through the sample. Figure 3 (panel a) represents a schematic experimental arrangement, which is generally utilized for the absorbance studies through the detection of IR radiation transmitted from a sample. Here, we have shown that when an IR beam is made to impinge on a sample, it undergoes partial reflection due to the cell boundaries, marked by reflection intensity IR , absorption by sample molecules, with absorption intensity IA prior to transmission through the sample with intensity IT . The absorbance from Eq. (2) using Beer-Lambert law shows the linear dependence on the concentration of the sample and the optical path length, as shown in Fig. 3 (panel b). Therefore, IR spectroscopy can be used to identify and quantify the concentration of a substance in a sample.

Fig. 3 a Optical layout of an experimental scheme for absorbance studies; b linear dependency of absorbance on the sample thickness, d and concentration, C using Beer Lambert’s law

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The peak intensity in the IR absorbance spectrum depends on the concentration of particular molecules in the sample. In consideration of Maxwell equations, the field associated with light propagating in a medium is given by [1]: E(r, t) = E 0 e2πinkr −iωt

(3)

Here, E(r, t) is the amplitude of electric field at position ‘r’ in the medium of refractive index ‘n’ at time ‘t’ and ‘E 0 ’ is the field amplitude of the incident light beam. If there is an absorbing medium, then the light gets weaker as it penetrates through the medium. It can be explained after replacing ‘n’ by ‘nc ’, the complex refractive index of the medium, where the imaginary part of the refractive index κ (Eq. 4), accounts for the decay of the field during propagation through the absorbing medium (Eq. 5). We have plotted the real and imaginary parts of the refractive index of water as a function of IR wavenumber, shown in Fig. 4. n c = n + iκ

(4)

E(r, t) = E 0 e−2πκkr e2πinkr −iωt

(5)

The refractive index dispersion data of water for IR radiation has been adopted from Downing et al. [28]. The dispersion in κ values with IR and wavenumber is evident, which implies that the same sample can act as an absorbing medium for some range of IR wavenumbers whereas non-absorbing or weakly absorbing at the other wavenumbers. The absorption of light, as it propagates through the medium can find its root into energy dissipation through molecular oscillators using the harmonic oscillator model for molecular vibrations. The IR wavenumber or IR frequency (ν I R ) that a molecule absorbs and frequency of vibration of normal mode can be given by Fig. 4 Variation in real and imaginary parts of the refractive index of the water sample as a function of IR wavenumber

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Eq. 6 [1, 2, 22]. ωI R

1 1 νI R = = = λI R c 2π c



k m

(6)

Here, c is the velocity of light, λ I R is the IR wavelength, m is the reduced mass, and k is the force constant. The wavenumber of IR radiation and the force constant of molecules varies according to the change in the chemical environment of the sample. So, the strength of intermolecular interactions varies within the sample that originates a particular width of the peak in the IR spectrum. Also, more is the mass of the atoms involved in the functional group of a molecule, lower would be the resonance IR frequency. IR spectroscopy usually can be operated in reflection, transmission, and absorption modes by using different types of accessories with the FTIR spectrometer. The increasing interest of reflection spectroscopy within the research community lead the development of a new IR spectroscopic technique based on total internal reflection phenomenon named as attenuated total reflectance IR (ATR-IR) spectroscopy [1, 2, 5, 22, 32]. Here, we will briefly review the fundamental aspects of the reflection phenomenon occurring at the boundary between the two media of different refractive index. When an electromagnetic wave travels from one medium to another, the extent of reflection and transmission of light intensity at the boundary between the two optically transparent media is associated with their respective refractive indices. The panel a of Fig. 5, an IR radiation is heading towards the medium with refractive index of n2 from a medium with refractive index of n1 and it is impinged at the boundary between the two media. In the cartesian coordinate system, the z-axis is considered as the normal to the interface in xy-plane whereas the xz-plane is considered as the plane of incidence. The incident and the transmitted beams make an angle of incidence θ and angle of refraction θr with respect to the surface normal, the two of which are related through Snell’s law as follows [1, 2, 22]:

Fig. 5 a Optical representation of light propagation from one medium of refractive index n1 to the second medium with refractive index n2 as a function of angle of incidence with respect to the surface normal with a model system of IR propagation at ATR crystal/sample interface; b s- and p-polarization of the light with respect to the plane of incidence

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n 1 sin θ = n 2 sin θr

(7)

For normal incidence, the degree of reflection and transmission at the boundary is given in terms of Fresnel’s formulae as follows [1, 2]: r12 =

n1 − n2 n1 + n2

(8)

t12 =

2n 1 n1 + n2

(9)

where, r12 and t12 are the reflection and transmission coefficients for a ray propagating from medium one (n1 ) to medium two (n2 ). In the case of oblique incidence, the degree of reflection and transmission is different for the field components of light parallel and perpendicular to the plane of incidence, named as p-polarized and s-polarized light respectively described pictorially in Fig. 5 (panel b). For s-polarized incident light, the reflection and transmission coefficients are [1]:

s r12

=

s = t12

n 1 cos θ − n 1 cos θ +

 

n 22 − n 21 sin2 θ

(10)

n 22 − n 21 sin2 θ

2n 1 cos θ  n 1 cos θ + n 22 − n 21 sin2 θ

(11)

Similarly, for p-polarized light:

p

r12 p

 n 22 cos θ − n 1 n 22 − n 21 sin2 θ  =− n 22 cos θ + n 1 n 22 − n 21 sin2 θ

t12 =

2n 1 n 2 cos θ  n 22 cos θ + n 1 n 22 − n 21 sin2 θ

(12)

(13)

In reference to Snell’s law Eq. 7, when a light beam travels from a denser medium to a rarer medium, the transmitted ray bends away from the surface normal. If the angle of incidence defined as the critical angle, θc is reached, the angle of refraction reaches 90°, where the critical angle is given by [1, 2, 6, 7, 22, 27]: θc = sin−1



n2 n1

 (14)

Interestingly, for θ > θc , the transmitted beam no longer exists at the boundary and the incident ray undergoes total internal reflection. We have represented three

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Fig. 6 Reflectance at a Ge/water and b ZnSe/water interface as a function of angle of incidence for s- and p-polarized light at two different IR wavenumbers of 2700 cm−1 (κ = 0 for water) and 1630 cm−1 (κ = 0 for water) represented by solid and dotted curves respectively

situations in terms of a ray diagram in Fig. 5 (panel a). In ATR-IR spectroscopy, the IR spectrum is recorded by passing the IR beam through the ATR crystal placed in contact with the sample, where the IR beam propagates through the ATR crystal and get total internal reflection at the interface between ATR crystal and sample. During total internal reflection, an evanescent wave is generated outside to the reflecting surface of the crystal, which penetrates the sample and thus some intensity of the IR radiation is absorbed by the sample. So, the intensity of the reflected IR beam is attenuated after total internal reflection with respect to the intensity of the incident IR beam.  s 2    and R p = r p 2 as a We have plotted the variation in reflectance, Rs = r12 12 function of angle of incidence for s- and p-polarized lights in Fig. 6, panel a and b respectively, to extract the detailed understanding of the reflectance phenomenon at the interface from denser to rarer medium. The solid and dotted lines represent the variation of reflectance for Ge/water (black) and ZnSe/water (red) interfaces at two different IR wavenumbers i.e., at 2700 cm−1 and 1630 cm−1 respectively. At the normal incidence, the reflectance in s-polarization is equal to that in the ppolarization. For each interface, the reflectance reaches a maximum value, once the angle of incidence is greater than the critical angle of incidence which is characteristic of the refractive index contrast at the interfaces. In order to draw the reflectance plots (Fig. 6, panel a and b), we have used the n1 real values of the refractive index for the ATR crystal and n2 as a complex refractive index for the sample medium under investigation using IR spectroscopy. The refractive index values for Ge and ZnSe have been taken from the handbook on optical constants of solids by E. D. Palik and Connolly et al. respectively [29, 30]. In the Fig. 4, where we have plotted the dispersion in real and imaginary parts of the refractive index of water as a function of IR wavenumber, it is evident that the water sample behaves as a non-absorbing medium at 2700 cm−1 with κ = 0 and absorbing medium at 1630 cm−1 with κ = 0 respectively. Evidently, from the plots in Fig. 6 (panel a and b), we have observed that the reflectance after the critical angle reaches a maximum value of unity, that is total

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internal reflection (TIR) is observed for the non-absorbing IR wavenumber (solid curve), whereas due to the absorption of the evanescent field in the rarer medium, the maximum value of reflectance is less than unity, hence ATR is observed for the absorbing IR wavenumber (dotted curves). Along with this, it has been observed that before the critical angle has been reached, the solid and dotted curves in the reflectance plots Fig. 6a, b are not completely overlapped. This can be understood from the dispersion in refractive index values for water samples at two different IR wavenumbers 2700 and 1630 cm−1 , which we have considered here while plotting the reflectance curves in s- and p-polarizations. Also, unlike the s-polarization, the reflectance in p-polarization mode has reached to zero value for a certain angle of p incidence at which the numerator in the expression for the reflection coefficient r12 (Eq. 12) becomes zero and is termed as Brewster’s angle of incidence. The IR radiation absorbed in the sample medium through the generated evanescent wave gets weaker with respect to the penetration depth. The electric field component of this IR radiation (E) decays exponentially along the distance z from the interface and is given by [1, 2, 6, 7, 22, 27]:   z E(z) = E 0 exp − dp

(15)

Here, E 0 represents the amplitude of the electric field of IR radiation in the initial stage of the generated evanescent wave at the interface, whereas d p indicates the penetration depth of the evanescent wave in the sample. The penetration depth can be defined as the distance at which the amplitude of the electric field of IR radiation  drops down by 1e times of the E 0 i.e., and it can be given as [1, 6, 7, 27, 31]: ⎡



d p = ⎣2π n 1 ω I R sin2 θ −



n2 n1

2

⎤−1 ⎦

(16)

where, ω I R is the wavenumber of the incident IR radiation, θ is the effective angle of incidence, while n 1 and n 2 are the respective refractive indices of ATR crystal and the sample medium. Figure 7 (panel a) represents the evanescent field penetration profile in water medium at two different interfaces; Ge/water and ZnSe/water interface with 45° angle of incidence of IR beam at 1000 cm−1 wavenumber. We have also plotted the variation in penetration depth at the ATR crystal/water interface as a function of angle of incidence, shown in Fig. 7 (panel b). From the plot, it has been observed that the penetration depth has a decreasing profile with the increase in angle of incidence. Owing to the refractive index contrast between Ge and ZnSe [22, 24, 25], it has been observed that the evanescent field has a higher penetration depth of 1.31 μm at the ZnSe/water interface in comparison to the 0.62 μm value at the Ge/water interface. This implies that the incident IR field has a strong tendency to interact with molecules lying more towards the bulk water medium at the ZnSe/water crystal

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Fig. 7 a Evanescent field penetrated the water medium and b variation in the penetration depth with the incident angle; a comparative study at Ge/Water and ZnSe/Water interfaces

interface in comparison to the Ge/water interface, which seems more suitable for the characterization of molecules present at the surface comparatively. Till now, we have observed that for an angle of incidence greater than the critical angle, a non-absorbing IR beam will undergo total internal reflection, and an absorbing IR radiation will follow attenuated total reflectance at the ATR crystal/sample interface. Now, in the calculation of penetration depths in the TIR mode, n1 and n2 will be the real refractive indices of the ATR crystal and the sample respectively (Eq. 16). Whereas in the case of ATR mode, it has been suggested that we can evaluate the penetration depth values either by taking n2 as the real values of the sample refractive index or a modulus value of penetration depth can be calculated with n2 as a complex number. The ATR approach can be employed experimentally in two different modes: (i) single reflection mode and (ii) multiple reflection mode (Fig. 2). The effective path length (L e f f ) of the input IR radiation within the sample depends on the product of the total number of internal reflections (N R ) that take place in the ATR crystal at the interface in contact with the sample and penetration depth which is given as [27]: Lef f = NR × dp

(17)

In the case of multiple reflection mode of ATR, the number of reflection (N R ) can be estimated by the length of the crystal (l AT R ), the thickness of the crystal (t AT R ) and effective angle of incidence of IR radiation beam (θ ) as [27]: NR =

l AT R 2 × t AT R × tan θ

(18)

The light-matter interaction process occurs during the IR radiation absorbed by the molecules and it induces the dipole moment (μind ), which is given as [27]: μind = α  E

(19)

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where, α  is the linear polarizability and E is the electric field component of incident IR radiation. The polarization vector (P) for the incident electric field E can be given as [27, 32]: P = ε0 χ (1) E

(20)

Here, ε0 is the vacuum permittivity and χ (1) is defined as the linear electric susceptibility. Thus P gives the average number of dipoles induced per unit volume and it varies linearly with the dielectric susceptibility of the sample and the incident electric field. The light-matter interaction process based on Eq. (20) which is a linear optical process can be studied by applying ATR-IR spectroscopy [26, 27, 32, 33]. The wavelength-dependent propagation of IR radiation through the sample induces the response in the form of dispersion and the wavelength-dependent transfer of IR energy to the sample yields the absorption. Thus χ (1) in Eq. (20) is a complex tensor quantity and can be defined as a function of the complex refractive index of the sample [22, 27, 31, 33]:     χ (1) = r eal χ (1) + iIm χ (1)   = n 2 − 1 + i[κ]2

(21)

It is evident from Eq. (21) in collaboration with Eqs. 4 and 5 that the absorption of IR radiation by the sample completely on the imaginary part of linear   relies electronic susceptibility tensor i.e., Im χ (1) and the spectral profile of Im χ (1) in the IR absorption spectrum depends on the resonant vibrational modes of the molecules present  sample and the number of molecules present in the region  in the [32, 33]. The Im χ (1) is directly proportional to the IR absorbance by ATR-FTIR spectroscopy (A AT R ) and it is defined as [27, 32, 33]:   Im χ (1) = c AT R A AT R c AT R ∝

2n 1 cos θ N R d p ωI R

(22) (23)

where c AT R represents the ATR correction factor. The oscillator strength of the absorption band can be quantified by data fitting of the observed spectral profile in Lorentzian, Gaussian, or Voigt functions. The fit of the Voigt function profile contains the contribution of both Lorentzian and Gaussian profiles and defined as the convolution of Lorentzian and Gaussian functions [33]:   2 ω I R −ωq ) ( exp −    (q )2  Im χ (1) = Aq  2 ω −ω q 1 + ( I R 2q ) (q )

(24)

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Here, Aq ,  q represents the amplitude, half-width at half maxima of the absorbance peak with ωq as the wavenumber of the qth resonant mode. The ATR-FTIR spectroscopy can also be utilized as a structural probe to get the information of the sample by having polarized light as an incident IR radiation [34]. The use of polarized light as an incident IR radiation provides the selectivity to collect the vibrational spectrum of a sample with different components of an electric field directed in the different planes i.e., perpendicular (electric field of s-polarized IR radiation) and parallel (electric field of p-polarized IR radiation) to the plane of incidence, respectively (Fig. 5, panel b). Here, we will explain the efficacy of polarized ATR-FTIR spectroscopy in the calculation of the orientation tilt angle of the molecules at the ATR crystal/sample interface, and the corresponding theoretical model is well adopted in literature as presented below [1, 34]. We can calculate the electric field components of s- and p-polarized lights at the interface as follows [1, 34]:   sin2 θ − n 231 cos θ E x =     1 − n 231 1 + n 231 sin2 θ − n 231 2

(25)

2 cos θ E y =   1 − n 231

(26)

2n 232 sin θ cos θ E z =     1 − n 231 1 + n 231 sin2 θ − n 231

(27)

E x , E y , and E z are the electric field amplitudes of the incident IR-beam in the lab Cartesian coordinate system, and are dependent on the incident angle and refractive indices of the two media forming the boundary. n 31 and n 32 are relative refractive indices of the ATR crystal and sample medium with respect to air, defined as follows: n 31 =

n3 n1

(28)

n 32 =

n3 n2

(29)

Let α be the tilt angle of the IR-dipole moment with respect to the surface normal, which can be related to the dichroic ratio (DR) as follows [34]: DR =

E y2 sin2 α As (ω)  = 2 2 A p (ω) E x sin α + 2E z2 cos2 α

(30)

Here DR is referred as the absorbance ratio in two different polarizations, As (ω) and A p (ω) are the measured sample absorbance at a particular wavenumber ω in s-

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Fig. 8 Theoretical curves for orientation studies using ATR-IR spectroscopy showing variation in dichroic ratio as a function of tilt angle α of the IR dipole moment at 1000 cm−1 for the Ge/water and ZnSe/water interfaces

and p-polarization of incident IR radiation beam respectively. Figure 8 represents the theoretical curve showing the variation of DR with tilt angle α for the water layer at two different crystals with 45° as the angle of incident IR beam. It is evident from the plot that the orientation curves for the water molecule tilt angles at Ge and ZnSe interface are significantly different. A zero dichroic ratio corresponds to normally oriented IR-dipole moment with respect to the surface; whereas DR value of 1.07 and 1.26 corresponds to flat conformation of the water dipole i.e., parallel to the Ge/water and ZnSe/water surface respectively. One can obtain the orientation information of an IR dipole from the intersection of the respective theoretical curve with the experimentally obtained DR value. Thus, the ATR technique of IR spectroscopy probes the molecular interactions and their impacts in the bulk medium as well as the orientation of molecules at the boundary between the ATR crystal and the sample under investigation [17, 18, 33, 34].

4 Recent Trends in the Application of ATR-FTIR Spectroscopy for Molecular Characterization in Aqueous Media The preceding sections briefly introduced the background and some elementary instrumental and theoretical segments of the ATR-FTIR vibrational spectroscopy. The understanding of inter- and intra-molecular interactions hold the key to extract the detailed physical and chemical behavior of various compounds in the aqueous phase. In the current section, we intend to focus on certain contemporary research contributions inspecting the fundamentals of molecular vibration by employing the ATR-FTIR vibrational spectroscopic tool.

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4.1 Probing Biomolecular Self-Assembly Process The existence of all bio-systems has relied primarily upon their evolution and adaptation to the changing environment by forming intricately complex architectures through the self-assembly process. Biomolecules like, amino acids, peptides, proteins, lipids, nucleic acids, etc. arrange and consolidate into higher-order assemblies involving cellular organelles, membranes, muscles, bones, and organs [35–37]. They may also accumulate in a disordered random fashion to form aggregated-like structures within the biological system which are responsible for various diseases for example, the formation of amyloid aggregates from amino acids, peptides, or proteins, causing several degenerative disorders (like Parkinson’s disease) [38–40]. Inspired by nature’s ingenious process, self-assembly has been thoroughly employed in the field of building energy devices, remediation technologies, designing model biomaterials, nanofabrication, sensing, imaging, etc. [36, 37, 39, 41]. The formation of higher-dimensional structures due to the self-assembly process ensues from the molecular-level interactions including coordination bonds, stacking interactions, hydrogen bonding, electrostatic, and hydrophobic forces [35–37]. These molecular level interactions are predominantly governing the morphology of the selfassembly process. The surrounding conditions further influence their organizing strength, which involves concentration, solution pH, temperature, ionic strength, solvent composition, additives, etc. [36, 42]. In the recent past, a lot of research activities have been focused towards understanding the fundamentals of biomolecular self-assembly process and its relevance for design and fabrication of various nanotechnological platforms [36, 37, 39, 43– 46]. Here, we are covering some of the selected studies focused in the direction of amino acid and peptide self-assembly process using the ATR-FTIR spectroscopic technique. Valery et al. [47] characterized the pH-dependent conformational changes of the decapeptide self-assembly process. It is found that the switching of a decapeptide (Triptorelin) from a globular conformation at pH > 7.5 to an extended state below pH 6.5. It is supported by the electron micrograph studies shown in panel a of Fig. 9. Triptorelin decapeptide used for the study is pE1 -H2 -W3 -S4 -Y5 -(D)W6 -L7 -R8 -P9 G10 -NH2 with three aromatic (Y5 , W3 , W6 ) and three ionizing moieties (R8 and Y5 with pKa > 10, H2 pKa 6.1). The studies were carried out in the pH range from 5 to 8. The protonation state of histidine (-H2 -) with the mentioned pH range was probed by ATR-FTIR spectroscopy. From the spectra shown in panel b of Fig. 9, the absorption peak at ~1097 cm−1 depicted the positively charge imidazolium moiety of –H2 – at low pH of 5.5, while it deprotonates at higher pH of 8.5 with a peak absorbance at ~1106 cm−1 . The ATR spectra in the amide I region in panel c of Fig. 9 shows the formation of β-sheet structures through hydrogen bonding at lower pH state, which gets weaker at high pH value. The observations from ATR-FTIR spectroscopy indicated that the conformational fate of the triptorelin decapeptide is primarily subjected to a single –H2 – protonation. This mechanistic study pin-pointed the viability of such pH-dependent peptide as a molecular switch for designing responsive nanomaterials.

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Fig. 9 Self-assembled conformation of triptorelin decapeptide as a function of pH. a Electron micrograph image of the small nanotubes at low pH (black arrow) coexisting with large nanotubes (red arrow) formed at low and high pH respectively. b ATR-FTIR absorption spectra in amide I region of small (green line, pH 5.5) and large (blue line, pH 8.5) nanotube structures. c ATRFTIR spectra depicting pH-dependent ionizing states of Histidine (H2) residue of the decapeptide. Adapted with permission from [47]. Copyright, 2015, Macmillan Publishers Limited

Tomar et al. [26] carried out a pH-dependent ionizing study based on the l-Phe self-assembly to identify the role of electrostatic interaction in the emergence of varied aggregation states in the aqueous system with ATR-FTIR spectroscopic tool. The ionization of l-Phe at 100 mM concentration was probed at pH values 1.5, 5.8, and 12.2 as depicted in Fig. 10. Pertaining to pH sensitivity, diamond crystal assembly was utilized for spectral acquisition at extreme pH values. As noted from the ATR-FTIR spectra in Fig. 10 panel a, l-Phe acquires a cationic state at pH 1.5 and display vibrational modes of protonated amine groups at ~1560 and ~1643 cm−1 and a vibrational feature of a neutral form carboxylic acid (COOH) at ~1720 cm−1 . At pH value of 12.2 (Fig. 10, panel c), the anionic state of the amino acid displays neutral amine bending mode β(NH2 ) (~1355 cm−1 ), and negatively charged carboxylate groups as COOs – and COOas – modes positioned at ~1410 cm−1 and ~1560 cm−1 , respectively. The IR absorption spectra of l-Phe at a pH value of 5.8 shows the zwitterionic form of the amino acid (Fig. 10, panel b). The ATR-FTIR spectra of varying ionization states were compared with the SEM results observed for the drop-casted samples which presented a distinct morphological verification for the l-Phe sample at neutral pH (fibrillar state) and cationic and anionic pH values (flake structure) (Fig. 10, panel d, e, f), correlating well with the ATR-FTIR results. It was concluded that the inhibition of the fibrillar morphology of l-Phe beyond neutral pH state arises due to inter-molecular electrostatic interactions that dominate over the π–π stacking interactions among aromatic moieties. From these studies, it could be summarized that the information about the chemical profile and interactive forces provided by the ATR-FTIR technique can directly correlate with the morphological changes occurring during the self-assembly process.

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Fig. 10 In-situ ATR-FTIR absorption profiles for the varying ionizing states of l-Phe amino acid (100 mM) in aqueous solution with pH values of a 1.5 (cationic), b 5.8 (zwitterionic), and c 12.2 (anionic), respectively. The molecular structural form of l-Phe in aqueous media at different pH values are shown in the inset. Corresponding SEM images of dried l-Phe samples at d pH 1.5, e pH 5.8 and f pH 12.2 (2 μm scale). Adapted with permission from [26]. Copyright, 2019, The Royal Society of Chemistry

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4.2 Characterizing Engineered Nanoparticles and Biomolecule Interactions Nanoparticles are referred to as the self-organized materials that form nanometersized particles ranging up to about 100 nm [48]. Accelerated growth in the development of the engineered nanomaterials has led to the amalgamation of this technology with different interdisciplinary scientific areas. It has expanded its enrollment in pharmaceutics, biomedical, biomaterial, electronics, and purification applications [49, 50]. This has led to an increase in the direct impact and association of various nanomaterials with living species as well as the environment in which they thrive [51]. They often interact with the cells and their constituents like amino acids, proteins, DNA, lipid membrane, etc. The nano-bio interactions restructure and transform the fundamental properties of the nanomaterial surface like size, morphology, surface functionality, and other mechanical properties, which affect their dynamics and viability in vivo [52, 53]. Biomolecules also acquire distinct chemical structure and behavior upon interacting with the nanoparticles [51, 53]. The molecular forces which operate during the nano-bio interactions majorly involve van der Waals forces and electrostatic interactions, and certain short-range interactions including steric forces or interactions with neighboring solvent molecules [52, 53]. Probing such molecular interactions among the nanomaterials and biomolecules gives an insight towards the structural stability and the dynamical activity of the biomolecule associated nanoparticles in the aqueous solution [53]. Xu and Grassian [54] explored the interaction of TiO2 nanoparticles with BSA protein by varying the pH values of the solution (pH 7.4, 4.5 and 2.0) which modeled the pH conditions of human blood, lung and stomach fluid environment, respectively. In-situ adsorption and coverage profile of BSA over the TiO2 nanoparticle surface was studied using ATR-FTIR spectroscopy using AMTIR (Se/As/Ge glass) crystal. Here the role of phosphate in BSA adsorption over TiO2 nanoparticles is investigated (shown in Fig. 11) in the amide region with time. BSA protein presents two characteristic vibrational peaks at ~1651 cm−1 (amide I) and ~1548 cm−1 (amide II). A peak at ~1397 cm−1 is attributed to the C–O carboxylate stretch mode of BSA. Thus, the ratio of amide I/II peaks qualitatively determines the fate of BSA protein over the TiO2 surface. At pH 2.0, this peak ratio gets disturbed, which showcases that the BSA proteins denature at acidic pH value when it gets adsorbed at the TiO2 nanoparticle surface. Conversely, BSA conformation at pH 7.4 and 4.5 stays consistent with time (Fig. 11, panel a). In the presence of phosphate in solution, profile for both phosphate and BSA evolve in the spectra indicating their co-adsorption over TiO2 . Even at the acidic pH 2.0, co-adsorbing phosphate groups prevents the protein’s secondary structure from denaturation after adsorption (Fig. 11, panel b). The finding of the work epitomized the pH-dependent nanoparticle-protein interactions influenced by the extraneous salts. The intermolecular associations of the nanoparticle and single amino acid were selectively studied in aqueous media by Tomar et al. [27] using ATR-FTIR spectroscopy, wherein the interaction of synthesized metal-oxide nanoparticles (i.e.,

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Fig. 11 ATR-FTIR normalized spectra of the adsorption of BSA on TiO2 nanoparticle surface at varying time scales (10, 30, 60 and 90 min in red, green, blue, purple lines respectively) a without phosphate and b with phosphate at the pH values of 7.4 (top), 4.5 (middle) and 2.0 (bottom spectra). Adapted with permission from [54]. Copyright, 2017, American Chemical Society

cupric oxide (CuO) and zinc oxide (ZnO)) have been examined with three different amino acids, l-Leucine (l-Leu), l-Cysteine (l-Cys), and l-Serine (l-Ser). In this work, the effect of the differential behavior of CuO and ZnO nanoparticles individually with three amino acids is seen over the biomolecular fingerprint region of the amino acid functional moieties in zwitterionic state at pH 5.8. The spectral analyses of metal-oxide nanoparticles were made solely based on their impact over the carboxylate and ammonium functional groups of the respective amino acids taken for the study. For l-Leu amino acid (10 mM), the fingerprint region (Fig. 12, panel a) showcased that the ZnO nanoparticles presented a significant impact over the pristine l-Leu IR spectrum in comparison to CuO nanoparticles with a visible dominant peak at ~1106 cm−1 and slight broadening observed around ~1600 cm−1 , assigned to the antisymmetric carboxylate stretching mode (COOas – ) of the amino acid. Considering the l-Cys amino acid (0.1 mM) (Fig. 12, panel b), the IR absorption spectra displayed a huge impact on ZnO than CuO nanoparticles with a high-intensity peak of bending anti-symmetric (βas (NH3 + )) vibrational mode of an amino group (~1652 cm−1 ), and a shoulder peak at ~1585 cm−1 of COOas – . CuO nanoparticles were only seen to affect the amino acid structure through the CH2 wagging vibrational mode. For the case of l-Ser amino acid (Fig. 12, panel c), both the oxide nanoparticles in the IR absorption

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Fig. 12 FTIR-ATR absorption spectra of a l-Leu (10 mM), b l-Cys (0.1 mM) and c l-Ser (100 mM) in the absence and presence of CuO and ZnO nanoparticles at ZnSe-water interface with water signal subtraction in the fingerprint region (1000–1800 cm−1 ). Adapted with permission from [27]. Copyright, 2019, Springer Nature Singapore Pte Ltd.

spectra in the fingerprint region displayed no significant changes. The overall spectra of l-Ser get diminished after oxide nanoparticles addition. The present study demonstrated the role of molecular-level functional group specificity of the amino acids for certain metal-oxide nanoparticles, depicting differential behavior of the synthesized nanomaterials with biomolecules.

4.3 Investigating Binary Solvent Mixtures for Industrial Applications A binary solvent mixture or solution is the combination of dissimilar substances that confers unconventional properties to the system. The components of the binary solution involve in molecular-scale associations predominantly in the form of hydrogenbonds that either self-associate or interact with secondary polar or non-polar components present within the solution [55, 56]. This classifies certain properties to the binary system such as density, polarity, refractive index, viscosity, surface energy, and

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Fig. 13 a Adsorbate ATR-FTIR vibrational profile of the n-propanol and water binary mixture with varying mole fraction (y) values of n-propanol (1–0). The CH-region (2750–3000 cm−1 ) intensities are amplified by 5 for clear representation. b Adsorbed effective thickness measured from the peak intensity values obtained from ATR-FTIR spectra separately of n-propanol and water molecules over SiO2 . Adapted with permission from [68]. Copyright, 2012, American Chemical Society

different thermal properties, which distinguishes it from pure solutions [55, 57–60]. These characteristics are influenced by several external factors like temperature, ionic strength, mechanical disturbances, etc. [61–63]. Comprehension of these parameters offers useful insights about binary mixtures in the domains of chromatographic purification or adulteration processes, biomolecular coupling reactions, drug designing, microfluidic devices and hydrodynamic cavitation in microchannels [62, 64–66]. Several studies have been performed aiming at the interaction processes occurring among the water molecules and different organic liquids, including alcohols, acetone, dimethylsulfoxide (DMSO), acetonitrile, esters, ethers, dimethylformamide (DMF), and amine-based solvents [57, 59, 61–63, 67]. Barnette and Kim [68] probed the adsorbate thickness of the binary mixture of n-propanol and water layered on the fused silica surface. Panel a of Fig. 13 display the ATR-FTIR spectra of n-propanol with water as a function of varying n-propanol vapor mole fraction (y). The observed results depicted that the spectral intensity of alkyl stretching mode stays relatively even for the ypropanol between 1 and 0.4. Below ypropanol value of 0.36, the intensity gets significantly decreased. However, contrastingly in the higher wavenumber region, the OH-stretch vibrational mode observed at ~3550 cm−1 is seen to show a slight red shift along with a rise in the intensity in its vibrational mode towards peak centered at ~3400 cm−1 as ypropanol decreases beyond 0.6. Thus, as evident from the spectra, the presence of water molecules becomes dominant. The authors utilized the ATR intensity profile of the n-propanol-water binary mixture to evaluate the effective thickness of the adsorbed layer. Figure 13 (panel b) discretely shows that as the ypropanol decreases beyond 0.4 value, the effective thickness of the n-propanol layer over the crystal surface also decreases reaching ~0.23 nm thick layer (equivalent to 0.4 mL of n-propanol). It could be conversely realized for an increase in the multilayer formation of water molecules which rises to ~6.5 nm (>20 mL) of the adsorbed layer over the crystal surface. Therefore, considering a direct association

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Fig. 14 DMF-water mole fraction series of the binary mixture solution in the a OH-stretch region, and b CH-stretch region, observed by ATR-FTIR vibrational spectroscopy. The DMF concentration proportion varies from 0.0 to 1.0 calculated in terms of mole fraction. Adapted with permission from [33]. Copyright, 2020, AIP Publishing

of the IR intensity with the number of molecules at the surface, adsorbate thickness and molecular propensity could be conveniently found using ATR-FTIR vibrational technique. Tomar et al. [33] in their work considered the interactions among the two polar solvent molecules, DMF and water as a binary mixture. DMF is an organic polar solvent that is intensely used in the pesticide industry, coatings, adhesives, peptide coupling, etc. [66, 69, 70]. DMF solvent contains a formamide group that could act as a strong model molecule to decipher the hydroxyl and amide group linkages in aqueous systems, mimicking the process of peptide or protein hydration [69, 70]. Tomar et al. analyzed the effect of the DMF mole fraction (0.0–1.0) in the CHand OH-stretching regions of the DMF-water binary solution [33]. The ATR-FTIR spectral fitting assigned three vibrational modes in the OH-stretch region at peak positions ~3226 cm−1 for strongly H-bonded water molecules, and ~3410 cm−1 and ~3560 cm−1 peaks representing weakly H-bonded states of water molecules (see Fig. 14, panel a). As the DMF proportion increases within the DMF-water binary mixture, the designated peaks in the OH-stretch region decrease in their intensities and get shifted. This spectral behavior indicates that the DMF is forming hydrogen bond-based associations with the water molecules which influences their bond strengths and alter their number leading to the peak shifts and intensity decrement as observed from the spectra. Similarly, the CH-stretch region probing of the DMF-water binary solution with increasing DMF concentration in the bulk phase is shown in Fig. 14, panel b. With increasing DMF proportion, the characteristic vibrational contributions from the DMF-functional groups were observed which consequently shifted. These include the formyl CH group at ~2884 cm−1 shifted to 2855 cm−1 , and the cis-CH3 group shifted from the peak position ~2940 cm−1 to 2924 cm−1 . This shift arises predominantly due to the formation of hydrogen bonds among the two DMF molecules and that of DMF with the neighboring water molecules. They inferred from the ATR-FTIR and complementary studies that the

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DMF molecules tend to cluster among themselves at higher concentrations at the crystal surface projecting significant spectral changes.

4.4 Application in Forensic Sciences Over the course of a few years, ATR-FTIR spectroscopy has acquired the potency in differentiating the constituents and estimating their degradative states at the molecular level. It has been therefore applied not only for the biomolecular probing but also in the domain of forensic chemometrics, identifying toxic chemicals, and the bio-threat or warfare agents for forensic analysis [71–74]. Considering the simplified mode of working of the ATR-FTIR technique, it is now being widely commercialized as a hand-held portable testing tool for on-site evaluation purposes especially in the field of forensic sciences [75]. Many reports reveal its importance in quantitative and trace-level identification of elements with a non-destructive approach which include discriminating paper elements and its age, drug profiling and quantitation, probing sunscreen stains, predicting the sex of an individual through fingernail analyses, etc. [71, 72, 76–78]. Zou et al. [79] unraveled the potential of the ATR-FTIR spectroscopic tool along with the Raman spectroscopy to identify and differentiate between the human blood and semen sample as well as their deterioration state. Spotting of the biofluid presence and their preservation is pivotal for further examination of the crime scene conditions. In this work, the authors utilized a single reflection ATR-FTIR spectrometric geometry with a horizontal ZnSe crystal mount accessory. The sample slides containing the body fluid sample were attached to the crystal surface via clamping. The ATR spectra were taken for a human blood sample at varying time scales (fresh, after 1/4th day, 1 day, and 2 days) shown in Fig. 15, panel a. The fresh blood was analyzed in a liquid state while for the aged blood samples, they were observed in the dried state. The vibrational profile of the blood sample shows prominent peaks in the

Fig. 15 ATR-FTIR vibrational spectra of a human blood sample at time scales (fresh, 1/4, 1, and 2 day(s)), and b human semen samples at varying time scales (fresh, 5/4, 5, and 30 days). Adapted with permission from [79]. Copyright, 2016, The Royal Society of Chemistry

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amide I (~1635 cm−1 ), amide II (~1531 cm−1 ) and amide III (~1238 and ~1304 cm−1 ) vibrational modes from the peptide chains, respectively. The vibration pattern from the C–O bond and the bending mode of C–O–H vibration is projected at 1082 and 1165 cm−1 respectively. Other peaks including O–H stretch mode (~3278 cm−1 ) and alkyl stretch modes (~2872, 2931, 2958 cm−1 ) were also noted. Sample aging is observed by calculating the peak IR absorbance ratios of 1531 and 1635 cm−1 peak positions which tends to increase as the sample ages from 1/4th—1 day, and then gets equilibrated after the first day. Likewise, the vibrational spectra of human semen samples (Fig. 15, panel b) were evaluated at different time rates, i.e., fresh sample, sample after 5/4th day, 5, and 30 days, and approximately similar peak positions were observed on comparing with the blood samples. Both the samples displayed approximately similar peak patterns considering the contribution from the protein albumin that is an intrinsic part of both. However, contrasting to the blood sample spectra, semen samples show an appearance of the 1057 cm−1 peak position assigned to the asymmetric mode of P–O–C vibration. For the human semen sample examination at different times, the IR intensity ratios of the peaks at position 1240 and 1633 cm−1 were considered which reduced in its intensity as the sample ages, indicating the degradation of the sample components. Fresh samples were dominated by the broad water absorbance peaks since they were evaluated in the liquid state. While using the ATR technique to distinguish between the two biofluids (Fig. 15, panel a and b), the fresh blood sample retained a 1531 cm−1 peak which is absent from the fresh semen sample spectra. Further, the vibration mode from C–O stretch and C–O–H bending mode is observed for blood samples while asymmetric P–O–C vibration mode is distinctive for the human semen samples. This work established the basis of the combination of ATR-FTIR mid-infrared radiation-based technique and Raman spectroscopy to carry out a chemical-free evaluation of the on-scene procured samples depending on their degradative timeline. This could affirm in correlating the suspected individuals with crime events. Mamede et al. [80] estimated the compositional changes and identification of the type of bone from the perspective of the archeological forensics of the burned bone samples. Utilizing the vibrational spectroscopic tools, the authors studied the role of OH vibration modes in spectroscopically establishing a distinction between the archeological remains of recently burned and ancient burned fossils of bones and teeth (Fig. 16). Platinum single-crystal ATR assembly was utilized in this study. Both the sets presented sharp phosphate signatures (603 and 1035 cm−1 ) that are indicative of their high crystalline property (Fig. 16, panel a, b and c). However, a distinction among the two sets arises in observing the OH stretch contribution (at ~3572 cm−1 ) and OH liberation peak (~630 cm−1 ) from the recent skeletal remains (Fig. 16, panel d and e) instead of the fossil samples (panel a) which is very less. From the ATR spectra, OH/P ratio values were considered as a landmark to analyze the burned bone or teeth samples. From the observed spectral features, it is inferred that the mineralization of the fossil bone matrix leads to the replacement of the hydroxyl groups with different anions with time. This crystallinity pattern of the recently burned and fossilized samples differs and the occurrence or absence of the OH mode in the vibrational spectra is an indication of their discrimination.

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Fig. 16 Vibrational spectra of archeological fossil samples in range a 380–4000 cm−1 , and b 380– 1750 cm−1 . ATR spectra of archeologically genuine burned samples in range of c 380–4000 cm−1 , d 380–800 cm−1 and e 3400–3700 cm−1 examining the OH vibrational modes. Adapted with permission from [80]. Copyright, 2018, American Chemical Society

4.5 Examining Surface Reactions at Solid/electrolyte Interface The electrochemical devices work on the principle of converting chemical energy into electrical energy or vice-versa. The topic has received a lot of attention in various practical approaches from the past few decades for providing efficient, clean and portable power sources [81–86]. The rechargeable batteries like Li-ion batteries (LIBs) are one such product of electrochemical devices that exhibit high energy density, high open-circuit voltage, stable cyclability after prolong use and have emerged as an aid in industrial applications [87–89]. The electrochemical reactions within the sandwich of electrode and electrolyte yield products that severally affect the intercalation of ions due to the formation of solid electrolyte interphase (SEI) during several cycles and impact the performance of such devices [90–93].

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The construction of SEI upon contact with the electrode consists of reduction products of electrolyte (residues) involved during various cycles of lithiation/de-lithiation [92, 93]. The molecular insight of adsorbed moieties at the SEI requires interfacial spectrometer techniques that can distinguish in determining the involved role of electrolytes, electrodes and ions in the electrochemical processes [92, 93]. ATR-FTIR technique offers such substantiality in experiments to study the interactions of the chosen molecules and solid surface under a wide range of conditions [16, 24, 94–98]. Shi et al. [96] investigated the Li-ion electrolyte reduction process at two different metal electrodes surfaces (Au and Sn) by in-situ ATR-FTIR spectroscopy. The electrochemical cell used in the study consists of Au and Sn foil as a working electrode and lithium foil as both the counter and reference electrode. The electrolyte composition of lithium hexafluorophosphate (LiPF6 ) and ionic liquid of ethylene carbonate (EC) and diethyl carbonate (DEC) in a proportion of 1 M LiPF6 /EC: DEC (=1:2 v/v) were used to elucidate the involved surface chemistry of the reduction process with a different metal electrode. The working electrode was pressed against a 45°-cut Ge prism. Assembly of all the three electrodes and electrolytes were kept in a Teflon electrochemical cell. The ATR-FTIR measurements were acquired after providing electrode potential in the range from the open-circuit voltage (OCP) to various potential values before Li-deposition for 2 h. The in-situ studies showed different pathway reaction of electrolyte in contact with used metal electrodes. Authors identified (Fig. 17, panel a, b and c) insoluble lithium ethylene dicarbonate (LiEDC) forms on the Au electrode with the appearance of 1115 cm−1 at ~0.6 V whereas relatively soluble dioahexane dicarboxylate (DEDOHC) appears on the Sn electrode (dominant peak centered at 1744 cm−1 ) at ~1.25 V with common lithium propionate (characteristic peak positions observed at 2920, 2851, and 1565 cm–1 ) formed on both the electrodes. Based on the FTIR observations, it is concluded that two different reaction mechanisms i.e., a non-catalytic reaction path (Au electrode) and a catalytic reduction path (Sn electrode), to clarify the observed surface dependence. The study shows important implications in understanding the surface modification dependence upon the selection of polymer binder, electrolyte and electrode in the SEI formation in the LIBs. Shi et al. [16] studied on the electrode/electrolyte interface and focused on investigating the decomposition process of SEI on the graphite electrode during the electrochemical cycling procedure. The work is performed by using in-situ ATR-FTIR spectroscopy with varying penetration depth. The electrochemical cell utilized for this study consisted of a silicon working electrode, lithium electrode as both the counter and reference electrode and an electrolyte. The electrolyte composition of 1 M LiPF6 (EC: DEC = 1:2 v/v) was used and the detailed description of the cell is mentioned in their study [16]. The angle of incidence of the IR beam varied from 45° to 65° with respect to the ATR prism. The penetration depth of the evanescent wave decreased on increasing the incidence angle of IR on the sample (refer Eq. 16), which provided subtle information up to 65 nm depth at ~65◦ covering the SEI deposited on the electrode/electrolyte interface. In contrast, the diffuse layer (∼0.5 μm) at ~45◦ probed the electrolyte reactivity processes in the bulk phase.

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Fig. 17 In-situ ATR-FTIR spectra of a on Au electrode acquired after applying a potential of open circuit potential (OCP), 0.6, 0.4, and 0.2 V. b Sn electrode acquired after applying a potential of OCP, 1.4, 1.0, and 0.75 V (both electrodes are kept in contact with EC/DEC 1 M LiPF6 electrolyte). c Sodium propionate (reference spectrum for lithium propionate), DEDOHC, and LiEDC. Adapted with permission from [96]. Copyright, 2015, American Chemical Society

It is revealed that the disappearance of C–H signal at 2986 cm–1 above the incidence angle of 45◦ (Fig. 18) corresponded to hydrocarbon chains towards the bulk electrolyte. However, other peaks start to disappear above 60◦ with simultaneous occurrence of new peaks at 3008, 2975, 2961, 2943, 1664, and 1310 cm–1 assigned to LiEDC, as shown in Fig. 18 during long-term cycling. A marginal difference of diffuse layer (bulk electrolyte) to near-surface of electrode/electrolyte distinctly

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Fig. 18 In-situ ATR-FTIR spectra of lithiated silicon electrodes acquired after applying 5 mV potential (spectra collected after applying potential for 1 h) with variation in incidence angle from 40◦ –65◦ . Adapted with permission from [16]. Copyright, 2017, American Chemical Society

provides the flexibility of retrieving SEI information under different conditions of potential and cyclability of the electrolyte to understand its surface mechanism. In this complete study, the retrieved results with in-situ ATR-FTIR showed DEDOHC as the major SEI product on native silicon oxide film during the initial cycling process that contributes towards the capacity losses. While for long-term cycling, LIEDC came out as a potential reduction product on the lithiated silicon electrode that participated in the passivation process over the cracked surfaces (after consuming electrons). Thus, these studies [16] successfully achieved the SEI product information during cyclic lithiation/de-lithiation as verified by cyclic voltammograms (CV) and showed the enormous ability of ATR-FTIR spectroscopy as a technique to elucidate the detailed molecular-level information in tracing out the surface mechanism within various electrochemical devices. Kollath et al. [98] focused on improving the silicon electrode electrochemical performance by probing the physicochemical properties of a binder polymer in contact with the silicon electrode by using ex-situ ATR-FTIR spectroscopy. This study was further verified and conducted with the analysis of XAS, AFM, and TGA to evaluate the performance of LIBs. The authors used poly(1-pyrenemethyl methacrylate) (PPy) as a binder due to a dual conductive-additive functionality of this polymer to preserve the mechanical integrity of composite electrodes. The powdered PPys immersed in the electrolyte solution (1.2 M LiPF6, EC: DC: fluorinated carbonate (FEC) in the ratio 2.1:4.9:3 v/v) for 24 h, filtered, washed with DMC and dried for 24 h in the glove box. The obtained FTIR spectra (Fig. 19, panel a) of PPy exposed to the electrolyte, suggested no chemical degradation of PPy in the electrolyte and include 1775 and 1810 cm−1 attributed to solvent trapped in the PPy structure. The molecular

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Fig. 19 Ex-situ FTIR spectra of a electrolyte and PPy powder soaked before and after in the electrolyte and drying at room temperature for 24 h, b Si(100) electrode and Si(100)/PPy cycled electrodes. Adapted with permission from [98]. Copyright, 2018, Elsevier B.V.

investigation of Si(100) and Si(100)/PPys after 8 cycles (Fig. 19, panel b) shows a reduction reaction product that contains intense peaks assignments of asymmetric C–H peaks (2850, 2930 cm−1 ), Si–O (1020, 778 cm−1 ) and O–C stretching of Si– O–CH3 (1160 cm−1 ). But peak distribution comparison of Si(100) and Si(100)/PPys below 1300 cm−1 shows variations in the film growth at the interface. A study via ATR FTIR spectroscopy significantly indicates the importance of uniform binder distribution in the composite electrode association with mechanical integrity to achieve a stable interfacial behavior during long term cycling.

5 Conclusion In the current chapter, we have explored the basic theoretical and experimental aspects of the ATR-FTIR vibrational spectroscopy. A convenient evaluation of samples in both solid and liquid state with a non-destructive and non-invasive approach ensures the versatility of this vibrational spectroscopic technique. The ATR-FTIR spectral acquisition of different compounds quickly aids in identifying the sample’s composition and the proportion of the constituents through their fingerprint signatures irrespective of their organic or inorganic chemical identity. To gain a fundamental understanding of ATR-FTIR spectral evaluation, we have systematically discussed five different application areas in-depth, covering distinct facets of molecular assembly, molecular interactions, electrochemical behavior, and real-time implementation of the ATR-FTIR technique in the forensics. With the involvement of chemometrics for data interpretation, polarization schemes for identifying molecular orientation, and

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devising a portable hand-held ATR probe for on-site identification, ATR-FTIR spectroscopy is developing at a faster pace. It is commonly being used in conjunction with the other techniques to attain a more refined analysis in the research and at the industrial scale for measurements and quality control. Therefore, further advances could be anticipated in the infrared vibrational spectroscopic field that would improve the availability, easy usage, and the analytical approach in the molecular spectroscopy. Acknowledgements The authors acknowledge research support from the Center for Biomedical Engineering and Department of Physics, Indian Institute of Technology Ropar for SEED Grant and Defence Research and Development Organisation (ERIP/ER/1500487/M/01/1602).

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Two Dimensional Infrared Spectroscopy: A Structure Sensitive Technique with Ultrafast Time Resolution Deborin Ghosh, Samadhan Deshmukh, Srijan Chatterjee, Sushil Sakpal, Tapas Haldar, Ambuj Dhakad, Somnath Kashid, and Sayan Bagchi

Abstract In the recent years, 2D IR spectroscopy has evolved as a structure sensitive experimental technique to study ultrafast structural and conformational dynamics across broad fields of science. This article explains the 2D IR experimental methodology and provides a qualitative understanding of the basics principles and spectral signatures of the various dynamical processes. A comprehensive review of various 2D IR investigations provide an in-depth understanding of the applications of this spectroscopic technique in chemistry, materials, and biology. Recent technological advances are discussed to explain the future scope of 2D IR in deciphering the molecular structure-function relation. Keywords 2D IR spectroscopy · Cross peaks · Chemical exchange · Vibrational coupling · Ultrafast dynamics

1 Introduction The advent of femtosecond laser sources and other technological breakthroughs allowed the design of structure sensitive non-linear spectroscopic methodologies with ultrafast time resolution. Multidimensional spectroscopy techniques, namely two dimensional electronic spectroscopy (2D ES) [1], two dimensional infrared spectroscopy (2D IR) [2], two dimensional sum frequency generation (2D SFG) [3, 4], mixed frequency 2D spectroscopy [5, 6], have been developed. Each of these

D. Ghosh · S. Deshmukh · S. Chatterjee · S. Sakpal · T. Haldar · S. Kashid · S. Bagchi (B) CSIR-National Chemical Laboratory, Physical and Materials Chemistry Division, Pune 411008, India e-mail: [email protected] S. Deshmukh · S. Chatterjee · S. Sakpal · T. Haldar · S. Kashid · S. Bagchi Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India A. Dhakad School of Chemical Sciences, National Institute of Science Education and Research (NISER), PO-Bhimpur-Padanpur, Via-Jatni, District-Khurda, Bhubaneswar 752050, India © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 D. K. Singh et al. (eds.), Modern Techniques of Spectroscopy, Progress in Optical Science and Photonics 13, https://doi.org/10.1007/978-981-33-6084-6_2

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multidimensional techniques have proven to be powerful spectroscopic tools to interrogate the structure and dynamics across broad fields of science. Here, we will focus on 2D IR spectroscopy. This chapter will include the experimental methodology of 2D IR, the basic principles of 2D IR, the technological advances in 2D IR, and the applications of 2D IR spectroscopy in various fields of science. 2D IR spectroscopy began its development some twenty years ago [2] and since then it has served as a powerful experimental method to address several aspects of ultrafast structural and conformational dynamics in chemical and biological systems. Although an infrared absorption spectrum contains the complete knowledge of structures and dynamics, the spectral responses to the electromagnetic fields are interpreted based on statistical models, which get ever more challenging with the increasing complexity of the systems. In NMR spectroscopy, a successful approach to disentangle the overlapping features in a spectrum has been to increase the dimensionality of the spectroscopic technique. Based upon a sequence of infrared laser pulses, 2D IR is closely related to two dimensional NMR (2D NMR) method. Similar to 2D NMR, 2D IR can spread the spectral information in two dimensions and serve as an ideal experimental technique to identify interactions between vibrational modes and to measure the temporal evolution of the vibrational frequencies. Unlike 2D NMR, which is typically limited to detection of species interconverting on millisecond and slower timescales, 2D IR spectroscopy has the advantage of an inherently fast, subpicosecond timescale that ensures detection of states that rapidly interconvert and enables direct measurement of fast structural and environmental fluctuations. 2D IR spectroscopy is well suitable to study condensed phase samples where conformational fluctuations occur on fast timescales. Based on the vibrational frequencies that are highly sensitive to molecular environments, 2D IR spectroscopy provides information about the local sites and their fluctuations within complex systems. 2D IR spectroscopy conveys rich information on molecular systems such as homogeneous and inhomogeneous spectral broadening effects, vibrational anharmonicity, spectral diffusion, intermode coupling strength and its temporal variation, energy relaxation, chemical exchange and conformational interconversions. Over the past two decades, 2D IR spectroscopy has been extensively used to study the structure and dynamics of small peptides, proteins, DNA, and lipid bilayers. Moreover, 2D IR has been utilized to interrogate ultrafast energy transfer in materials and biology, hydrogen-bond (H-bond) making and breaking in liquid water and in biomolecules, and solvation dynamics arising from solute-solvent interactions. The purpose of this article is to provide a comprehensive understanding of the 2D IR experimental technique to determine equilibrium structures and ultrafast timescales of various chemical and biological processes. Mathematical expressions and equations have been avoided throughout the article to provide a simple and qualitative, yet detailed picture regarding the multiple light-matter interactions in 2D IR experiments. The spectral signatures of different ultrafast processes in the 2D IR spectrum have been illustrated. Several 2D IR works have been reviewed to explain the important roles played by this nonlinear experimental technique towards our understanding of the molecular structure, energy relaxations, ultrafast fluctuations, and conformational dynamics. The organization of this article is as follows: it

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starts with a description of the experimental set-ups of 2D IR spectroscopy. The next section provides a detailed understanding of the basic principles of the light-matter interactions and the possible excitation pathways involved. This is followed by the description of the unique spectral signatures of the different chemical processes in the 2D IR spectrum. Next, a comprehensive review of various studies in chemistry, materials and biology using this experimental technique has been put together. This is followed by a section about the recent technological advances in 2D IR. Finally, we conclude by stating the future direction and scope of this spectroscopic technique.

2 Experimental Methodology 2D IR spectroscopy involves three light-matter interactions. The commonly used 2D IR experimental set-up is shown in Fig. 1. A Ti:Sapphire regenerative amplifier, seeded by a Ti:Sapphire oscillator, produces femtosecond pulses centered at 800 nm with a repetition rate of 1 kHz. The amplified pulses are used to pump the white light seeded optical parametric amplifier (OPA). The signal and the idler beams of the OPA are combined using difference frequency generation to generate the IR pulses of typically 50–100 fs duration. Chalcopyrite crystals (typically AgGaS2 or AgGaSe2 ) are used for difference frequency generation. In recent times, pulse trains with higher repetition rates are gaining popularity for IR light generation. Ytterbium based oscillators have been used to generate IR at 100 kHz, which paved the way to performing 2D IR experiments at 100 kHz [7, 8]. The higher repetition rate can dramatically reduce the acquisition time of 2D IR spectra. The pulse sequence used in a 2D IR experiment is shown in Fig. 2. Each pulse has a corresponding wave vector denoted as k1 , k2 , k3 which describes the direction of the pulse. The three light matter interactions and the subsequent emission of the signal from the sample involve three time intervals. The interval between pulses 1 and 2 is denoted by τ (coherence period), that between pulses 2 and 3 is denoted by T w (waiting time or population period), and that between pulse 3 and the detected signal

Fig. 1 Schematic representation of experimental set-up for 2D IR spectroscopic technique

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Fig. 2 Typical pulse sequence used in 2D IR experiment

is denoted by t (detection period). In practice, 2D IR experiments are realized in two different optical geometries, BOXCAR geometry and pump-probe geometry. It is also important to remember that both of these methods can be used in either transmission or reflection mode. In this article, we will be mostly focusing on transmission mode spectra. The BOXCAR geometry involves four-wave mixing where the IR pulse generated using difference frequency generation is further divided into four pulses (Fig. 3). Three of these pulses with wave vector k1 , k2 , and k3 are arranged in a manner such that they are focused into the sample coming from three corners of the square. The signal is emitted from the fourth corner of the square with a new, unique wave vector ks . The BOXCAR geometry provides control over the polarization of each pulse which can be utilized to explore the polarization dependent excitation pathways in a chemical system [9]. One benefit of the BOXCAR geometry is that the signal, emitted in a different direction as compared to the input IR pulses, is background free. The fourth beam, which does not pass through the sample, acts as a local oscillator (LO) to enable heterodyne detection. This allows phase information to be retained in the experiment. The LO also aids in signal detection as the LO amplifies the signal. 2D IR spectra produced with a pump-probe geometry use pulse shaping approach which involves modulating the phase and amplitude of individual frequencies to produce the pulse pair required for 2D IR spectroscopy. A typical geometry for the mid-IR pulse shaper is shown in Fig. 4. In this approach, the mid-IR beam is separated in a strong pump pulse and a weak probe pulse. The pump pulse is then passed through the acousto-optics modulator (AOM). AOM creates two temporally

Fig. 3 Schematic representation of BOXCAR geometry

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Fig. 4 Schematic representation of the mid-IR pulse shaper geometry

separated pump pulses by the application of the pulse shaping approach. Two pump pulses and the probe pulse are then focused into the sample. Unlike the BOXCAR geometry, both pump pulses have the same wave vector direction which allows for ease in alignment but limits the polarization dependent studies that can be performed. The resultant signal from the sample, obtained either by BOXCAR or pumpprobe geometry, is further dispersed through the help of a monochromator and finally detected by a liquid nitrogen-cooled MCT array detector.

3 Basic Principles Vibrational modes of molecules, sensitive to the local environment, are powerful reporters of chemical structure and dynamics. Interactions of a vibrational mode with the solvent molecules alter vibrational frequency of the mode, making the molecular vibrations sensitive to the fluctuations of the local environment. 2D IR spectroscopy, depending on the lifetime of the vibrational mode, allows a quantitative investigation of the vibrational dynamics across timescales ranging femtosecond to nanosecond. 2D IR spectroscopy is a third order nonlinear spectroscopic technique that involves three interactions between ultrashort femtosecond laser pulses and the chemical system (sample). In response to the inherent electric field of the interacting laser pulse, a chemical system emits a macroscopic polarization. As 2D IR involves three light-matter interactions, the macroscopic response is a function of three input pulses. The third order nonlinear polarization can be expressed as a function of the electric fields (E n ) associated with the input pulses at time t 1 , t 2, and t 3 and the thirdorder system response function, R(3) (t 1 , t 2 , t 3 ), of the chemical system. The R(3) contains all the possible excitation pathways. In case of multiple vibrational modes

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within the bandwidth of the femtosecond excitation pulses, each mode will have their own responses. Therefore, it is important to consider all the energetic pathways available during the experiment along with the spectral signature of each of the pathways. Although there are different ways of creating these light-matter interactions, as discussed in the experimental set-up section, the underlying principle remains the same. As mentioned above, different Liouville pathways are involved when a chemical system interacts with multiple infrared pulses [10]. However, the interactions of the three IR pulses with the sample and the subsequent generation of the 2D IR signal can be qualitatively understood with a three-level energy diagram (v = 0, 1 and 2). With the molecules starting in the ground vibrational state, the first pulse excites the molecules to generate a coherence between the ground state (v = 0) and the first excited state (v = 1). The second pulse comes after time interval τ (coherence time) and transfers this coherence into a population of either of the two vibrational states. The third pulse, coming at an interval of T w (waiting time) after the second pulse, creates another coherence either between v = 0 to v = 1 or v = 1 to v = 2 states. After the three pulses interact with the sample, a signal field, encoded with the molecular response of the system, is emitted from the sample, and reads out the final oscillating frequencies. In a typical 2D IR experiment, T w is fixed while τ is scanned; each value of τ producing a spectrum as detected by the MCT array detector. The 2D IR data from a single experiment at constant T w thereby consists of a 2D array of time (τ ) and frequency. Numerical Fourier transform of the acquired data along the time (τ ) axis produces the 2D IR spectrum. A two dimensional spectrum is constructed with oscillating frequencies of the first coherence (denoted by ωτ ) on one axis and that of the final coherence (denoted by ωt or ωm ) on the other axis. The responses can be observed as peak pairs in the 2D IR spectrum. Depending on the states involved in the coherence, the 2D IR spectrum consists of a negative diagonal peak due to emission involving 0–1 transition (ground state bleach or excited state emission) and a positive off-diagonal peak due to emission involving 1–2 transition (excited state absorption). The off diagonal peak is red-shifted from diagonal peak by the anharmonicity of the vibrational probe. The intensity of the diagonal and off-diagonal peak decreases with increasing waiting time and is directly related to the vibrational lifetime of the vibrational mode being excited during the experiment. The axis is also identified as a pump and probe axis based on the analogy to the pump-probe spectroscopy. It should be noted that a three level system has been considered because femtosecond IR pulses are limited by broad bandwidth and can in turn excite both ground and the first excited states. However, this simple understanding of the 2D IR spectrum in terms of a diagonal and an off diagonal peak is limited for a single vibrational mode and the simplest of experimental geometries. Presence of multiple vibrational modes within the bandwidth of the excitation pulses would lead to multiple diagonal peak pairs. This will be discussed in detail in the next section of the article. In addition, vibrational coupling, chemical exchange, and population transfer, if present, would lead to cross peaks in the 2D IR spectrum. Some of these cases will be discussed in the latter part of this article.

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4 2D IR Spectrum A single vibrational mode is manifested as a peak pair in the 2D IR spectrum (Fig. 5). At zero waiting time (T w ), the initial and final coherence frequencies (ωτ and ωt ) involved in the ground state bleach (blue contour in Fig. 5) or the excited state emission (red contour in Fig. 5) signal of the vibrational mode in a certain molecule within the ensemble are the same. This leads to a single point along the diagonal on the 2D IR spectrum. However, as 2D IR is an ensemble measurement and the molecules within the ensemble exist in slightly different environment, the vibrational mode of interest produces a diagonally elongated frequency distribution covering such microenvironments. This width of a 2D IR peak along the diagonal signifies the heterogeneity in the microenvironment and thus provides the inhomogeneous line width (black arrow in Fig. 5). Homogeneous line broadening mechanisms, namely lifetime and dephasing, causes a small broadening in the peaks perpendicular to the diagonal (red arrow in Fig. 5). The corresponding width of the peak perpendicular to the diagonal provides the homogeneous linewidth. A similar peak, arising from excited state absorption, is observed in the 2D IR spectrum at the same ωτ , but red-shifted along ωt by the diagonal anharmonicity of the vibrational mode. With increasing waiting time, structural evolution occurs due to inherent dynamics of the system and the surrounding solvent molecules. As vibrational frequency is sensitive to the surrounding environment, such conformational and solvent fluctuations at larger waiting times (T w ) make the final coherence frequencies different from the initial ones. This causes broadening of the diagonally elongated peak. At large enough waiting time, when the molecule samples out all the possible structural conformations, the 2D IR spectrum becomes circular. Therefore, the conformational fluctuations are encoded in the 2D IR spectrum at different waiting times through the change in the 2D IR peak shapes. This process of dynamics dependent evolution of spectral shape is known as spectral diffusion. The fluctuation timescales can be obtained from the analysis of the 2D IR spectral lineshapes using ellipticity [11], nodal line slope [12], or the center line slope methods [13, 14]. When femtosecond IR pulses excite multiple energetically close vibrational modes, multiple pairs of diagonal peaks are obtained in the 2D IR spectrum. Dipeptides, where one of the two carbonyl modes is isotopically labeled, have shown two

Fig. 5 Pictorial presentation of 2D IR spectrum of a single vibrational mode at different waiting time (T w )

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distinct peak pairs in the 2D IR spectrum [15, 16]. A similar spectrum containing multiple diagonal peak pairs can also be obtained in case a certain vibrational mode exists in multiple conformations, thereby leading to distinct spectrally resolved population distributions. For example, vibrational mode shows distinct spectral signatures in the presence and absence of hydrogen bonding [17–19]. Similar spectral signature can be obtained when the vibrational probe exists in two different conformations with the protein environment [20, 21]. In addition, cross peaks between the diagonal peak pairs can be observed in the 2D IR spectrum. The cross peaks in a 2D IR spectrum are the signatures of vibrational couplings [22], chemical exchange [23], and energy transfer [24] between the vibrational modes. The characteristics of the cross peaks will depend on either the orientation between two the vibrational modes or the interconversion rate between the two different populations of the vibration. Energy and population transfer being time dependent, cross peaks arising from these processes are absent at T w = 0 and become more prominent with increasing waiting times. Whereas, off-diagonal peaks linking the vibrational modes through coupling are waiting time independent and are clearly visible at T w = 0. Hence, origin of cross peaks can easily be separated. Structural information can be generated from off-diagonal peaks of vibrational coupling. The angle between the transition dipole moments of the two coupled vibrational modes can be extracted from the relative amplitudes of the diagonal and cross peak pairs as a function of input pulse polarization [25, 26].

5 Applications of 2D IR In the last two decades, 2D IR has been utilized to obtain structural information through dipolar couplings in small model compounds and to study fundamental processes in the fields of chemistry, materials, and biology. The fundamental processes include energy transfer and relaxation, chemical exchange, hydrogen bond making and breaking, interfacial dynamics, and charge transport [27–30]. Here, we review some of the important 2D IR results which allowed us to obtain interesting insights about the above mentioned fundamental processes. It has been revealed that vibrational energy transfer in silicon nanoparticles, occurring in tens of picoseconds timescale, is diminished when doped with small amounts of boron and phosphorous [31]. Berry pseudo-rotation, the pairwise exchange between axial and equatorial ligands, was experimentally validated in five coordinated ruthenium complex [32]. Very fast making and breaking of intermolecular and intramolecular hydrogen bonding as well as interconversion between free ion and contact ion pair have key importance in biological systems. Structural interconversions of acetonitrile in methanol, methyl and ethyl acetate in methanol, 2-methoxyphenol in toluene (Fig. 6), hydrogen bonding delocalisation in liquid NMA and β-isocyanoalanine in fluorinated alcohols, and ion pair dynamics of LiNCS in benzonitrile, Li+ and SCN− in dimethylformamide, SeCN− ion in N,N-dimethyl formamide (DMF) have been

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Fig. 6 Shows the 0–1 region of the T w dependent 2D vibrational echo spectrum for deuterated 2methoxyphenol in toluene. The growth of the cross-peaks changes the shape of the spectrum, which becomes square after substantial exchange. Adapted with permission from Ref. [34]. Copyright, 2006, American Chemical Society

experimentally visualized [23, 33–39]. Evidence for the presence of the elusive CH…O hydrogen bond and n-π* interaction in liquid solution at room temperature has been identified [17, 40]. Solvent molecules in the close vicinity of the solute are of key importance because of structural reorganisation. The interaction of solvent molecules with the solute alters the reaction rate, yield of product, and stabilisation of transition state. Dynamics of hydration structure around a small vibrational probe have been reported in neat solvent and in binary solvent mixtures [19, 41]. Preferential solvation and cosolvent exchange dynamics for vitamin biotin labelled with a transition-metal carbonyl probe in water has been investigated [42]. The effect of molecular nature of the vibrational probe on vibrational dynamics has also been reported in aqueous solution using 2D IR [43]. As a powerful method to monitor ultrafast fluctuation of molecular structure and symmetry under equilibrium condition, 2D IR has revealed solvent-mediated molecular symmetry breaking dynamics

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in [Co(CN)6 ]3− highly symmetric molecular species [44]. 2D IR spectroscopy has been used to investigate the dynamics in a fragile glass forming liquid [45]. Metal ion dependent vibrational relaxation in EDTA complex using carboxylate group as a vibrational reporter has been reported [46]. Solvation dynamics in green solvents like ionic liquids and DES have been interrogated [47–51]. Water dynamics has gathered considerable interest in the scientific community because water in any confined environment behaves differently from bulk water. To understand the dynamics of confined water, scientists have been studying the hydrogen bond dynamics of water at the interface of reverse micelles and water for a significant amount of time using 2D IR and pump probe spectroscopy [52, 53]. Dynamics of nanoscopic water in aerosol-OT reverse micelles have been studied and the water dynamics have been separated into two different ensembles, namely interfacial water and bulk like core water [52]. Water dynamics at the surfactant interfacial boundary layer in large Aerosol-OT reverse micelles has been found to be slower than in bulk water [53]. Studies on vibrational relaxation of water interacting with ionic surface and molecular anionic H-bond dynamics in water provided interesting insights about water dynamics [54, 55]. Confined water dynamics was further investigated inside the nanoporous silica materials were studied using SeCN− as a probe [56]. The water of hydration inside the crystals defines the physical property of that mineral. Water dynamics inside minerals like gypsum and basanite have been interrogated [57]. In addition, reorientational relaxation in DMSO/water, an industrially relevant solvent, has been found to depend on the concentration of water in the binary solvent mixture [58]. Using isotope dilution of water, it has been found that the OH frequency shifts arise from the changes in molecular electric fields acting on the proton [58]. Study of interfacial energy dynamics of D2 O in air/water interface has shown a sub-picosecond transfer of energy between hydrogen bonded interfacial water molecules and the OD groups pointing out from the water surface [59]. To understand the role of water in biology, dynamics of water in enzyme active sites have been investigated on NO bound ferric haem of the catalase enzyme from Corynebacterium glutamicum in H2 O and D2 O [60]. Another example of studying water dynamics in a biological context is the work on interfacial water dynamics in the lipid membrane. It has been found that hydrophobic fragments affect the ultrafast rotational dynamics of water [61]. The impediment has been explained using excluded volume effects in hydration [62]. 2D IR spectroscopy has been applied to carbonyl and phosphate vibrations intrinsically located at the lipid–water interface to examine the effects of DMSO on the H-bond dynamics [63]. In another work (Fig. 7), the effects of membrane peptide concentration on the picosecond interfacial H-bond dynamics reveal a non-monotonic dependence of water orientation and dynamics as a function of transmembrane peptide:lipid ratio [64]. There are several examples of measuring structural, conformational, and solvation dynamics of biological molecules using 2D IR. In small molecule-DNA interactions, a therapeutic molecule, Hoechst33258, has been found to prefer to A-tract sequence relative to a suboptimal alternating A-T sequence [65]. This technique has been applied to screen 2016 2D IR spectra of 12 double-stranded DNA oligonucleotides obtained in the presence and absence of Hoechst 33,258 to efficiently retrieve the

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Fig. 7 a–c Ester C=O 2D IR spectra of dimyristoyl-snglycero-3-phosphocholine (DMPC) bilayer, 1:50 lipid/peptide, and 1:10 lipid/peptide samples, respectively, at T w of 150, 1200, and 2500 fs. Blue data points denote the maximum 2D IR intensities at each excitation frequency; light blue lines are the linear fits to these points from which the center line slope is extracted. d CLS decays and exponential fits for the DMPC (black), 1:50 (green), and 1:10 (red) spectra. Decay y-axes are offset for the sake of clarity. Error bars represent the 95% confidence interval of the linear CLS fit. The center line slope value is unitless. Adapted with permission from Ref. [64]. Copyright, 2020, American Chemical Society

base composition of a DNA sequence and discriminate ligand–DNA complexes from unbound sequences. [66]. The effect of oligomer length on the vibrational mode coupling and energy relaxation mechanisms of AT-rich DNA oligomers in double and single-stranded conformations demonstrated vibrational coupling between base and backbone requires formation of the double-helix structure while vibrational energy management is an inherent property of the nucleotide [67]. It has been observed that rapid energy transfer processes take place between base and backbone, mediated by additional modes located on the deoxyribose moiety within the same nucleotide [68]. 2D IR spectroscopy of the vibrational modes of the DNA bases reveal signature off-diagonal peaks arising from coupling and energy transfer across Watson–Crick paired bases that are unique to double-stranded DNA [69]. It has been demonstrated that water molecules in the first two layers as the predominant source of electric

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fields at the surface of native salmon DNA and the electric fields fluctuate on a 300 fs time scale with an amplitude of 25 MV/cm due to thermally excited water motions [70]. Thermal dehybridization of DNA oligomers has been observed using 2D IR spectroscopy by tracking the four DNA bases independently [71]. Ultrafast 2D IR study of an RNA double helix in aqueous environment has shown that the equilibrium sugar-phosphate backbone of RNA and its hydration shell is distinctly different from hydrated DNA [72]. The 2D IR spectra of RNA display a greater number of backbone modes than that of DNA, with distinctly different lineshapes of the diagonal peaks [73]. Quantification of the lipid membrane interior mobility was quantified using N3 alkyl as probe by has illustrated that the spectral diffusion of N3 -alkyl reflects the dynamics of the environment, even in the media of very low polarity [74]. Vibrational frequency fluctuations of azide-derivatized amino acids have been investigated have suggested that the stretching mode of the covalently bonded azide group is sensitive to the fluctuations of hydrogen bond network system, as found for azide ions in water [75]. The changes in the secondary structure of the multifunctional calcium-binding messenger protein Calmodulin (CaM) as a function of temperature and Ca2+ concentration have been interrogated using 2D IR spectroscopy [76]. Structural dynamics within the distal cavity of wild type heme proteins and several mutants have shown spectroscopic signatures of interconversion between multiple protein substates [20, 21, 77, 78]. Complete molecular-level description of proteins including conformational heterogeneity and their rates of interconversion (has been studied using picosecond inherent timescale of 2D IR spectroscopy and the high spatial resolution afforded by the small size of IR chromophores. Ability to introduce the vibrational probe at different specific sights of the protein has allowed rigorous characterization of dynamics in localized environments and mapping out the dynamics throughout proteins with atomic spatial resolution and femtosecond temporal resolution [79–85]. The change in dynamics of an enzyme active sites upon binding to different inhibitors bound or upon select mutations have been correlated with the changes in the kinetic isotope effect to understand the role of such motions play in the enzyme-catalyzed hydrogen transfer reaction [86, 87]. Side-chain structure and dynamics of Histidine as well as ultrafast fluctuations of high amplitude electric fields in lipid membranes have been investigated using 2D IR spectroscopy [88, 89]. The mechanistic aspect of potassium (K) channels, responsible for the selective permeation of K+ ions across cell membranes has been studied [90, 91]. The ultrafast time resolution of 2D IR spectroscopy has provided an instantaneous snapshot of the multi-ion configurations and structural distributions that occur spontaneously inside the semisynthetic channel. 2D IR spectroscopy has also been used to investigate materials chemistry. Amide and carboxylate groups on spherical and aggregated nanoparticles characterized surface field enhancements, cross peaks, and different line broadening mechanisms of nanoparticle capping layers [92]. Surface-enhanced two-dimensional infrared (SE 2D IR) vibrational spectroscopy has been used to examine molecules in thin films of different thicknesses, ranging from a monolayer to ~ 100 nm, including diagonal and cross peaks measurements [28]. The structural dynamics of planar thin films

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of the room temperature ionic liquid with controlled nanometer thicknesses have been observed to be slower than the dynamics of the bulk liquid [93]. 2D IR spectroscopy demonstrated that the metal-polymer structure formed in paint films adopt either a coordination chain or an oxo-type cluster and the presence of water governs the relative concentration of these coordination structures [94]. The solvation structure and dynamics of lithium ions in carbonate electrolytes has revealed chemical exchanges between carbonate solvent molecules in the first and outer solvation shells [95]. Two-dimensional infrared (2D IR) spectroscopy used to study a thin film of the Pb-I-SCN layered perovskite using near Brewster’s angle reflection pump−probe geometry demonstrates that the lattice structure is dynamically disordered in contrast to structurally isolated layer [96]. 2D IR spectroscopy in combination with ab-initio molecular dynamics simulations revealed different rotational processes of the organic cations within the methylammonium (MA) lead iodide [97]. The sensitivity of a silane probe to the spectral diffusion on the nanometer length scale within the porous silica nanoparticle with different pore size has shown slower solvent dynamics near the surface-bound probe than that of the bulk solvent [98]. The process of gelation and aging in silica sol-gel has been observed through monitoring the change in the peak shape of a surface-bound Si–H vibration [98]. Covalently bound silicon hydride has been used to measure the structural dynamics of an oligomeric cross-linker and a cross-linked elastomer in the solvent and solvent free condition. The observed structural dynamics of polymer in similar timescale of solvent dynamics has indicated the contribution of structural motion of the polymer to the dynamics in its state swollen with solvent [99]. 2D-IR spectroscopy has observed a condition of decreased charge carrier mobilities in poly(3hexylthiophene-2,5-diyl) (P3HT) films. The solvent vapor annealing causes increase in hole mobility which also consistent with the loss of dynamics in the 2D IR measurement that suggest the damping of ring torsion vibrational modes in the 100–200 cm−1 regime [100]. 2D-IR measurement on the structural dynamics of polyaniline in its semiconducting undoped and conducting doped forms has revealed ultrafast dynamics only for the undoped semiconducting films [101]. To investigate solvent dynamics adjacent to the surface of a confined volume, a pore within a porous silica gel has been measured using a 2D-IR spectroscopy [102]. Water dynamics in the hydrated Nafion-212 with sodium counter ion has demonstrated that water in the Nafion core acts essentially like bulk water, and the interfacial water dynamics are the result of interactions with an interface of sulfonate anions [103]. Ultrafast infrared spectroscopy has been used to measure the vibrational dynamics of Pt–H on a nano-structured platinum surface. The vibrational lifetime and anharmonic shift is larger for the Pt–H than that of the Pt–CO which indicates the coupling of H and CO with metal [104].

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6 Recent Developments in 2D IR Several recent developments of 2D IR spectroscopy will be discussed in this section. As discussed above, 2D IR was mainly developed to study samples in the condensed phase, where multiple IR pulses are focused into the sample. This limits the exploitable overlapped path length through samples to a few millimeters. To circumvent this limitation, 2D IR experiments have been performed with a hollow waveguide which enable acquisition of 2D IR spectra of low concentration gas-phase sample [105]. Exploring the dynamics of a chemical system is limited by the vibrational lifetime of the probe. The short lifetimes of the common vibrational probes do not allow 2D IR to interrogate chemical/biological processes which are much slower than the lifetime. To overcome this shortcoming, vibrational probes with long lifetimes have been explored [83]. Alternatively, a new spectroscopic approach is to excite the molecules into the first electronic excited state (S1 ) which allows access to longer lifetimes. Vibrationally promoted electronic resonance spectroscopy (VIPERS) has been reported which utilizes a UV/Vis pulse to excite the molecules to S1 [106]. A combination of AOM based pulse shaping, rotating wave frame and phase cycling has been utilized to select specific Liouville pathways [107]. Full spectrum 2D IR has been developed covering the entire mid-IR spectral range which helps to decouple energy transfer or vibrational coupling and allows three dimensional molecular conformations to be directly determined [108]. Broadband IR light sources, e.g. a laser plasma source for femtosecond mid-IR pulses with bandwidth spanning the entire vibrational IR spectrum, are being developed that can be used for ultrafast spectroscopy across the entire mid-IR spectrum [109]. Several surface sensitive 2D IR techniques have been reported which employ surface enhancement, methods for studying electrochemical interfaces (2D ATR IR), and extensions for resolving nonequilibrium processes (transient 2D IR) [110]. 2D ATR IR, combining spectro-electrochemistry and 2D IR spectroscopy, is based on ultrathin conductive layers of noble metals and indium–tin oxide (ITO) as working electrodes on a single-reflection attenuated total reflectance (ATR) element in conjunction with ultrafast, multidimensional ATR spectroscopy [111]. Transient 2D IR is a UV pump narrowband-IR-pump broadband-IRprobe experiment of fifth order in the laser field to measure 2D IR spectra of transient species [112]. Obtaining 2D IR spectra of heterogeneous samples like perovskites and metal organic frameworks is generally hindered by severe light scattering. Use of choppers and shutters at strategic positions in the BOXCAR geometry has been reported to eliminate light scattering from heterogenous sample [112]. Additionally, polarization selective experiments have been reported to separate out solvent reorientation dynamics and solvent induced spectral diffusion [113]. Data analysis has recently incorporated neural network modelling to extract the dynamical timescales from 2D IR spectrum [114].

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7 Closing Remarks 2D IR spectroscopy began its journey from applications on small molecular complexes, dipeptides, and other model compounds. However, in the recent years, the spectroscopic technique has been applied to larger and more complex chemical/biological systems to measure dipolar orientation, structural fluctuation, and conformational dynamics. 2D IR is no longer limited to samples in the condensed phase; technological advances have allowed the application of 2D IR in thin films and gas-phase samples. Technology development continues to impact 2D IR spectroscopy. Advances in mid-IR laser source technologies, pulse shaping, and novel combinations of 2D IR with other experimental techniques will continue to push 2D IR to be applied on more heterogeneous systems to obtain information about structure and ultrafast dynamics. These information, when coupled to computational results would provide molecular insights towards structure–function relation of the system of interest. Additionally, further advances in detection systems, novel techniques for generating, amplifying, and compressing mid-IR pulses would likely lead to newer innovations in the field of 2D IR spectroscopy. In a nutshell, a multi-disciplinary approach from the fields on chemistry, physics, and biology in future will lead to new generation technological advances and applications of 2D IR spectroscopy.

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Exploring Non-covalent Interactions by Jet-Cooled Electronic and Vibrational Spectroscopy Prakash Panwaria and Aloke Das

Abstract Gas-phase laser spectroscopy in a supersonic jet is an important technique to probe the intrinsic nature and strength of weak non-covalent interactions present in molecular systems relevant to biology and materials. In this chapter, a brief review of supersonic expansion, laser desorption, time of flight mass spectrometry, and various laser spectroscopic techniques based on resonantly enhanced multiphoton ionization (REMPI) and laser-induced fluorescence (LIF) has been presented. Further, some representative results on the interplay between various non-covalent interactions, as well as the inherent nature and strength of an unconventional non-covalent interaction, namely Se hydrogen bonding interaction, have been described. An in-depth understanding of various non-covalent interactions is crucial for the optimum design of drugs and functional materials. The future of gas-phase laser spectroscopy lies in obtaining insight on the non-covalent interactions directly from the study of larger peptides or equivalent molecular systems. Keywords Supersonic jet · UV/IR laser spectroscopy · Laser desorption · Time of flight mass spectrometry · Non-covalent interactions · Unconventional hydrogen bonds

1 Introduction The subtle interplay between various types of non-covalent interactions is the key to the specific structures and functions of biomolecules, various supramolecular assemblies, drugs, etc. [1–13]. A detailed understanding of these non-covalent interactions and their interplay is essential for designing advanced functional materials and improved drugs. Non-covalent interactions range from conventional and unconventional hydrogen bonding [4–22], π-stacking [7, 23–31], cation-π [32–39], anion-π [40–43], chalcogen and pnictogen bonds [44–58], halogen bonds [55, 58–65], n → P. Panwaria · A. Das (B) Department of Chemistry, Indian Institute of Science Education and Research, Dr. Homi Bhabha Road, Pashan, Pune 411008, Maharashtra, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 D. K. Singh et al. (eds.), Modern Techniques of Spectroscopy, Progress in Optical Science and Photonics 13, https://doi.org/10.1007/978-981-33-6084-6_3

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π* interaction [66–79] etc. Among all these non-covalent interactions, the conventional hydrogen bond is the most abundant and well-studied interaction. Conventional hydrogen bonding, which is denoted by X-H…Y with both X and Y as strong electronegative atoms, is generally dominated by electrostatic interaction [14–16, 80–85]. On the other hand, either X or Y or both X and Y are very weak in electronegativity in the unconventional hydrogen bonding interaction. Although conventional hydrogen bonding is very much popular due to its presence in nucleic acids and proteins, it has been found recently that unconventional hydrogen bonding is not very far away from its conventional counterpart in terms of its strength and application. In general, unconventional hydrogen bonds were not explored earlier in detail, probably due to the conventional wisdom that these hydrogen bonds are very weak in nature. However, the study of these unconventional hydrogen bonds, considering various elements (C, P, S, Se, etc.) apart from the conventional electronegative elements in the periodic table as hydrogen bond donors and acceptors, has been increased to a great extent after the re-definition of the hydrogen bond by the IUPAC committee in 2011 [16, 50, 80, 86–93]. In general, various experimental techniques such as solution-phase FT-IR spectroscopy, Nuclear magnetic resonance (NMR) spectroscopy, and X-ray crystallography are used to characterize different types of hydrogen bonds as well as other non-covalent interactions present in small molecules to large supramolecular assemblies and biomolecules [89, 92, 94–98]. Solution-phase FT-IR spectroscopy is a prevalent technique, which exploits, in general, the red-shift in the frequency as well as the intensity of the vibrational transition of the hydrogen bond donor as a characteristic signature of the strength of the hydrogen bond present in a molecular system or complex [99]. The presence of the hydrogen bond in a system is also characterized by the chemical-shift of the NMR of the proton of the hydrogen bond donor as well as the hydrogen bond acceptor atom [100]. Temperature-dependent NMR, as well as 2D-NMR spectroscopy, render additional confirmation in support of the presence of the hydrogen bond in a system [101]. X-ray crystallography is another powerful method to determine the presence of the hydrogen bonding and other non-covalent interactions by measuring the distances and angles of the atoms in consideration for the attractive interaction [96–98]. Despite the versatility of the X-ray crystallography technique on determining the positions of the atoms in the crystal structure with atomic resolution in small molecules as well as large macromolecules such as proteins, nucleic acids, etc., this technique suffers some limitation in terms of accurate identification of the noncovalent interactions, especially, the hydrogen bonding. In the crystal structure, two atoms can come close to the distance within the sum of their van der Waals (vdW) radii due to the crystal packing forces, as well as the optimization of some other interactions between neighboring units. Consequently, parameters of these non-covalent interactions in terms of distances and angles vary widely from one protein to another. Further, it is quite uncertain to confirm the presence of the hydrogen bond in the X-ray crystal structure as the position of the hydrogen atom is not known there. Similarly, in solution phase FT-IR and NMR spectroscopy, the non-covalent interactions under investigation are perturbed by the intermolecular interaction with the solute as well

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as solvent molecules. Thus, the intrinsic nature and optimum strength of the noncovalent interactions, as well as their very much existence on their own, should be studied in an isolated condition, i.e., supersonic jet, in the absence of any solvent and other external perturbation [102–106]. The experiments performed in a supersonic jet go hand in hand with the quantum chemistry calculations. Hence, the performance of various quantum chemical theories can also be verified directly by comparing the theoretical results with the experimental ones. In this chapter, we have discussed the investigation of the intrinsic properties of various non-covalent interactions and their subtle interplay probed by isolated gas phase laser spectroscopy. More specifically, jet-cooled electronic and vibrational spectroscopy combined with quantum chemistry calculations performed on small molecules and its complexes, mostly from the author’s laboratory, along with a brief highlight from the other research groups in the world are described here.

2 Experimental Details 2.1 Supersonic Jet Expansion Technique Supersonic jet-cooling technique is in the central part of the gas phase laser spectroscopy of isolated molecules and its weakly bound complexes held by various non-covalent interactions [102–106]. An amalgamation of the supersonic jet technique with high-resolution tunable UV/Vis and IR lasers leads to well-resolved electronic and vibrational spectra of molecules and complexes. In general, solution-phase UV-Vis electronic spectra of molecular systems measured at room temperature are very broad. Hence, information on the vibronic structure of the molecules gets lost underneath the broad background. The broad background in the solution phase electronic spectra emerges due to collisional broadening as well as a significant population of many rotational energy levels along with a small population of some of the excited vibrational levels. On the other hand, mostly the lowest rotational (J" = 0) and vibrational (v" = 0) energy levels of the molecules in the ground electronic state get populated in a supersonic jet, and the molecules remain in an isolated gas phase without any collision with themselves as well as solvent [107, 108]. Consequently, very high-resolution electronic spectra featuring sharp vibronic bands of the molecules are obtained. Additionally, the ultracold environment of the supersonic jet allows the formation of weakly bound complexes of different sizes and measurements of their high resolution electronic and vibrational spectra, which are not possible in the solution phase [103]. There are many excellent reviews in the literature describing the detailed principle and application of the supersonic jet-cooling technique in molecular spectroscopy [102–106, 109–114]. In supersonic jet-cooling technique, the vapor of sample molecules of interest seeded in a carrier gas (He, Ne, Ar, etc.) of ~2–3 bar of pressure is expanded into a high vacuum chamber (10−6 –10–7 mbar) through a small

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orifice of ~0.5–1 mm diameter (Fig. 3). A supersonic molecular beam is produced while the orifice diameter (D) is greater than the mean free path (λ) of the sample molecules at the orifice (D  λ). Under the condition of the supersonic molecular beam, enormous collisions between the sample molecules and carrier gas at the orifice cools down the internal degrees of freedom of the molecules of interest. Generally, the translational temperature of the molecular beam goes down to ~1 K, while the rotational and vibrational temperatures reach to 3–5 K and 30–50 K, respectively. The density of the molecules, i.e., the number of collisions, is maximum near the orifice and decreases with the distance x from the orifice. Thus the cooling depends on the (x/D) ratio, where D is the orifice diameter [103–109]. The extent of cooling in the supersonic beam is expressed in terms of Mach number, (M), which is the ratio of the velocity of molecules in the supersonic beam to speed of the sound. In the supersonic beam, M is greater than 1, and thus, the name supersonic originates from there. The following equation gives the relationship between M and (x/D): M = A(x/D)γ−1 , where γ is the ratio of the heat capacity (Cp /Cv ) and A is a constant that depends on γ [104]. It is important to note that the molecules in the supersonic beam remain in the gas phase even the temperature is far lower than the freezing point of the sample. This is because the number density of the sample molecules in the supersonic beam rapidly decreases after a minimal distance from the orifice, and condensation cannot happen due to the lack of any significant three-body collision there [103, 104]. Supersonic expansion is isentropic under reversible adiabatic conditions, i.e., in the absence of any shock wave, shear force, heat source or sink, etc. Thus, the following isentropic equation of the ideal gas can be used to find out the temperature, pressure, and density of the expanding gas in the supersonic jet, T = T0



P P0

(γ −1)/γ

 =

ρ ρ0

γ −1

=

1+

1 − 1)M 2

1 2 (γ

where T 0 , P0, and ρ0 are temperature, pressure, and density of the gas in the high-pressure reservoir, respectively, while T, P, and ρ are the same quantities of the expanded gas after supersonic expansion [104].

2.2 Vaporization of Samples for the Gas-Phase Experiment Samples, which are studied using gas-phase laser spectroscopy, are generally solid or liquid. There are two methods for the vaporization of the sample molecules. One of those is thermal heating, and another one is laser desorption. However, the volatile liquid samples, which have significant vapor pressure at room temperature are required to cool at low temperature to control the sample vapor pressure used in the experiment.

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Thermal Heating

Thermal heating is the most convenient way to bring the sample molecules from the solid/liquid state to the gas phase. Thus, the solid/liquid samples having moderate vapor pressure at room temperature can be slightly heated to generate enough vapor pressure for obtaining a good amount of signal in the supersonic jet experiment. Generally, the sample is taken in a small stainless steel container placed behind the pulse solenoid valve having an orifice of ~0.5–1 mm diameter [115]. The sample holder, as well as the pulse valve, is heated at the required temperature using a resistive heater. The pulse valve is heated by 10–20 °C higher than the sample container to avoid clogging the sample inside the pulse valve. However, the commercial solenoid valves cannot be heated at a temperature higher than 110–120 °C. Moreover, nonvolatile samples, including biomolecules such as amino acids, peptides, etc. cannot be heated at higher temperature as those will be fragmented upon heating. The alternative technique used for the vaporization of the non-volatile samples without any fragmentation is laser desorption, which has been discussed in the next section.

2.2.2

Laser Desorption

Laser desorption circumvents the fragmentation of the fragile non-volatile sample molecules as the heating is done using a narrow pulse width laser beam [116]. Thus, the fragmentation of the samples is minimized by a significant reduction of the time scale of the heating. Generally, a laser of nanosecond (ns) pulse width is used for the desorption of the sample molecules. In a typical 10 ns laser pulse, the temperature jump is about 1000 K with the rate of increment of the temperature ~1011 K/s [68, 106, 117–119]. This technique is generally used to desorb the sample molecules from the matrix of suitable material that assists the desorption [111, 117–126]. The matrix should be a non-reactive, non-polar, and good heat-conducting material [106, 117, 124, 127]. In the case of the supersonic jet experiment, the sample is positioned after the pulse valve. There are various ways to prepare the sample for laser desorption. 1. The sample could be dissolved in benzene or methanol, and the resulting mixture could be coated on the surface of a metallic rod or pellet for the desorption [128]. However, a homogeneous film of the sample is preferable for the reduction of the laser pulse to pulse fluctuation of the density of the sample vapor during the laser desorption. 2. The fluctuation in the signal intensity due to significant variation in the sample vapor in successive laser pulses can be reduced to some extent by dissolving the sample in a liquid having high viscosity such as glycerol [124]. 3. The solid sample could be ground to a fine powder and pressed homogeneously on the surface of a graphite rod [111, 118, 119, 129–132]. 4. The pulse to pulse fluctuation of the density of the sample vapor can be alleviated to a significant extent by preparing a pellet by pressing a finely crushed mixture of the sample and graphite powder (varied percentage) in a hydraulic press of

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Fig. 1 A schematic diagram of the laser desorption assembly, which is incorporated in the jet-cooled laser spectroscopy set-up shown in Fig. 3

pressure ~3000 kg/cm2 . This method of sample preparation is mostly followed for the laser desorption in the gas-phase laser spectroscopy experiment performed in recent times [111, 121, 131–133]. To maintain a constant amount of the desorbed molecules throughout the experiment, a fresh surface of the sample pellet is also introduced for each pulse of the desorption laser by allowing the pellet to perform either back and forth translational or rotational motion or both. In the case of the translational motion, the sample pellet is placed in a sample holder attached to an XYZ manipulator connected with a motorized assembly for the translation of the pellet along the Z-axis. On the other hand, the pellet is directly mounted with the shaft of a motor for rotation of the pellet at a certain speed inside the vacuum [68, 72, 134]. A schematic diagram of the desorption assembly with the rotatable sample pellet is provided in Fig. 1. The pellet is positioned near the orifice of the pulse valve, having the horizontal distance between the orifice of the pulse valve and the edge of the pellet of ~1 mm. The vertical distance between the surface of the pellet and the orifice is optimized at ~2 mm. The optimization of the positioning of the sample pellet with respect to the orifice of the pulse valve is crucial to keep a balance between the supersonic cooling and density of the sample of the molecular beam. Generally, the desorption laser (pulse width 10 ns, Pulse repetition rate = 10 Hz, pulse energy ~600 μJ) at 532 nm is slightly off-focused on the edge of the pellet (towards the orifice) through an optical fiber. A laser beam of 1064 nm is also used for the desorption, but it is more convenient to align the 532 nm beam as it is visible. The desorption laser beam is kept perpendicular to the axis of the molecular beam. The laser desorbed sample molecules are en routed in the supersonic beam of the carrier gas (Ar) of about 5–6 bar of pressure and internally cooled through extensive collision with the carrier gas near the orifice.

2.3 Isolated Gas-Phase Laser Spectroscopic Techniques In general, conventional solution-phase UV-Vis/IR absorption spectroscopic techniques cannot be used in isolated gas-phase spectroscopy as the number density of the sample molecules in the supersonic jet is extremely low. In the case of the

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solution phase UV-Vis/IR spectroscopy, the absorbance of the sample molecules is directly monitored by measuring the intensity of the transmitted light with respect to the incident light passing through the sample, as a function of the frequency of the tunable UV-Vis/IR radiation [135]. The absorbance of the sample is given by the following Beer-Lambert law: A = εcl where A is the absorbance of the sample, ε is the molar absorptivity or molar extinction coefficient of the sample, c is the concentration of the sample, l is the path length traveled by the light in the sample. In the case of the supersonic molecular beam, it is not possible to measure the insignificant change in the intensity of the incident light after it passes through such an extremely diluted sample. Thus, it is required to have alternative detection methods, which can be employed for the isolated gas-phase spectroscopic techniques. It is the action spectroscopy, which can indirectly measure the absorption of the UV and IR radiation by the sample molecules in the supersonic molecular beam [106]. Here, we will discuss various action spectroscopy techniques such as laser-induced fluorescence (LIF) excitation spectroscopy, resonantly enhanced multiphoton ionization (REMPI) spectroscopy, UV-UV, and IR-UV holeburning spectroscopy, resonant ion-dip infrared spectroscopy (RIDIRS), and fluorescence dip infrared spectroscopy (FDIRS) [111, 112]. Schematic diagrams of all these different spectroscopic techniques have been shown in Fig. 2. Two major detection schemes are used to study gas-phase spectroscopy of isolated neutral molecules. One of those is based on fluorescence, and the other one is on resonance-enhanced multiphoton ionization.

2.3.1

Laser-Induced Fluorescence (LIF) Excitation Spectroscopy

LIF excitation spectroscopy is an indirect technique to measure the electronic absorption spectra of molecular systems studied in the supersonic jet [113, 136–139]. It involves the excitation of the sample molecules from the lowest vibrational level (v" = 0) of the ground electronic state (S0 ) to different vibrational levels of the excited electronic state (S1 ) using a tunable UV laser. Total fluorescence from different vibrational levels of the excited electronic state is collected by a photomultiplier tube (PMT) as a function of the excitation wavelength of the UV laser. Total fluorescence measured from different vibrational levels of the S1 state reveals the absorption of the molecules from the v" = 0 level of the S0 state to the corresponding vibronic levels. LIF excitation spectroscopy provides vibrationally resolved electronic spectra of the molecular systems, which yields the vibrational structure of the molecules in the S1 state. LIF excitation spectroscopy is essential to measure the electronic absorption spectra of the molecules which have very high ionization potential and hence are difficult to study through resonance-enhanced multiphoton ionization spectroscopy.

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Fig. 2 A schematic presentation of different laser spectroscopic techniques. See the text for the detailed description

2.3.2

Resonance Enhanced Multiphoton Ionization Spectroscopy

Resonance Enhanced Multiphoton Ionization (REMPI) spectroscopy is another technique to measure the electronic spectra of molecules and weakly bound complexes in the isolated gas phase. This technique is very much useful to obtain the electronic spectra of the molecules which do not fluoresce. In REMPI spectroscopy, supersonically cooled molecules in the lowest vibrational level (v" = 0) of the ground electronic state (S0 ) are excited to different vibrational levels of the first excited electronic state (S1 ) using a tunable ultraviolet (UV) laser. Subsequently, the second photon of the same wavelength of the same laser ionizes the molecules, and the ion signal is detected as a function of the wavelength of the first photon of the laser. Thus, the REMPI spectrum is essentially an absorption spectrum for the S1 ← S0 electronic transition where ionization of the molecules via resonant S1 state is monitored instead of direct measurement of their absorption. This technique is called 1-color resonant 2-photon ionization (1C-R2PI) or (1 + 1) REMPI spectroscopy [140–143]. In the REMPI technique, the total energy of the two photons has to be higher than the ionization potential of the molecular system. Thus, the 1C-R2PI method may not be suitable to measure the electronic spectra if the sum of the energy of the two photons used for the S1 ← S0 electronic excitation and subsequent ionization

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is less than the ionization energy of the molecule. In that case, the second photon of higher energy from a different laser fixed at a specific wavelength is used to ionize the molecule while the wavelength of the first photon is tuned. However, 2nd photon energy is chosen so that the two-photon energy will be a few 100 cm−1 higher than the ionization threshold of the molecule to prevent dissociation of the molecule or the complex in the ionic state. This technique is called 2-color resonant 2-photon ionization (2C-R2PI) or (1 + 1 ) REMPI spectroscopy [144, 145]. Here, the two laser beams should be temporally as well as spatially overlapped, and the pulse energy of the excitation laser beam should be as low as possible to avoid twophoton ionization from the first laser beam. The R2PI spectroscopy is coupled with time of flight mass spectrometry to measure mass-selected electronic spectra of the molecules and complexes.

2.3.3

UV-UV Hole-Burning Spectroscopy

This technique is used to discriminate the presence of different conformers of molecules/complexes in the jet-cooled experiment [142, 146, 147]. Two UV lasers are used in this experiment. Here, the first UV laser (pump/hole-burning laser of pulse energy ~300–400 μJ) is scanned in the range of the electronic spectrum of the molecule and the second UV laser (probe laser of pulse energy ~100–150 μJ), which is fired 50–100 ns after the hole-burning laser, is kept fixed at a particular vibronic transition of the electronic spectrum. The probe laser beam counter-propagates with the pump laser beam and the two laser beams are spatially overlapped. The Pump laser depletes the population of the ground state of the molecules, which is monitored by the probe laser. All the vibronic transitions belonging to the same conformer show depletion in the intensity in the hole-burning spectrum as they arise from the same ground state. In the REMPI spectrum, depletion is observed in the ion signal of the probe laser, whereas depletion in the fluorescence signal of the probe laser is observed in the LIF spectrum.

2.3.4

Resonant Ion-Dip Infrared Spectroscopy (RIDIRS)

This technique is used to measure conformation-specific infrared (IR) spectra of molecules and complexes by probing their characteristic vibrational frequencies. RIDIRS is an indirect IR absorption measurement technique, which is very important to determine the structures of different conformers present in the jet-cooled experiment. In this experiment, counter-propagating UV and IR laser beams, which are spatially overlapped, are mutually orthogonal to the molecular beam axis. The pump IR laser is fired 100–200 ns prior to the probe UV laser, which is fixed to a particular transition in the electronic spectrum (R2PI) [148–154]. The IR laser is scanned in the region of the vibrational frequencies of the functional groups such as –OH, –NH, –SH, –CH, C=O, etc. The UV ion signal depletes whenever the IR laser frequency resonates with any vibrational frequency of the molecules or complexes.

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The pulse energy of the IR laser beam used is ~2–3 mJ, while the UV laser pulse energy is maintained at 200–300 μJ. If the detection method used is fluorescence instead of multiphoton ionization, then we measure the dip in the fluorescence signal instead of the ion signal, and the technique is called Fluorescence Dip Infrared Spectroscopy (FDIRS) [154].

2.3.5

IR-UV Hole-Burning Spectroscopy

IR-UV hole-burning spectroscopy can also be used to discriminate the presence of different conformers in the experiment. Thus, this technique provides the same information, which is available from the UV-UV hole-burning spectroscopy. In this technique, the IR laser, which is fixed at one of the vibrational transitions of the molecule, precedes the UV laser by ~100 ns. As the IR laser depletes the population of the ground state by the vibrational excitation of the particular mode, the intensity of all the bands in the electronic spectrum belonging to the same conformer gets reduced when the UV laser is scanned throughout the whole electronic spectral region [106, 112]. On the other hand, the intensity of the electronic bands corresponding to different conformers remains unaffected.

2.4 Experimental Set-Up for Jet-Cooled LIF and Mass-Selected REMPI Spectroscopy A schematic diagram of a generalized set-up for performing gas-phase laser spectroscopy experiments employing LIF and REMPI schemes coupled with supersonic jet-cooling has been provided in Fig. 3 [115, 155]. The REMPI technique is further combined with Time of Flight (TOF) mass spectrometry. The set-up comprises two differentially pumped high vacuum chambers linked through a skimmer of 2 mm diameter hole and 25 mm length. A commercial pulse solenoid valve (10 Hz rep. rate, opening time ~200 μs) of orifice diameter of 0.5–1 mm attached with a stainless steel sample holder connected with a long stainless steel pipe is mounted with an XYZ manipulator. The whole pulse valve assembly is housed in the bigger chamber (supersonic expansion chamber) pumped by a 10-in. diffusion pump or similar capacity turbomolecular pump. The XYZ manipulator is extremely important to align the orifice of the pulse valve with respect to the skimmer, which is generally kept separated from the pulse valve by ~2.5 cm. A two-stage ion source consisting of a repeller plate, an extraction grid, and an accelerating grid following Wiley-McLaren design having a 1 m linear time of flight (TOF) tube is incorporated in the smaller size vacuum chamber (ionization chamber) pumped by a 6-in. diffusion pump or a turbomolecular pump of similar capacity [156]. The TOF tube is placed perpendicular to the axis of the molecular beam. A schematic of the jet-cooled LIF/REMPI Time of Flight Mass spectrometry assembly is provided in Fig. 3.

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Fig. 3 A schematic drawing of the jet-cooled laser-induced fluorescence (LIF) excitation coupled with resonance enhanced multiphoton ionization-time of flight mass spectrometer set-up

The sample of interest (solid or liquid) having moderate vapor pressure is taken in the sample holder positioned behind the pulse solenoid valve and heated at 80– 100 °C to generate sufficient vapor pressure necessary for performing the experiment. Highly volatile samples are cooled in an appropriate freezing mixture kept outside the vacuum chamber and connected with the pulse valve. A carrier gas (He/Ne/Ar) at a backing pressure of 2–3 bar is flown through the sample holder. The sample vapor seeded in the carrier gas is supersonically expanded through the orifice into the high vacuum chamber. The pressure of the expansion chamber is maintained at ~ 2 × 10–6 mbar while the pulse valve is on. In the case of the laser desorption, as mentioned in Sect. 2.2.2, the laser desorption assembly is placed after the pulse valve (see Fig. 1), and the backing pressure (mainly Ar) of the carrier gas is increased to 5–6 bar [68, 72, 157]. The central coldest portion of the pulsed molecular beam is collimated through the skimmer and reached the middle point, between the repeller plate and extraction grid, called the ionization region. A vacuum of ~ 6 × 10–7 mbar is preserved in the ionization chamber during the experiment. The pulsed UV laser beam (10 Hz rep. rate, pulse width = 10 ns) intersects the pulsed molecular beam at the right angle in the ionization region. A digital delay generator is used to synchronize the interaction of the pulsed molecular beam with the pulsed laser beam at the ionization region. The ionization of the molecular beam is performed employing 1C-R2PI or 2C-R2PI technique using a tunable UV laser. The ions generated in the ionization region are extracted in the direction perpendicular to both the molecular beam and laser beam by applying a potential field between the repeller plate and the extraction grid, which are set at ~ + 3000 V and ~ + 2700 V, respectively. In contrast, the accelerating grid is kept grounded. The separation between the repeller plate and the extraction

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grid, as well as the extraction grid and the accelerating grid, is 1 cm. The ions are finally detected by a dual chevron type microchannel plate (MCP) detector placed at the end of the TOF tube. The detector is housed in a small chamber pumped by a turbomolecular pump (speed ~ 70 lit/s), and the operating pressure is maintained at ~ 2 × 10–7 mbar. The detector voltage is set at ~ -2800 V. A grounding mesh is placed just before the detector to ensure that the ions coming out of the accelerating grid travel the field-free region of the TOF tube with the same kinetic energy. The ions with different masses reach the detector at different times, and thus the time of flight of the ions can be converted into their masses. The ion signal from the detector is further amplified using a preamplifier and fed into a digital oscilloscope. The TOF mass spectrum of the molecule or complex is recorded at a single wavelength of the UV laser, and the mass-selected electronic spectrum is obtained by selecting a particular mass channel on the oscilloscope trace and tuning the wavelength of the laser. Data acquisition from the oscilloscope and simultaneous scanning of the laser wavelength is done through a USB interface using LabView based programs. In the case of the UV-UV and IR-UV double resonance spectroscopy techniques, synchronization of the timing sequence of the lasers is controlled by the digital delay generator. In the case of the LIF spectroscopy, the UV/IR laser beam intersects the molecular beam in the perpendicular direction at a distance of ~1 cm from the orifice of the pulse valve. Fluorescence is collected from the intersection point in the direction mutually orthogonal to the direction of the molecular beam and the laser beam through a lens to the photomultiplier tube (PMT) [158, 159]. The signal from the PMT is transferred to a digital oscilloscope, and the LIF spectra are acquired in a computer through a USB interface using Labview based programs. The results, which will be discussed in this chapter, are mostly derived from the mass analyzed REMPI spectroscopy technique while the samples are vaporized by thermal heating. The experimental results are analyzed with the help of quantum chemistry calculations performed using Gaussian 09 [160], Q-Chem [161], NBO [162], and Gamess-USA [163] software packages.

3 Results and Discussion 3.1 Interplay Between Multiple Non-Covalent Interactions The fine interplay between multiple non-covalent interactions dictates most of the structures of biomolecules and materials. A few best examples of the molecular systems having such interplay are DNA, proteins, etc. [1, 4, 164, 165]. A delicate balance between base-pairing through strong (conventional) hydrogen bonds and base stacking provides a very distinct double-helical structure of the DNA. Similarly, a subtle co-operation between the strong hydrogen bonding interactions in the backbone and various other non-covalent interactions between backbone-sidechain and sidechain-sidechain of proteins renders the folding motifs of their particular

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secondary structures. The intrinsic nature of this delicate interplay of the non-covalent interactions can be better understood or tested by studying some judiciously chosen model complexes in an isolated condition, i.e., in the absence of any other interactions present there. Interestingly, heterocyclic aromatic molecules are the major building blocks of the biomolecules and materials. We have described here some of the heterocyclic complexes among many reported in the literature.

3.1.1

Indole…Pyridine Complex

Indole is one of the essential aromatic heterocycles due to its presence in the side chain of the tryptophan residue of proteins. Additionally, indole is a vital component of many important drugs approved in the market. Pyridine derivatives also have prodigious medicinal importance. A molecular-level understanding of the optimum binding motif and strength of various non-covalent interactions present in the heterodimers of aromatic heterocycles is extremely important for drug discovery. Das and co-workers have studied 1:1 indole…pyridine complex in a supersonic jet employing 1C-R2PI, IR-UV double resonance spectroscopy combined with quantum chemistry calculations [115]. TOF mass spectrum of mixed vapor of indole and pyridine seeded in Ar carrier gas measured using 1C-R2PI technique is provided in Fig. 4A. The mass spectrum shows the appearance of the indole monomer and its

Fig. 4 A TOF mass spectrum of indole in the presence of pyridine. Ind and Py stand for indole and pyridine, respectively; B Electronic spectra measured in the mass channels of (a) indole, (b) indole…pyridine dimer, and (c) (indole)2 …pyridine trimer employing 1C-R2PI spectroscopy. The dimer and the trimer spectra are magnified by 50 and 75 times, respectively. Adapted with permission from Ref. [115], copyright 2011, American Chemical Society

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higher-order clusters as well as the indole…pyridine dimer and (indole)2 …pyridine trimer. Water complexes of indole as well as indole…pyridine are also observed in the experiment. Figure 4B displays electronic spectra of indole, indole…pyridine, and (indole)2 …pyridine measured in their respective mass channels using the 1C-R2PI technique. The electronic spectrum of the indole…pyridine dimer shows two sets of band with one group (A) appearing near the origin band of the indole monomer while the other group (B) is quite away from the indole origin band. Interestingly, the electronic spectrum measured in the (indole)2 …pyridine trimer mass channel exactly reproduces the bands marked by the species A observed in the indole…pyridine dimer channel. Thus, the bands designated as the species A in the indole…pyridine dimer spectrum (Fig. 4Bb) originate due to the S1 ← S0 transition of the (indole)2 …pyridine trimer, which fragments after the 2-photon ionization and appears in the mass channel of the indole…pyridine dimer. The IR spectra measured by probing one of the electronic bands of each of the species A and B are distinctly different (Fig. 5A). A

Fig. 5 A IR spectra measured by probing the (a) 000 band of indole, (b) B00 +30 cm−1 band of indole…pyridine dimer, and (c) A00 +27 cm−1 band of (indole)2 …pyridine trimer in the N–H stretching frequency region; B IR-UV hole-burning spectra recorded in the indole…pyridine dimer mass channel by probing the vibrational bands at (a) 3411 cm−1 and (b) 3269 cm−1 . (c) R2PI spectrum measured in the indole…pyridine dimer mass channel. Adapted with permission from Ref. [115], copyright 2011, American Chemical Society

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comparison of the experimental and theoretical IR spectra presented in Fig. 5A indicates that the species A and B observed in the experiment are due to indole…pyridine dimer and (indole)2 …pyridine trimer, respectively. The IR-UV hole-burning spectra presented in Fig. 5B further confirm that all the electronic bands of the species A and species B are due to a single conformation of the indole…pyridine dimer and (indole)2 …pyridine trimer, respectively. The electronic origin band of the (indole)2 …pyridine trimer (A00 ) is red-shifted by 76 cm−1 while the same of the indole…pyridine dimer is red-shifted by 271 cm−1 . Theoretical calculations performed using wave function theory (MP2) as well as density functional theory (M05-2X, M06-2X) with different basis sets predict that the observed indole…pyridine dimer has a V-shaped structure stabilized primarily by a strong N-H…N hydrogen bond in addition to a secondary C-H…π and weak π-stacking interactions (Fig. 5Ab). The IR spectrum presented in Fig. 5Ab shows that the frequency of the N-H…N hydrogen-bonded N-H group of indole in the indole…pyridine dimer is red-shifted by 257 cm−1 . The most intriguing finding from the spectroscopic study of the indole…pyridine dimer is the observation of the structure which is stabilized due to the interplay between the electrostatic dominated strong N-H…N hydrogen bond, and dispersion dominated relatively weaker C-H…π and π-stacking interactions. This result demonstrates that a delicate balance between the strong and weak non-covalent interactions, which optimize the final shape of the molecular systems, is the key to the functional structures of biomolecules and materials. Energy decomposition analysis of the observed V-shaped structure of the indole…pyridine dimer obtained from Localized Molecular Orbital-Energy Decomposition Analysis (LMO-EDA) [166] indicates that the electrostatic (E ele ) and dispersion (E disp ) interactions in the dimer are comparable (Table 1). Thus, the indole…pyridine dimer is classified in the category of the mixed complex defined in the S22 database [167]. It could be mentioned here that the phenol dimer, which is a mixed complex reported in the literature [168–172], has a similar V-shaped structure stabilized by an interplay between a strong O-H…O hydrogen bond and a weak CH…π and π-stacking interactions. In the case of the (indole)2 …pyridine trimer, the observed structure could be any one of the three low energy conformers [(ind)2 .py-1, (ind)2 .py-2, (ind)2 .py-3], which has similar cyclic geometry stabilized by N-H…N, N-H…π, C-H…π, and C-H…N interactions (Fig. 5). The energetics as well as the N-H…N and N-H…π bound N-H stretching frequencies in the three conformers of the trimer are relatively close to each other. Table 1 Decomposition of the total interaction energy (E tot, kcal/mol) in the V-shaped indole…pyridine dimer, using the LMO-EDA method at the M05-2X/aug-cc-pVDZ level of theory. The interaction energy is decomposed into electrostatic (E ele ), exchange (E ex ), repulsion (E rep ), polarization (E pol ), and dispersion (E disp ) components. Adapted with permission from Ref. [115], copyright 2011, American Chemical Society) Indole…pyridine

E ele

E ex

E rep

E pol

E disp

E tot

−11.10

−6.47

21.57

−3.81

−7.77

−7.57

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Indole…Imidazole and Indole…(Pyrrole)2 Complexes

Indole…imidazole and indole…pyrrole complexes have been studied further by Das and co-workers not only to explore the similar interplay between multiple noncovalent interactions present there but also to mimic aromatic-aromatic interactions present in the side-chains of the aromatic amino acid residues in proteins [173, 174]. It has been demonstrated from extensive PDB analysis that aromatic-aromatic interaction exists in almost 50% of the proteins deposited there. Figure 6Aa shows the electronic spectrum measured in the mass channel of indole…imidazole dimeric complex. The electronic origin band (000 ) of the indole…imidazole complex appears at 34,933 cm−1 [173]. The spectrum exhibits many low-frequency transitions within 100 cm−1 blue side from the origin band. The UV-UV hole-burning spectrum (Fig. 6Ab) by probing the origin band demonstrates that all the bands in the electronic spectrum originate due to a single conformer of the dimer. Thus, the low-frequency bands riding on the origin bands are assigned as intermolecular vibrations for the stretching and bending of the N-H…N hydrogen bonds present there. The IR spectrum of indole…imidazole measured in the N-H stretching region using RIDIR spectroscopy is provided in Fig. 6Ba. The theoretical IR spectrum of the most stable conformer of the dimer calculated at the B97-D/6–311 + G(3df, 3pd) level of theory is presented in Fig. 6Bb. A reasonable agreement between the experimental and theoretical IR spectra bespeaks that the observed conformer of the dimer has a V-shaped structure stabilized primarily by a strong N-H…N hydrogen bond with secondary C-H…π and π-stacking interactions. The bands at 3270 and 3516 cm−1

Fig. 6 A (a) Electronic spectrum of indole…imidazole dimer, (b) UV-UV hole-burning spectrum by probing the 000 band of the dimer; B (a) IR spectrum in the N-H stretching region by probing the origin band of indole…imidazole dimer, (b) theoretical IR spectrum of the dimer calculated at the B97-D/6-311 + G(3df, 3pd) level of theory. Adapted with permission from Ref. [173], copyright 2012, American Chemical Society

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in the IR spectrum presented in Fig. 6Ba arise due to the N-H…N hydrogen-bonded N-H stretching of the indole moiety and the free N-H group of the imidazole unit of the dimer, respectively. The indole…imidazole complex is found to be similar to the indole…pyridine complex in terms of its structure and the interplay between different non-covalent interactions present there [115]. Additionally, the structural motif of the indole…imidazole complex observed from the experiment mimics the aromatic-aromatic interaction between the side chains of the tryptophan and histidine residues of proteins [175–177]. Indole…(pyrrole)2 trimeric complex studied by Kumar et al. represents aromatic trimeric interactions present in the aromatic side-chains of the amino acid residues in proteins [174]. Figure 7Aa shows the mass-selected electronic spectrum of indole…(pyrrole)2 trimer measured using 2C-R2PI spectroscopy. The UV-UV hole-burning spectrum presented in Fig. 7Ab substantiates the observation of a single conformer of the trimer in the experiment. Figure 7B shows mass-selected conformation-specific IR spectra of the indole monomer and indole…(pyrrole)2 trimer measured in the N-H stretch region using RIDIR spectroscopy. Theoretical IR spectra of two almost isoenergetic structures of the trimer, i.e., IP2-1 and IP2-2 calculated at the M05-2X/cc-pVTZ level of theory, are also presented in Fig. 7B. It is apparent from Fig. 7B that the IP2-1 is the observed structure, which has a cyclic geometry stabilized by three N-H…π hydrogen bonding interactions. Observation

Fig. 7 A (a) Mass-selected electronic spectrum of indole…(pyrrole)2 trimer using 2C-R2PI spectroscopy, (b) UV-UV hole-burning spectrum by probing the 000 band of the trimer; B IR spectra measured in the N–H stretching region by probing the origin bands of (a) indole, and (b) indole…(pyrrole)2 trimer using RIDIR spectroscopy. (c–d) Theoretical IR spectra of the IP2-1 and IP2-2 structures of the indole…(pyrrole)2 trimer obtained at the M05-2X/cc-pVTZ level of theory. Adapted with permission from Ref. [174], copyright 2012, American Institute of Physics

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of the cyclic aromatic trimer of the binding energy of about −20 kcal/mol in the experiment demonstrates that the aromatic trimeric interactions present in the side chains of proteins contribute significantly to the stability of proteins [174].

3.1.3

Indole…Furan and Indole…Thiophene Complexes

The T-shaped geometry of the aromatic dimers is one of the common structural motifs observed in biomolecules and materials [178, 179]. Benzene dimer is the most extensively studied aromatic dimer from both experimental and theoretical point of view [180–185]. Interestingly, the experimentally observed benzene dimer has a Tshaped geometry [183]. However, both parallel-displaced and T-shaped structures of the dimer are almost isoenergetic according to the CCSD(T)/CBS level calculation [178, 186]. It is further intriguing to note that the observed benzene dimer is not perfectly T-shaped, but the planes of the two benzene moieties are tilted with respect to each other to include secondary π-π interaction [184, 185]. It is also found that the benzene units in the crystal structure are oriented in a similar tilted T-shaped or herringbone geometry. Thus, it is again evident that the subtle interplay between various non-covalent interactions is the key to the optimum structure of any molecular system. The tilted T-shaped structure of the benzene dimer is an effect of the fine balance between the C-H…π hydrogen bonding and π…π interactions [184, 185]. Aromatic dimers comprising heterocyclic compounds are of special importance due to their prevalence in a wide range of molecular assemblies. The simplest Nheterocyclic compound, which replaces benzene, is pyridine. It has been reported from theoretical calculations that the preferred structure of the pyridine dimer and pyridine…benzene complex is parallel-displaced π-stacked [187]. The substitution of the nitrogen atom in the pyridine ring significantly reduces the π-electron density in the center of the ring. Hence, pyridine favors π-stacking configuration of the dimer instead of the T-shaped geometry through π-hydrogen bonding interaction. Thus, Sherrill and co-workers concluded that aromatic heterocycles are very poor π-hydrogen bond acceptors [26, 185, 187]. Das and co-workers have investigated the effect of heteroatoms (O, S) other than nitrogen present in the aromatic ring on the strength of the π-hydrogen bonding interaction [144, 188]. Figure 8A shows mass-selected electronic spectra of the indole monomer, indole…furan, and indole…(furan)2 complexes measured using 1C-R2PI spectroscopy by Das and co-workers[144]. The spectra of the dimer and trimer show a broad background, which is mostly due to the fragmentation of higher-order clusters of indole and furan into the mass channels of smaller clusters. The IR spectrum of the indole…furan dimer measured using RIDIR spectroscopy is presented in Fig. 8Bb. Theoretical IR spectra of a few possible low energy conformers of indole…furan dimer designated as T/ (N-H…π), HB (N-H…O), Syn-PD/ , Anti-PD are provided in Fig. 8B. The T/ conformer, which is the global minimum, has a tilted T-shaped structure held primarily by N-H…π hydrogen bonding interaction. The HB is the N-H…O hydrogen-bonded structure with V-shaped geometry, which gets additional

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Fig. 8 A Mass-selected electronic spectra measured in the (a) indole, (b) indole…furan dimer, and (c) indole…(furan)2 trimer mass channels using 1C-R2PI spectroscopy; B IR spectra obtained in the N–H stretching region by probing the origin bands of (a) indole and (b) indole…furan dimer using RIDIR spectroscopy. Adapted with permission from Ref. [144], copyright 2012, American Chemical Society

stability due to the presence of the C-H…π and π…π interactions. In the SynPD/ structure, the indole N-H group and the oxygen atom of furan are positioned on the same side, having a tilted parallel-displaced π-stacked structure. The N-H group and oxygen atom are on the opposite side with a parallel-displaced π-stacked geometry in the Anti-PD structure. A comparison of the experimental IR spectrum with the theoretical IR spectra of the four conformers of indole…furan (Fig. 8B) reveals that the observed dimer has a tilted T-shaped structure, i.e., T/ (N-H…π), which provides 40 cm−1 red-shift in the N-H stretch frequency with respect to that in the indole monomer. A remarkable point to note here is that the π-hydrogen-bonded (unconventional hydrogen bond) structure dominates over the conventional N–H…O hydrogen-bonded structure. Das and co-workers further extended their spectroscopic investigation on N-H…π interaction by replacing the heteroatom in the π-hydrogen bond acceptor aromatic ring by sulfur (S) atom [188]. Figure 9Ab shows the mass-selected IR spectrum of the single conformer of indole…thiophene complex measured by RIDIR spectroscopy. The theoretical IR spectrum of the dimeric complex calculated at the M05-2X/6– 311++G(2df,2pd) level of theory is provided in Fig. 9Ac. The observed conformer of the indole…thiophene complex has a tilted T-shaped structure bound by N-H…π hydrogen bonding interaction similar to that of the indole…furan complex. However, the tilt angle in the T-shaped structure of the indole…thiophene complex is larger than that of the indole…furan complex [144]. The red-shift of 56 cm−1 in the NH stretching frequency of the indole…thiophene complex with respect to that of

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Fig. 9 A IR spectra measured in the C-H and N-H stretching regions by probing the (a) 000 band of the indole monomer and (b) electronic band maximum of the indole…thiophene dimer at 35,075 cm−1 ; B Electrostatic potential map of pyridine, furan, and thiophene. Adapted with permission from Ref. [188], copyright 2012, American Institute of Physics

the indole monomer presented in Fig. 9Aa demonstrates that the π-hydrogen bond present in the indole…thiophene dimer is stronger than that in the indole…furan dimer. It is worth mentioning here that the π-hydrogen-bonded dimers reported in the literature, such as indole-benzene, pyrrole-benzene, (pyrrole)2 exhibit similar red-shift (~50 cm−1 ) in the N-H stretching frequency [189–192]. Electrostatic potential mapping of the three aromatic heterocycles pyridine, furan, and thiophene provided in Fig. 9B illustrates their different potentiality as a πhydrogen bond acceptor [188]. The sequence of the π-electron density in the center of the three heterocycles is the following: pyridine < furan < thiophene. In the case of pyridine, the nitrogen atom does not take part in the aromaticity but pulls out the π-electron density from the ring. However, the lone pair electrons on the oxygen and sulfur atoms in furan and thiophene, respectively, take part in their aromaticity and get delocalized in the ring. Due to less electronegativity and higher polarizability of the S atom in comparison to those of the O atom, the delocalization of the lone pair electrons in thiophene is more than that in furan. Hence, it has been proved from this study that five-membered aromatic rings containing heteroatoms are reasonably good π-hydrogen bond acceptors [144, 188].

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3.2 Unconventional Hydrogen Bond Although the conventional hydrogen bonds are well-understood in the literature, unconventional hydrogen bonds are yet to be explored in terms of their nature, physical origin, and strength [16, 20, 22, 93]. In the unconventional hydrogen bond, the hydrogen bond donor and acceptor atoms are weakly electronegative. C-H…Y (Y = O, N) is a relatively well-studied unconventional hydrogen bond, where the hydrogen bond donor atom (C) is very weak in electronegativity [10, 80]. It has been reported that C-H…O or C-H…N hydrogen bond is very weak in strength [80]. Desiraju and co-workers have demonstrated the significance of the C-H…Y hydrogen bonds in crystal engineering and supramolecular assemblies [10, 12, 16, 18]. In recent times, there are reports on spectroscopic studies of various unconventional hydrogen bonds having different unconventional atoms in the periodic table such as P, S, and Se as hydrogen bond acceptors [80, 86–88, 91–93, 145, 193, 194]. Surprisingly, it has been found from various gas-phase spectroscopy studies that these unconventional hydrogen bonds, i.e., N-H…S, N-H…Se, N-H…P, etc. are as strong as any conventional hydrogen bonds such as N-H…O, O–H…O, etc. [80, 86–88, 93, 145, 193, 194]. Sulfur centered hydrogen bonds are also extensively present in proteins and many other supramolecular assemblies [18, 20, 22, 88, 93, 111, 194]. Biswal et al. have made a seminal contribution to understanding the physical nature and strength of sulfur centered hydrogen bonds by studying various model complexes in a supersonic jet using different laser-based spectroscopic techniques and quantum chemistry calculations [86–88, 93]. They have studied several dimeric complexes of indole, p-cresol, p-fluorophenol with various solvents such as Me2 S, Et2 S, Et2 O, Me2 O, H2 S, etc. employing LIF excitation, R2PI, FDIRS, and RIDIRS spectroscopy techniques [20, 87]. It has been found that the IR red-shift in the N-H and O-H stretch frequencies in the N-H…S and O-H…S hydrogen bonds, respectively, are quite similar to that in any conventional hydrogen bond [20, 22, 86–88, 93, 195]. They have found from the energy decomposition analysis that dispersion interaction plays a significant role in the stability of these unconventional hydrogen bonds, and hence these non-covalent interactions are termed as dispersion stabilized hydrogen bonds [20, 22, 86–88, 93, 194, 195]. Observation of unusually strong N-H…S hydrogen bond has also been reported using room-temperature gas-phase Fourier transform infrared (FTIR) spectroscopy by Kjaergaard and co-workers [89, 90]. They have further demonstrated that phosphorous (P) can act as a strong hydrogen bond acceptor by studying the O-H… P hydrogen bond present in 1:1 complexes of trimethyl phosphine and various alcohols using matrix isolation as well as gas-phase FTIR spectroscopy [91, 92]. Later, Biswal et al. provided the spectroscopic evidence of the formation of strong N-H…Se hydrogen bonds by investigating the complexes of N-phenyl acetamide and 2-pyridone with dimethyl selenide [194]. Recently, Das and co-workers have performed a thorough investigation of the nature of the selenium hydrogen bonding by studying both N-H…Se and OH…Se hydrogen bonds using gas-phase laser spectroscopy and detailed theoretical calculations [145]. Figure 10A shows mass-selected conformation-specific IR

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Fig. 10 A IR spectra measured by probing the origin bands of (a) indole, (b) indole…dimethylselenide complex, (c) phenol, and (d) phenol…dimethylselenide complex; B Decomposition of the interaction energies of various complexes of indole using ALMO-EDA method at the B97-D/6– 311++G(d,p) level of theory. The structures of various complexes of indole with their binding energies are provided below the ALMO-EDA plot. Adapted with permission from Ref. [145], copyright 2017, Royal Society of Chemistry

spectra of indole-dimethyl selenide (indmse) and phenol-dimethyl selenide (phdmse) complexes in the N-H and O-H stretching region using RIDIR spectroscopy. There is a nice corroboration of the experimental IR spectra with the theoretical IR spectra of the two complexes calculated at the B97-D/6–311++G(d,p) level of theory. The Redshift of 154 cm−1 in the N-H stretching frequency in the indmse complex with respect to the indole monomer and the red-shift of 240 cm−1 in the O-H stretching frequency in the phdmse complex with respect to the phenol monomer demonstrate the observation of very strong N-H…Se and O-H…Se hydrogen bonds in the experiment [145]. Finding of S or Se centered hydrogen bonds of strength similar to any conventional hydrogen bonds in terms of the IR red-shift cannot be explained with the conventional perceptiveness of the electrostatic interaction as both S and Se are very weak in electronegativity compared to O and N atoms. The dispersion interaction, albeit, increases in the S and Se centered hydrogen bonds (i.e., N-H…S, N-H…Se) as we move from the O or N-centered hydrogen bonds (i.e., O-H…O, N-H…N) [87,

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115, 145, 173, 196]. However, it is reported in the literature that the IR red-shift in the stretching frequency of the hydrogen-bonded functional group arises from the electrostatic, polarization, and charge transfer interactions while the dispersion interaction contributes to the overall stability of the system [80]. Absolutely localized molecular orbital energy decomposition analysis (ALMOEDA) [197, 198] of different conformers of indole…dimethylselenide (indmse1, indmse2), indole…dimethylsulfide (indms1, indms2) and indole…dimethyloxide (indmo) presented in Fig. 10B illustrates that there is a linear relationship between the IR red-shift of the N–H stretching frequency and charge-transfer interaction present in the complexes. It should be pointed out here that indmse2 and indms2 were not observed in the experiment, although those conformers are stable low energy minima. The reason for not observing these two conformers could be due to their low interconversion barrier with the global minima. In the ALMO-EDA method, total interaction energy (E) is decomposed into frozen density (E Frz ), polarization (E Pol ), and charge transfer (CT) components, where E Frz consists of electrostatic, repulsion, and dispersion energy. This result demonstrates that the CT interaction in combination with the electrostatic and polarization interactions has a significant contribution to the observed IR red-shift in the stretching frequency of the hydrogen bond donor in the case of the unconventional hydrogen bonds [145]. Das and co-workers further modeled single water-mediated selenium hydrogen bonding interactions present in proteins by studying 1:1:1 trimeric complex of indole, dimethyl selenide, and water using isolated gas-phase UV-IR double resonance spectroscopy [193]. Here, indole and dimethyl selenide represent the side chains of the tryptophan and selenomethionine amino acid residues of proteins, respectively. It has been found from detailed PDB analysis that there were 7526 single water-mediated Se hydrogen bonding interactions present in proteins. Figure 11Bc shows a massselected conformation-specific IR spectrum of indole…dimethylselenide…H2 O measured in the N-H and O-H stretching frequency region using RIDIR spectroscopy. A comparison of the experimental IR spectrum of the trimer with the theoretical IR spectrum (Fig. 11Bd) reveals that the observed trimer has a cyclic structure where H2 O makes a bridge between indole and dimethyl selenide through N-H…O and O-H…Se double hydrogen-bonding interactions. It could be noted that the observed structure is stabilized by both conventional (N–H…O) and unconventional (O–H…Se) hydrogen bonding interactions. The hydrogen-bonded N-H and O-H stretching frequencies in the trimer appear at 3399 and 3414 cm−1 , respectively. Interestingly, the most stable structure of the indole…dimethylselenide…H2 O complex observed in the experiment corroborate the structural motifs of the single water-mediated Se hydrogen bonding interactions found between the amino acid residues in proteins and Fig. 11A shows a representative analogy of this [193].

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Fig. 11 A A representative example of single water-mediated selenium hydrogen bonding interaction between the indole moiety of the tryptophan and selenomethionine residues in the PDB: 3D5P; B Mass-selected IR spectra of (a) indole and (c) indole…H2 O…Me2 Se complexes. Theoretical IR spectra of the (b) indole monomer and (d) indole…H2 O…Me2 Se complex calculated at the ωB97X-D/6–311++G(d,p) level of theory Adapted with permission from Ref. [193], copyright 2019, American Chemical Society)

4 Conclusions and Future Outlook In this chapter, we have provided a brief overview of jet-cooled laser spectroscopic techniques with resonantly enhanced multiphoton ionization (REMPI) time of flight mass spectrometry as well as fluorescence detection scheme. Jet-cooled spectroscopy is unique in rendering high resolution electronic and vibrational spectroscopy of molecules and weakly bound complexes in an isolated gas phase. Intrinsic physical properties of the molecular systems can be retrieved only from the data obtained from isolated gas-phase spectroscopy as it is performed in the absence of any perturbation from any solvent as well as intermolecular interactions. Moreover, the isolated gas-phase spectroscopy data are immensely valuable to benchmark various levels of theories employed for quantum chemistry calculations. Here intrinsic nature, physical origin, and optimum strength of different non-covalent interactions, as well as the subtle interplay between those interactions, have been explored by studying various model complexes of biological relevance in the gas phase. The inherent existence of various unconventional non-covalent interactions present in proteins and other supramolecular assemblies can only be validated by studying those interactions in the supersonic jet where additional interactions from surroundings or media are entirely absent. Gas-phase laser spectroscopy coupled with laser vaporization/desorption technique enables us to study non-volatile larger molecular systems such as peptides and

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other biologically relevant molecules in the gas phase. However, the major challenge is to extend this study to the gas phase spectroscopy of larger peptides with a significant number of amino acid residues and still acquiring meaningful information on the non-covalent interactions present there as well as their structures. Although the presence of the multiple numbers of N-H and C=O groups in larger peptides can broaden the IR spectra, it will be possible to have isotope labeling of particular N-H or C=O group to probe the specific interaction present in the system. Furthermore, state-of-the-art supercomputing facilities can handle the quantum chemical calculations of the large conformational landscape of the larger peptides and hence aid the interpretation of the structures observed in the experiment. In the theoretical front, it is also necessary to develop a better model to improve the calculation of the scaling factors to correct the harmonic frequencies obtained from the calculations as anharmonic frequency calculations are computationally expensive. Acknowledgements We would like to thank our graduate students, postdocs, and collaborators who have contributed to the results from our laboratory, discussed in this chapter. Financial support received from Indian Institute of Science Education and Research Pune, Department of Science and Technology, India (Grant No. SR/S1/PC/0054/2010), and Science and Engineering Research Board, India (Grant No. EMR/2015/000486) is gratefully acknowledged.

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Classical- and Heterodyne-Detected Vibrational Sum Frequency Generation (VSFG) Spectroscopy and Its Application to Soft Interfaces Subhadip Roy, Subhamoy Saha, and Jahur Alam Mondal

Abstract Chemistry at interface, i.e., at the junction between two immiscible bulk media is remarkably different from either of the bulk phases. This is primarily due to an anisotropic environment at interface where reactants behave differently than they do in isotropic bulk media. Interface-selective spectroscopy can provide valuable insight into such interface specific phenomena at the molecular level. Presently, vibrational sum frequency generation (VSFG) is the best suited spectroscopic technique to unravel interfacial molecular properties, especially for the “soft interface” such as liquid surface where linear optical spectroscopy and particle scattering based techniques cannot be applied either due to the lack of interface-selectivity or due to requirement of stringent experimental conditions. VSFG is inherently interfaceselective and probes molecules by their vibrational transitions, revealing site-specific molecular properties. This Chapter provides a brief description of the theory of VSFG followed by the origin of its interface-selectivity and detection of VSFG signal by the conventional (classical-VSFG) and heterodyne-detection (HD-VSFG) methods. Unlike classical-VSFG, HD-VSFG records unambiguous absorption spectra and absolute orientation of interfacial molecules. Finally, we discuss the application of narrowband classical-VSFG and broadband HD-VSFG techniques to aqueous interfaces of biological and environmental relevance. Specifically, the surface properties of ions, molecules, lipids, pollutants, and the associated perturbation of H-bonding and orientation of interfacial water. Keywords Sum frequency generation (SFG) · Heterodyne detection · HD-VSFG · Aqueous interface · Perfluorinated-POP · Structure-making ion · TMAO · Lipid

1 Introduction An interface is a few molecular layers thick junction between two immiscible bulk media, where the inversion symmetries of bulk media are broken. Since S. Roy · S. Saha · J. A. Mondal (B) Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Homi Bhabha National Institute, Mumbai 400085, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 D. K. Singh et al. (eds.), Modern Techniques of Spectroscopy, Progress in Optical Science and Photonics 13, https://doi.org/10.1007/978-981-33-6084-6_4

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water is the most abundant liquid on earth, aqueous interfaces are wide-spread in biotic and abiotic systems including ocean surface (air-water interface), mineral surface (gas-solid or liquid-solid interface), cell and cellular organelles (lipid or protein-water interface). Because of the anisotropy and rapidly varying number density of water, most ions and molecules exhibit an orientational preference and non-uniform distribution (adsorption/depletion) across the interface, giving rise to interface-selective physicochemical processes. Molecular level understanding of such interfacial processes requires selective probing of the interface with molecular precision. Conventional spectroscopies such as absorption, fluorescence and Ramanscattering are not interface-selective and hence, the inherently weak signal of interfacial molecules (due to extreme thinness of the interface, ~a few nm) gets buried into the huge signal of the bulk phase. Particle scattering based surface-selective spectroscopies such as X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy as well as nano-scale microscopy imaging techniques including scanning (or transmission) electron microscope are incompatible with most liquid surfaces because of the dynamic nature of molecules on liquid surface and the need of ultrahigh vacuum for such techniques. It is important to note that though near ambient pressure (~few mbar) XPS (NAP-XPS) overcomes the stringent requirement of ultrahigh vacuum to some extent, in general the photoelectron spectra are not sensitive to the structural change of interfacial water. Therefore, the atmospherically and biologically relevant interfaces such as the marine boundary layer, aqueous aerosol droplet, lipid membrane surface, which involve water as the liquid phase, broadly classified as “soft interfaces”, needs special endeavor to achieve molecular level understanding. Even-order nonlinear spectroscopy such as vibrational sum frequency generation (VSFG), which is inherently interface-selective and applicable at ambient condition to any kind of interfaces accessible by light, can provide the much-needed molecular spectra of an interface [1, 2]. Recent up-gradation of classical-VSFG to phase-sensitive/heterodyne detected-VSFG, [3, 4] enables one to record the accurate absorption spectra of interfacial molecules along with their absolute orientation. Here, we provide a brief description of the theory of SFG and its interfaceselectivity followed by the instrumentation of the classical- and heterodyne-detection (HD) methods. Applications of classical- and HD-VSFG techniques have been illustrated through the elucidation of surface prevalence and orientations of molecules, ions and surfactants at the air-water interface and its effect on the structure and orientation of the interfacial water.

2 Theory of SFG SFG is one of the outcomes of nonlinear interaction of light with matter. In a material, the negatively charged electrons of its constituent atoms/molecules are bound by electrostatic attraction with their positively charged nuclei. Interaction of light (an oscillating electric field) with a material (molecule), induces  an oscillation of its electrons, generating an oscillating induced dipole moment μind . μind is proportional to the electric field of the light (E), μind = α E, where α is the proportionality

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constant, known as molecular polarizability. μind is not necessarily in the direction of E and hence α is vectorial in nature. For high intensity light (intensity ∝ |E|2 ), specifically when the amplitude of the light electric field is comparable to the atomic field - the electric field with which the electrons are held by the atomic nuclei (typically, 5 × 1011 V/m), μind deviates from the linear dependence of E; it becomes a higher-order polynomial of E, as expressed by Taylor series. μind = α E + β E 2 + γ E 3 + . . .

(1)

where, the coefficients β, γ are the corresponding molecular hyperpolarizabilities. For a macroscopic system, the induced dipole moment is expressed as the induced dipole moment per unit volume, known as polarization ( P) such that P = N μind  = o χ (1) E + o χ (2) E 2 + o χ (3) E 3 + . . . = P (1) + P (2) + P (3) + . . .

(2)

where, N is the number of molecules per unit volume (or per unit area for surface);  indicates the ensemble average over orientational distribution; o is the electric constant in vacuum; χ (1) , χ (2) , χ (3) are electric susceptibilities corresponding to the molecular polarizabilities α, β and γ , respectively. Thus, χ (n) provides a macroscopic description of the susceptibility of a material towards polarization via the effective sum of the corresponding molecular (hyper) polarizabilities. Similar to molecular (hyper) polarizability, χ (n) is also vectorial in nature. The higher order polarization terms, P (2) , P (3) , etc., become increasingly important for light from a pulsed laser. For instance, Light from a femtosecond (fs) pulsed laser (1.0 W, 1.0 kHz, 50 fs pulse duration) produces an energy density of the order of ~1019 W/m2 (i.e., a field strength of ~8.5 × 1010 V/m) when focused on a material with a lens of 50 cm focal length. In general, the spectroscopy associated with the higher order polarizations ( P (2) , P (3) , etc.) are known as nonlinear spectroscopy. Sum frequency is generated by the oscillating dipole corresponding to the secondorder polarization, P (2) . Assuming two intense lights of angular frequencies ω1 and ω2 , falling on a material, P (2) can be expressed as, P (2) (t) = o χ (2) E(t)2

(3)

  where, E(t) = (E 1 e−iω1 t + E ∗1 eiω1 t ) + E 2 e−iω2 t + E ∗2 eiω2 t . By successive expansion, P(2) (t) takes the form,   P (2) (t) = o χ (2) 2E 1 E ∗1 + 2E 2 E ∗2 + (E 21 e−2iω1 t + E 22 e−2iω2 t + E ∗1 e2iω1 t + E ∗2 e2iω2 t )   + 2E 1 E ∗2 e−i(ω1 −ω2 )t + 2E ∗1 E 2 ei(ω1 −ω2 )t   + 2E 1 E 2 e−i(ω1 +ω2 )t + 2E ∗1 E ∗2 ei(ω1 +ω2 )t 2

2

(4)

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Fig. 1 Direction of SFG signal following the phase matching in a non-collinear reflection geometry

As can be seen from Eq. 4, second order polarization creates oscillating electric field at frequencies ω1 +ω2 (i.e. sum frequency), ω1 −ω2 (i.e., difference frequency), 2ω1 or 2ω2 (i.e., second harmonic of ω1 or ω2 ), and at zero frequency (DC field, i.e., optical rectification). At a given instance, only one of the components exists predominantly whose phase matching condition, i.e., momentum conservation is satisfied [5]. For non-collinear incidence of ω1 and ω2 on a surface (Fig. 1), the phasematching condition for the sum frequency signal produced in the reflection geometry is η S F k S F sin Θ S F = ηV I S k V I S sin ΘV I S + η I R k I R sin Θ I R , where, η, k and Θ are the refractive index, wave vector and incident/generation angle of the corresponding lights [6, 7]. Thus, sum frequency is generated at a particular direction (Θ S F ) which is different from the reflection angles of ω1 and ω2 . This has a technical advantage in selective detection of the SFG signal by spatial separation from the ω1 and ω2 .

2.1 SFG and Interface-Selectivity Following Eq. 4, the amplitude of the second-order polarization corresponding to the generation of sum frequency is (2) E1 E2 P (2) S F G = 2o χ

(5)

As the applied fields (E 1 and E 2 ) and resultant polarisation ( P (2) S F G ) are vectors, not necessarily parallel to each other, χ (2) should be treated as a ‘tensor’ which relates the incident and resultant vector fields. In fact, χ (2) is a third-rank tensor, having 33 = 27 different components in Cartesian system, and hence, 27 different combinations of incident and induced field vectors (Fig. 2a). On incorporation of dummy Cartesian coordinates (i, j, k), P (2) S F G can be expressed as, (2) (2) P i,S F G = o χi jk E j,1 E k,2

(6)

where, (i, j, k) represents (x, y, z). Equation 6 indicates that the fields E 1 and E 2 applied along j and k, respectively induce a polarization along i. For an isotropic

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(2) Fig. 2 a 27 possible combinations of χi(2) jk , b SFG is produced only at the interface where, χi jk  = 0. At the air-water interface, due to C∞v symmetry, only seven non-zero components (four independent components) of χi(2) jk exist which are coloured blue in panel ‘a’

ˆ medium, χi(2) jk is independent of direction and hence on inversion operation (i), Eq. 6 changes as,     (2) (2) ˆ iˆ P i,S F G = i o χi jk E j,1 E k,2   (2) (2) (2) − P i,S F G = o χi jk (−E j,1 ) −E k,2 = + P i,S F G

(7)

(2) Equation 7 holds, only if χi(2) jk = 0. In other words, χi jk = 0 for centrosymmetric (2) media and there is no SFG ( P i,S F G = 0) in isotropic bulk media, under electricdipole (ED) approximation. The inversion symmetry necessarily breaks down at interface, making it anisotropic. Therefore, χi(2) jk  = 0, only at the interface where the sum frequency is generated. This makes SFG an inherently interface specific optical phenomenon under ED approximation (Fig. 2b). As χi(2) jk is a third-rank tensor, the total polarization is given by,



x,y,z x,y,z x,y,z

P (2) S F G = o

i

j

χi(2) jk E j,1 E k,2

(8)

k

However, an interface, e.g. air-water interface is isotropic with respect to the surface normal, having C∞ rotation axis along z-direction. Therefore, following the symmetry consideration, out of 27 only four independent components of χi(2) jk sustain [6, 8, 9]. (2) (2) (2) (2) (2) (2) χx(2) x z = χ yyz , χx zx = χ yzy , χzx x = χzyy and χzzz

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2.2 SFG Intensity and Vibrational Spectra The SFG signal, generated at an interface, depends on the material parameter, χi(2) jk . In other words, sum frequency signal carries interfacial material response via χi(2) jk which is correlated with second-order molecular hyperpolarizability (βlmn ) and hence with the vibrational/electronic transition of interfacial molecules. For vibrational measurement, spatiotemporal overlap of a high power fixed frequency visible light (E V I S , i.e., E ω1 ) and a tuneable infrared frequency (E I R , i.e., E ω2 ) on a material surface (e.g., on the surface of water) will generate the SFG signal following Eq. 6. (2) When the VIS and IR fields are expressed in terms of their actual magnitudes, P i,S FG takes the form, (2) (2) P i,S F G = o χi jk (K j E V I S )(K k E I R )

(9)

K j and K k are the Fresnel factors which define the amplitude coefficients of the reflected VIS and IR light, repectively. Using a similar Fresnel factor or ‘L-factor’, (2) P i,S F G can be correlated to the sum frequency electric field (E i,S F G ) as follows [6, 10, 11]. (2) (2) E i,S F G = L i P i,S F G = L i o χi jk (K j E V I S )(K k E I R )

(10)

Thus, the intensity of the emitted sum frequency light which is related to the square of the electric field will be 2 2 (K E )(K E ) I S F G ∝ E i,S F G ∝ L i o χi(2) j V I S k I R jk

(11)

Equation 11 is frequently expressed in the simplified form 2 I S F G ∝ χi(2) jk I V I S I I R

(12)

As discussed in Sect. 2, χi(2) jk represents the ensemble averaged second-order molecular hyperpolarizabilities (βlmn ) such that, χi(2) jk =

Ns βlmn  o

(13)

where Ns is the number density of the surface molecules contributing to SFG signal,  indicates the ensemble average orientational distribution. (l, m, n) signify the generic indices corresponding to molecular coordinate (a, b, c). At interface, orientation of a molecule is usually expressed by the angle of its principal axis with the surface normal. It is very unlikely that the molecular symmetry axes (a, b, c) will coincide with the laboratory-frame coordinate (x, y, z). Hence, to correlate χi(2) jk with the molecular property βlmn , a coordinate transformation is necessary which is

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carried by Eulers’ angle transformation [10, 12, 13]. Non-zero components of βlmn are associated with the vibrational modes of the molecules. The expression for βlmn , obtained from quantum mechanical perturbative treatment, [1, 14] is as follows, βlmn =

Av ωv − ω I R − iΓv

(14)

where, ωv and Γv are the resonant frequency and the natural linewidth of the vth vibrational transition. ω I R is the tuneable IR frequency. Av is the amplitude coefficient proportional to the product of Raman and IR transition moments [1, 15]. Therefore, Av Ns o (ωv − ω I R − iΓv ) Ns Av (ωv − ω I R ) Γv Ns Av = +i 2 2 o (ωv − ω I R ) + Γv o (ωv − ω I R )2 + Γv2 



(2) = Re χi(2) jk + I m χi jk

χi(2) jk =

(15)

where, (ωv − ω I R ) Ns Av and o (ωv − ω I R )2 + Γv2 Γv Ns Av Im[χi(2) jk ] = o (ωv − ω I R )2 + Γv2 Re[χi(2) jk ] =

In the above equation, χi(2) jk realizes the resonance condition of VSFG. When the incident IR frequency (ω I R ) is equal to the vth vibrational transition frequency (ωv ) of interfacial molecules (i.e., ω I R = ωv or ωv − ω I R = 0), the imaginary component, 

becomes maximum and shows an absorptive band shape with the variation Im χi(2) jk 

of ω I R . The real component, Re χi(2) jk = 0 at ω I R = ωv , and shows a dispersive band shape with ω I R . The sum frequency intensity (I S F G ), IS F G

1 Ns Av 2 (2) 2 ∝ χi jk ∝ o (ωv − ω I R )2 + Γv2

(16)

 also shows an absorptive band shape similar to the Im χi(2) jk , which means the intensity of the sum frequency signal is maximum at vibrational resonance (ω I R = ωv ). This is known as ‘resonantly enhanced’ VSFG (Fig. 3). Moreover, to have non-zero value of χi(2) jk , Av must be non-zero, i.e., the vibrational transition should be both Raman and IR active. In other words, the vibrational modes of interfacial molecules must be IR and Raman active to generate VSFG signal.

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Fig. 3 Energy level diagram of the coherent interaction between ω I R and ωV I S , leading to the generation of SFG at ω S F = ωV I S + ω I R . For ω I R = ωv , i.e., the incident IR is in resonance with the vibrational transition of interfacial molecules, the SFG intensity is resonantly enhanced. Dashed horizontal lines indicate virtual states

χi(2) jk , in addition to the resonant component discussed above, contains a nonresonant component (χ N(2)R ), which is invariant with respect to the IR frequency, and hence, appears as a constant background. (2) χi(2) jk = χ N R +

Av Ns o (ωv − ω I R − iΓv )

(17)

In that case the sum frequency intensity, IS F G

2 (2) Ns Av ∝ χ N R + ωv − ω I R − iΓv

(18)

For multiple vibrational transitions, the resonant component is generally considered as the sum of all possible vibrational modes such that,

IS F G

2 Ns Av (2)  ∝ χ N R + ωv − ω I R − iΓv v

(19)

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As shown in Eq. 19, the SFG-intensity spectrum, recorded with classical-VSFG spectrometer, contains not only the real and imaginary components of resonantalso the non-resonant-χi(2) χi(2) jk , but jk which appears as constant background with 

(2) the Re χi jk signal. In the case of weaker resonant components (weak signal), the relative contribution of χ N(2)R can

be significant, which changes the line-shape of the I S F G signal from that of the Im χi(2) jk signal. This issue becomes further complicated for multiple overlapping bands. The SFG signal and the incident ω1 and ω2 can be categorized by the direction of their electric fields with reference to the plane of incidence. Electric field vectors that lie with the plane of incidence are ‘P-polarized’ and those perpendicular to the same are known as ‘S-polarized’. S-polarized light contains electric field only along y-axis; whereas, the P-polarized light contains two electric field components along x and z axes, respectively (Fig. 1). Polarization of the SFG light entirely depends on (2) the non-zero χi(2) jk ; selective measurement of a specific component of χi jk is possible by proper choice of polarization combination of ω1 , ω2 and the SFG light. The SFGintensity spectra at different polarization combinations (SSP, SPS, PSS and PPP) are used to deduce information about orientational/conformational distribution of interfacial molecules [8].

3 Narrowband Classical-VSFG Spectrometer Figure 4 shows the typical optical layout of a narrowband classical-VSFG spectrometer. It is called ‘narrowband’ and ‘classical’ to indicate that the IR pulse used for VSFG has a narrow spectral width and the SFG signal is detected as the conventional intensity spectrum (I S F G ∝ |E S F G |2 ), respectively. The output from a laser, such as a mode-locked Nd:YAG laser (∼30 ps, 1064 nm, 50 Hz, 40 mJ/pulse), is divided into two parts; the first part is frequency doubled to 532 nm with a secondharmonic generator. The 532 nm beam is again split into two; one is used as the visible pulse (ωV I S ; ∼500 μJ/pulse) for SFG and the other is mixed with the second part of the 1064 nm fundamental to produce a tuneable infrared pulse in an optical parametric amplifier and difference frequency generator unit (OPA-DFG unit). The narrowband IR pulse (ω I R ; ∼260 μJ/pulse @ 3300 cm−1 ) is spatially and temporally overlapped with ωV I S on the sample surface (e.g., water surface) while the IR frequency is tuned stepwise to cover the spectral region of a desired vibrational band. The intensity of the SFG is detected by a single channel detector consist of a monochromator and photomultiplier tube (PMT). It is obvious that for a vibrational transition in the mid-IRregion, the corresponding SFG signal appears in the visible region (ω I R 3400cm−1 +ωV I S (532nm) = ω S F (450nm)), and hence, visible detector such as PMT is used which is easier to handle than that of IR-detector. To remove the influence of the energy profile of the input IR and visible light, the measured spectra are normalized with the non-resonant SFG signal from the surface

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Fig. 4 Typical optical layout of a narrow band classical-VSFG spectrometer, based on a picosecond Nd:YAG amplified laser system

of a gold mirror. Measurements at different polarization combinations (e.g., SSP, SPS, and PPP) are achieved by changing the polarization of the respective beams by placing appropriate half-wave plate and polarizer combination in the respective beam line. Because of the narrow spectral width of the IR pulse, SFG intensity spectrum is recorded by a single channel detector (monochromator with PMT), which takes quite a long time to acquire a spectrum of modest SNR even when the acquired data points are not close enough. This limitation is overcame by using a broadband IR pulse (say, from a femtosecond laser system) and by detecting the broad band SFG signal with a single shot multiplex detector (spectrograph and CCD). This is known as broadband classical-VSFG spectroscopy [16]. However, the bigger issue with the classical-VSFG spectroscopy is that it provides the SFG-intensity spectrum which is 2 proportional to the square modulus of χ (2) (Eq. 12), also known as χ (2) -spectrum. As discussed in Sect. 2.2, since χ (2) contains non-resonant background, the SFGintensity spectrum is deformed for weak signals of overlapping bands. Therefore, to extract the true vibrational features of interfacial molecules, it is necessary to fit the experimental SFG-intensity spectra with Eq. 19. However, the solutions of the fitting are not unique due to the lack of phase information, i.e., the sign of the transition amplitude ( Av ) of the vibrational bands. Secondly, the sign of χ (2) which reveals the preferred orientation of interfacial molecules is lost in the SFG-intensity spectrum as χ (2) gets squared. Third, the SFG intensity is proportional to the square of Ns (number density of molecules contributing to SFG); the nonlinear concentration dependence makes quantative interpretation of SFG-intensity spectra less straightforward.

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4 Broadband Heterodyne-Detected VSFG Spectrometer As shown in Eq. 15, the vibrational resonance of interfacial molecule is contained in the imaginary component of χ (2) . Therefore, it is desirable to experimentally record the Imχ (2) spectrum, preferably by using broad band IR and narrow band visible pulses, typically from a femtosecond laser system. As described by Tahara and coworkers, [4, 17] heterodyne detection of broad band SFG signal, can independently provide the imaginary- and real-χ (2) spectra of an interface. In this method, the SFG from a sample is mixed with another non-resonant SFG signal from a substrate, known as ‘local oscillator’ (LO). Sample SFG signal (SFS ) is delayed with respect to the LO (SFLO ) in time domain (described later) and the interference pattern following their dispersion in a spectrograph is detected with a charge coupled device (CCD). A typical layout of a femtosecond laser-based HD-VSFG spectrometer is shown in Fig. 5. Briefly, output from a Ti:Sapphire regenerative amplifier laser (800 nm, ∼50 femtosecond, energy ∼2 mJ/pulse, 1.0 kHz pulse repetition rate) is split into two parts of energy ∼1 mJ/pulse each. One part is passed through a narrow band pass filter (center wavelength 800 nm) that converts the broad band femtosecond pulse into a narrow band picosecond pulse (fwhm ∼16 cm−1 , energy ∼15 μJ/pulse), known as the visible pulse (ωV I S or ω1 ). The second part of the amplifier output (∼1 mJ/pulse) is frequency converted to a broad band IR pulse (ω I R or ω2 pulse; fwhm ∼300 cm−1

Fig. 5 Typical optical layout of broadband HD-VSFG setup, based on a femtosecond Ti:Sapphire amplified laser system

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@ 3400 cm−1 ) using an optical parametric amplifier (OPA) followed by a difference frequency generator (DFG). The ω1 pulse is passed through a translation stage (to adjust time zero relative to the IR pulse) and focused (L1; f = 50 cm) onto the sample surface. The ω I R is focused (L2; f = 10 cm; ∼6 μJ/pulse) onto the sample surface at the same spot as that of the ωV I S . The incident angles of the visible and IR beams are ∼52° and ∼60° respectively. The sum frequency, ω S F = ωV I S + ω I R , is generated at ∼54° in the incident plane. For the heterodyne detection of the sample sum frequency, it is mixed with an LOsignal which has a constant phase-difference from that of the former. In the present set-up (Fig. 5), the reflected ωV I S and ω I R from the sample surface are refocused (CM, f = 10 cm) on a GaAs (110) wafer to generate the LO signal (SFLO ). A constant phase-difference i.e., path difference (∼2 ps) is introduced between the SFS and SFLO , by selectively passing the former through a 1.0 mm thick anti-reflection coated glass plate (GP) located in between the sample and the concave mirror (CM). Since glass has higher refractive index than air, SFS is delayed in time domain (i.e., less velocity) compare to the reflected ωV I S and ω I R , and hence, to the SFLO . Both the signals are then collimated by a lens (L3, f = 10 cm) and focused at the slit of the spectrograph using another lens (L4, f = 6 cm). The mixing of temporally separated SFS and SFLO after the dispersion in spectrograph, creates an interference pattern which is detected by a thermoelectric cooled (−70 °C) CCD detector. For the intensity and phase calibration, a similar interference pattern is recorded by replacing the sample with a z-cut quartz. The relatively tight focusing, spatiotemporal overlap (focusing at same spot and with zero-time difference between the pulses) of ωV I S and ω I R and their coplanarity are some of the critical considerations for efficient generation of SFG signal from the sample (or reference) and LO-substrate and subsequent detection of the interferogram. The height of the sample and the z-cut quartz is maintained using a laser displacement sensor with a resolution of ~100 nm. The precise control of the sample height is necessary to retain the phase information of the sample SFG relative to that of the reference quartz.

4.1 Extraction of Imχ (2) and Reχ (2) Spectra from HD-VSFG Signal The interferogram of the SFG signals detected for the sample can be expressed as, 2 I HS D−S F G ∝ r L O E S eiωt + r S(V I S)r S(I R) E L O,S 2 ∗ ∗ iωt ∝ |r L O E S |2 + r S(V I S)r S(I R) E L O,S + E S E ∗L O,S r L O r S(V I S) r S(I R) e + E ∗S E L O,S r L∗ O r S(V I S)r S(I R) e−iωt Similarly, for the quartz,

(20)

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2 I HQD−S F G ∝ r L O E Q eiωt + r Q(V I S)r Q(I R) E L O,Q 2 2 ∗ ∗ iωt ∝ r L O E Q + r Q(V I S)r Q(I R) E L O,Q + E Q E ∗L O,Q r L O r Q(V I S) r Q(I R) e + E ∗Q E L O,Q r L∗ O r Q(V I S)r Q(I R) e−iωt

(21)

where, E Q ∝ FQ χ Q(2) E V I S E I R ; E L O,Sor Q ∝ FL O χ L(2)O E V I S E I R ; and E S =   FS χ S(2) E V I S E I R eiπ/2 eiϕ . Where, r S , r Q and r L O are the reflectivity coefficients of the sample, quartz and LO surfaces, respectively. ‘F’ terms indicate the product of Fresnel factors for individual beams, i.e., SFG, VIS and IR in respective medium. In case of sample (bulk χ (2) inactive), a phase difference of π/2 is generated with respect to the reference quartz or LO iπ (bulk χ (2) active), which is taken care by e / 2 = i. An additional phase factor, eiϕ is introduced to account for the phase-difference between sample and reference SFG signal that may appear due to difference in heights following evaporation of the liquid sample. This is the most important source of phase error in HD-VSFG measurement; difference in sample and reference heights must be minimized. Figure 6a depicts the interference fringe patterns from the reference quartz and air-water interface (sample), detected by HD-VSFG method. The weak fringe from the air-water interface is due to the lower magnitude of χ (2) from the water surface compared to

Fig. 6 a Heterodyne-detected SFG signal (interference fringe) from reference quartz and water (sample) surfaces. b Conversion of frequency domain spectrum into time domain by Fourier transformation analysis and extraction of cross term at (+t) by multiplying with a suitable filter function (purple box function). c Inverse Fourier transformation of the cross term to get back the frequency domain spectra

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the quartz. The frequency domain signals (interference patterns) are converted into time domain by Fourier transformation (Fig. 6b); the signal at t = 0 represents ∗ ∗ iωt the squared terms in Eq. 20, while the cross terms, E S E ∗L O,S r L O r S(V I S) r S(I R) e and E ∗S E L O,S r L∗ O r S(V I S)r S(I R) e−iωt appear at the time +t and −t, respectively. ∗ ∗ iωt is extracted by multiThe cross-term at +t i.e., E S E ∗L O,S r Ls O r S(V I S) r S(I R) e plying with a suitable filter function (purple box function in Fig. 6b). A similar ∗ ∗ iωt is also obtained from the quartz surface. The term E Q E ∗L O,Q r L O r Q(V I S) r Q(I R) e extracted heterodyne components are transformed back to the frequency domain by inverse Fourier transformation (Fig. 6c). To calibrate the intensity and phase of complex-χ (2) , the sample interferogram is normalized by the reference interferogram as follows,

I N or m = =

∗ ∗ iωt E S E ∗L O,S r L O r S(V I S) r S(I R) e ∗ ∗ iωt E Q E ∗L O,Q r L O r Q(V I S) r Q(I R) e ∗ ∗ ∗ r S(V I S) r S(I R) E S E L O,S

∗ ∗ ∗ r Q(V I S) r Q(I R) E Q E L O,Q  ∗   

χ (2) E V I S E I R eiπ/2 eiϕ FL O χ (2) E V I S E I R ∗ ∗ r r S(V S L O F S I S) S(I R) =   ∗ ∗ ∗ r Q(V FQ I S) r Q(I R) χ Q(2) E V I S E I R FL O χ L(2)O E V I S E I R  

∗ ∗ r r FS χ S(2) S(V I S) S(I R) iϕ = ie (22) ∗ ∗ r Q(V FQ χ Q(2) I S) r Q(I R)

Therefore, χ S(2) χ Q(2)

=

χ N(2)or m

= −ie−iϕ



FQ FS



∗ ∗ r Q(V I S) r Q(I R) ∗ ∗ r S(V I S) r S(I R)

 I N or m

(23)

Ignoring the weak frequency dependence of Fresnel Factors, [18] FQ /FS can be considered as positive constant. The reflectivity terms within parenthesis (Eq. 23), regarded as reflectivity correction factor (RCF), is also a real positive quantity. In actual experiment, the RCF can be determined as, RC F =  S F G L O,Quar t z /S F G L O,Sample ), where, S F G L O,Quar t z and S F G L O,Sample are the SFG signals (from LO only) produced by reflected ωV I S and ω I R from the quartz and sample surfaces, respectively. The RCF for the air-water interface (OH stretch region) is shown in the inset of Fig. 7b. χ Q(2) , being non-resonant, is also a real constant throughout the IR frequency region and it is positive in sign. Thus, the normalized χ S(2) spectrum, more specifically the Imχ (2) and Reχ (2) spectra, can be directly obtained from Eq. 23. Plots of the ‘imaginary’ and ‘real’ components of χ N(2)or m against the frequency of the IR light (ω I R = ω S F − ωV I S ) provide the Imχ (2) and Reχ (2) spectra of the sample. Figure 7b shows such spectra of the pristine airwater interface. To have improved SNR throughout a broad spectral region, say the

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Fig. 7 OH stretch spectrum of the pristine air-water interface in the SSP polarization combination: 2 a SFG intensity i.e., χ (2) spectrum measured with narrowband classical-VSFG spectrometer (Adapted with permission from [19]. Copyright 1993 American Physical Society) and b Imχ (2) (solid black) and Reχ (2) (black dotted) spectra measured with broad band HD-VSFG spectrometer. Inset: Reflectivity correction factor (RCF) as obtained experimentally for the air-water interface. c The squared-χ (2) spectrum (blue curve) deduced from the experimental Imχ (2) and Reχ (2) spectra shown in panel ‘b’. d Pictorial presentation of ensemble averaged orientation of water at air-water interface. The 3700 cm−1 band is not well resolved in panel ‘b’ and ‘c’ due to insufficient IR power in that region

2700–3700 cm−1 region which simultaneously covers the CH and OH stretch bands, the SFG signal has been recorded at three different center frequencies of the IR pulses (e.g., 3050, 3300 and 3550 cm−1 ); the combined spectra has been analyzed relative to the combined reference spectra from a z-cut quartz. The Imχ (2) spectrum thus obtained is essentially the surface analogue of IR (Imχ (1) ) and stimulated Raman (Imχ (3) ) spectra and hence, directly comparable to the corresponding bulk spectra. Moreover, the phase-information of vibrational transitions is retained in the Imχ (2) spectrum which can provide the absolute orientation of molecules at the interface [3, 4]. In addition, the ‘heterodyne detection’ provides signal amplification (by multiplication with LO-signal) that improves the SNR of the acquired spectrum. It is important to mention that the heterodyne detection of SFG is a phasecontrolled experiment. Therefore, the optical path of the sample and quartz, specifically the distance from sample/quartz surface to the detector, should be precisely maintained. In reality, however, the height of liquid sample can change significantly (relative to solid quartz surface) due to evaporation, creating an optical path difference (i.e. phase difference eiϕ as shown in Eq. 23) during acquisition. Generally,

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the height of a liquid sample is adjusted with the reference quartz using high precision optical displacement sensor (resolution ∼ 100 nm) coupled with the motorized sample stage (z-translational stage). Classical- and HD-VSFG techniques can directly measure liquid surfaces without much of a sample preparation. For instance, to measure water surface such as the pristine air-water or air-aqueous solution interfaces, ~5 mL Milli-Q water (18.2 M cm resistivity, pH = 5.8) or aqueous solution of electrolytes/solutes of desired concentration is taken in a Petri dish of ~4 cm diameter and placed on the sample stage as shown in Figs. 4 and 5. For long chain alcohol/lipid monolayer-water interfaces, stock solution of the alcohol/lipid is prepared in chloroform; ~10 μL of the stock is spread on the water in the Petri dish, while monitoring its surface tension. On evaporation of the chloroform at ambient condition, the self-assembled monolayer of alcohol/lipid is formed on water surface. The phase of the monolayer or the degree of adsorption of a solute is reflected by the surface tension values measured in situ.

5 Applications 5.1 Air-Water Interface: Does “Ice-like” Water Exist at the Air-Water Interface? The pristine air-water interface is one of the most extensively investigated soft interface due to the omnipresence of water and its vital roles in innumerable physicochemical processes. Often, the pristine air-water interface is considered as the bench mark in understanding the effect of ions, molecules and surfactants at aqueous interface. The OH stretch spectrum of water at the air-water interface, first measured by Shen and coworkers [19] using a narrowband classical-VSFG spectrometer (Fig. 7a), shows three distinct features of interfacial water: The red region of the SFG-intensity spectrum shows a prominent band around 3250 cm−1 followed by a stronger band around 3450 cm−1 ; while the extreme blue region shows a sharp band centered around 3700 cm−1 . These spectral features have been confirmed by numerous groups subsequently [20–24]. The sharp band at 3700 cm−1 and the strong shoulder at 3250 cm−1 are interface-selective features of water. Based on the frequency position of these bands, the 3250 cm−1 band was assigned to tetrahedrally coordinated “ice-like” structure of water in analogy with the tetrahedrally coordinated OH stretch band of ice, which appears dominantly around 3200 cm−1 . The 3700 cm−1 band is assigned to the non-hydrogen bonded water O–H or free O–H protruding towards the air, specifically known as ‘dangling OH’ [19]. The 3450 cm−1 band in analogy with liquid water is attributed to the “liquid-like” structure of water at the air-water interface. Later, the Imχ (2) spectrum of the air-water interface was measured by Shen and coworkers [3, 25] with narrowband phase-sensitive-VSFG as well as by Tahara and coworkers [4] with broadband HD-VSFG spectroscopy. The Imχ (2) spectrum showed a sharp positive band around 3700 cm−1 and a negative band within 3250–3600 cm−1 ,

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followed by a positive band around 3200 cm−1 . Morita and coworkers [26, 27] simulated the Imχ (2) spectra reproducing the experimental Imχ (2) including the positive band around 3200 cm−1 . The positive sign of the 3700 cm−1 represents that the dangling OH is indeed pointed towards the air (i.e. away from the aqueous phase) while the liquid like water has a net H-down orientation (Fig. 7d). Nevertheless, the origin of the 3200 cm−1 band was debated—unlike Shen and coworkers, Morita as well as Tahara and Bonn suggested that the 3200 cm−1 positive band is due to strongly coupled water pairs at interface [28]. Very recently, Tahara and Yamaguchi [29, 30] observed that the 3200 cm−1 positive band is nearly non-existent in the Imχ (2) spectra measured with HD-VSFG spectrometer of precisely controlled optical path (using a laser displacement sensor) and air-D2 O interface (instead of z-cut quartz) as the reference. In our laboratory, we used mathematical phase-correction of the experimental χ (2) spectra by internal referencing of zero Imχ (2) signal in the region of non-resonance (say around 2700 cm−1 for the OH stretch band of water) [31, 32]. The positive band around 3200 cm−1 is not observed in our phase–corrected spectra. Recently, we have also measured the air-water interface with high resolution (~100 nm) optical displacement sensor [33, 34] and the Imχ (2) does not show any distinct positive band around 3200 cm−1 (Fig. 7b). Very recently, Tian and coworkers [35–37] as well as other groups [38, 39] reported no distinguishable band around 3200 cm−1 for the air-H2 O interface. Thus, the emerging picture of water structure at the pristine air-water interface reconcile with the fact that the interfacial water has a continuous “liquid-like” structure with a net H-down orientation in the H-bonded region of the OH stretch band (3000-3600 cm−1 ); while in the non-H-bonded region (>3600 cm−1 ), the dangling OH is pointed towards the air (H-up), which originates from the top-most water layer (Fig. 7d). Along with Imχ (2) the HD-VSFG measurement provides the Reχ (2) spectrum of the air-water interface (Fig. 7b). The Reχ (2) spectrum shows a dispersive band shape along with a negative non-resonant back ground signal embedded into it. In the region below 3000 cm−1 , where the Imχ (2) signal is zero, the Reχ (2) spectrum is negative in sign and appears as a constant due to the non-resonant background. 2 The χ (2) spectrum (Fig. 7c) deduced from the experimentally measured Imχ (2) 2 and Reχ (2) spectra is in qualitative agreement with the χ (2) measured by narrow band classical VSFG spectrometer (Fig. 7a). Unlike the Imχ (2) spectrum, the sign of the squared-χ (2) spectrum is positive throughout the OH stretch region due to squaring of χ (2) . In summary, the HD-VSFG measurement is clearly an advantage and more informative in elucidating the structure and orientation of molecules at a soft interface relative to that of its classical-analogue.

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5.2 Air-Electrolyte Solution Interface: Surface Prevalence of Structure-Making Anion Surface prevalence and interfacial distribution of ions are of paramount importance to interfacial chemistry which encompasses a wide range of fields including atmospheric aerosol to cellular process in biological systems. Previous studies, mostly surface potential, surface tension, classical-VSFG, NAP-XPS measurements as well as MD simulation revealed that weakly hydrated and highly polarizable anions such as I− and Br− are adsorbed at the water surface and the counter-ion, e.g. Na+ for NaI, follows the anion [40–46]. Such non-uniform distribution of oppositely charged ions is considered to create an electric double layer (EDL) which increase the orientational order of water at the interface. On the other hand, strongly hydrated ions (structure makers) are believed to be repelled from the water surface. However, the question remains: how does an electrolyte with structure making ions (e.g., Na2 SO4 ) affects the interfacial water? If a structure making anion (e.g., SO2− 4 ) is repelled form the surface, is it the counter ion (say, Na+ ) that resides closer to the surface? When the counter ion is also strongly hydrated (e.g., Mg2+ ), what happens to the relative positioning of the cation and anion at the interface and how do they affect the H-bonding and orientation of the interfacial water? Classical-VSFG measurement inadequately answered these questions, because even if there is an increase in squared-χ (2) intensity (OH stretch), it only reveals a non-uniform distribution of cation and anion at the interface which may lead to increased ordering of the interfacial water. However, it does not disclose whether the cation or the anion is closer to the water surface, and whether the net orientation of the interfacial water is H-up or H-down. Recently, (phase-sensitive) HD-VSFG measurements of structure making electrolytes solu2− tions seem to suggest that the structure making anions such as SO2− 4 and CO3 are more strongly repelled from the interface than the mono and bivalent metal ions (Na+ and Mg2+ ) [47, 48]. Apart from the overall structure making/breaking nature of an anion, the role of atomicity of the anion (e.g., monatomic F− versus polyatomic IO− 3; both are structure maker but their atomicity are different) on its surface prevalence and perturbation of interfacial water are important aspects of specific ion effect at the interface. Figure 8a shows the OH stretch Imχ (2) spectra of the air-water interface in the presence of IO− 3 (0.3 M KIO3 ) along with the spectrum of the pristine air-water interface. IO3 – increases the negative Imχ (2) signal in the OH stretch region, particularly in the red region, 3000–3400 cm−1 . The structure breaking monatomic anion, e.g., I– (0.3 M CsI; effect of Cs+ and K+ ions are comparable), which is preferentially adsorbed at the water surface shows an opposite change in the OH stretch band: the negative signal around 3200 cm−1 is reduced compared to neat air-water interface with a negligible increase in negative signal around 3450 cm−1 (Fig. 8b). Thus, at low concentration of I– (~0.3 M), the OH stretch band of interfacial water becomes narrower (especially by the suppression around 3200 cm−1 ) rather than an increase in the OH stretch signal even at the peak. This observation suggests insignificant change of the orientational order of interfacial water, though the structure (i.e., band

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Fig. 8 Imχ (2) spectra of the a air-water-KIO3 and b air-water-interfaces. (inset) air-waterinterfaces. Concentration of electrolyte solution is 0.3 M each. The spectrum of the neat air-water interface is shown in each panel for reference. Adapted with permission from [50]. Copyright 2019 American Chemical Society

narrowing) is perturbed, presumably due to the hydration of interfacial I– anion [49]. A comparative glance to the spectral change reveals intriguing features of specific ion effect on the interfacial water: Even though, IO3 – is a strongly hydrated anion (believed to be repelled from water surface), the magnitude of spectral change around 3200 cm−1 is comparable to that of I– , a structure breaking surface active anion. Clearly, being repelled form the top surface does not mean non-perturbing to the interfacial water. If IO3 – is repelled from the water surface more strongly than that of its counterion K+ , there could be a separation of charge (positive electric field) at the interfacial region, which may lead to a net H-down orientation of water at interfacial region increasing the negative Imχ (2) signal as observed in Fig. 8a. However, this argument does not hold, because even for the strongly surface adsorbed I- anion, the separation of charge (between I– and Cs+ ) is not strong enough to change the relative orientation of interfacial water for the same electrolyte concentration (0.3 M). Therefore, it is highly likely that the water associated with the hydration of IO3 – anion contributes predominantly to the increased negative Imχ (2) signal at the air-water-KIO3 interface. Measurement of the air-water interface in presence of F– (0.3 M; same as that IO3 – ), another “structure maker” anion having Jones-Dole viscosity B-coefficient similar to that of IO3 – (B-coefficient [51] = 0.11 for F– and 0.14 for IO3 – ) sheds light into the origin of the perturbed water at the interface. F– anion does not show noticeable change in the OH stretch spectrum (inset Fig. 8a); the structure making anions even if are repelled from the top surface are not equally perturbing to the water in the interfacial region. Particularly, the polyatomic IO3 – anion is more perturbing to the interfacial water than the monatomic F– , though both of them are structure making, have the same net charge, and similar viscosity B-coefficient value. The origin of this ion-specificity is assigned to the polyatomic nature of the anion [52]. The perturbed interfacial water (increased negative Imχ (2) signal around 3250 cm−1 in presence of IO3 – ) is predominantly associated with the hydration shell of the anion,

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as suggested by the analysis of the hydration shell water of IO3 – and I– anions using Raman multivariate curve resolution (Raman-MCR) spectroscopy [52–54]. Under ED-approximation, appearance of the anion hydration shell water in SFG signal suggests that the inversion symmetry of the hydration shell is broken at the interface, i.e. the anions are asymmetrically hydrated in the interfacial region. Given the sharply varying number density of water across the interface, asymmetric hydration of anion in terms of water number density and orientational preference along the surface normal is quite reasonable.

5.3 Small Amphiphiles at Air-Water Interface Amphiphilic small molecule has a hydrophilic group along with a hydrophobic moiety of limited number of carbon atoms (~4 to 5 carbon). In bulk water such small hydrophobic groups are accommodated through the formation of cavity created by the water molecules that are strongly interacting with each other at the surface of the small hydrophobic group, known as ‘hydrophobic hydration’. Although strong water-water interaction provides an enthalpic favour to hydrophobic hydration, the cavitation energy (i.e., energy associated with the creation of the cavity) and the reduced entropy of the hydration water go against the dissolution of the hydrophobic group. However, at aqueous interface, these small molecules get readily adsorbed by protruding their hydrophobic part towards the hydrophobic air (i.e. away from the aqueous phase) while the hydrophilic moiety remains exposed to the aqueous phase, making the molecule preferentially oriented at the interface. The preferred orientation renders differential accessibility of the hydrophobic and the hydrophilic groups from the aqueous and the gas phases. This feature gives rise to interface-specific solutesolute and solute-solvent interactions which strongly contribute to the dynamics and kinetics of surface reactions. In light of HD-VSFG measurements, preferential orientations of some environmentally and biologically relevant small amphiphiles such as acetone, dimethyl sulfoxide (DMSO), propylene carbonate (PC), trimethylamine-Noxide (TMAO) at the air-water interface and the associated change of the interfacial water structure and orientation are discussed below. Figure 9 shows the Imχ (2) spectra of the air-water interface in presence and absence of acetone, dimethyl sulfoxide (DMSO), and propylene carbonate (PC). For all the amphiphile solutions, the Imχ (2) spectra show a negative band within 2800–3000 cm−1 regions (CH stretch region), while for the neat air-water interface, the Imχ (2) signal is zero in that region. The Imχ (2) bands correspond to the methyl symmetric stretch (CH3 SS ~2920 cm−1 for DMSO and acetone and ~2876 cm−1 for PC) and Fermi resonance (CH3 FR~2925 cm−1 for PC) of the adsorbed amphiphiles. The negative sign of CH3 SS band reveals that the methyl groups of the amphiphiles are oriented towards the air (methyl-up orientation) at the air-water interface. It is important to note that, opposite to water OH-stretch, the sign of the hyperpolarizability (β ccc ) of CH3 SS is negative and hence the negative sign of Imχ (2) indicates

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Fig. 9 Imχ (2) spectra of air-water interface in presence of a DMSO b acetone and c propylene carbonate (PC). The spectrum of the neat air-water interface (black curve) is shown in each panel for comparison. The bulk concentration of each amphiphile is 1.5 M. Adapted with permission from [33]. Copyright 2020 Anita Publications

methyl-up orientation [4] which is in agreement with the hydrophobic nature of methyl group. Perturbing influence of the amphiphiles (on interfacial water) increases with increasing their hydrophobicity, DMSO < acetone < PC. At air-water-DMSO interface, the OH stretch band shape (3000–3600 cm−1 ) is quite similar to that of the neat air-water interface, but the amplitude is marginally decreased. With acetone, the reduction of the amplitude (3000–3600 cm−1 ) becomes more noticeable, suggesting increased alteration of the net orientation of interfacial water in presence of acetone. For PC which has the larger alkyl group as well as more number of ‘O’ atoms, the effect is even more substantial with the appearance of a distinct positive band around 3200 cm−1 and subsequent reduction in amplitude around 3450 cm−1 . The change in the OH stretch band clearly suggests that the interfacial water increasingly adopts a net H-up orientation in the presence of PC. Compared to DMSO and acetone, PC possesses two additional ‘O’ atoms that are H-bond acceptor. The interfacial water hydrating such O-atom via H-bond donation may adopt a preferential H-up orientation. Moreover, the differential distribution of the amphiphiles across the interface contributes to the net orientation of the interfacial water [33].

5.4 Perfluorinated Persistent Organic Pollutant (POP) at Environmentally Relevant Aqueous Interfaces Unlike degradable pollutants which have a short-term effect on the environment, persistent organic pollutants (POPs), because of their resistance to degradation, have a long-term effect on aqueous interface and the associated biotic/abiotic systems.

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Perfluorinated organic compounds such as perfluoro carboxylic acid (PFCA) is one of the emerging POPs [55]. Perfluoro compounds are anthropogenic and ocean water is their primary sink. For the long-range transport and speciation of PFCA, its interaction with aqueous interface (ocean surface) and hydrogenated amphiphiles (e.g., fatty acid, alcohol, and lipid) and ions that are usually present at ocean surface are of critical importance. Classical-VSFG measurement of perfluoroheptanoic acid (PFHA; C6 F13 COOH)-water interface in the presence of hydrogenated amphiphiles such as octanol, heptanoic acid (HA), hexadecanol and palmitic acid (PA), reveals intricate details of the surface prevalence of PFHA and its interaction with other surfactants. As can be seen in Fig. 10, the PFHA-water interface (grey circle) does not show any vibrational feature in the CH stretch region due to the absence of C–H bonds in PFHA. The hydrogenated amphiphiles, 1-octanol and heptanoic acid show their corresponding CH stretch bands in SSP and PPP polarizations (CH2 SS = 2850 cm−1 ; CH3 SS = 2880 cm−1 ; CH3 FR = 2950 cm−1 ; CH3 AS = 2970 cm−1 ) [1, 57]. In presence of PFHA, the CH2 SS of 1-octanol disappears completely while the CH3 SS stretch band intensity is reduced to 50% (red circle, Fig. 10a). Thus, the absolute intensity of CH3 SS stretch is reduced but the intensity ratio, (ICH3 SS /ICH2 SS )1/2 which is a measure of the alkyl chain order of long chain amphiphiles/surfactants is increased. This result can be rationalized as follows: in the presence of PFHA, octanol molecules are partially lost from the water surface, but the remaining molecules are more ordered than that of the neat air-water-octanol interface. In the case of HA (Fig. 10c), presence of PFHA reduces the CH2 SS and CH3 SS band intensity to a smaller extent, but the

Fig. 10 SFG-intensity spectra in CH-stretch region for octanol-water (a, b) and heptanoic acidwater (c, d) interfaces in the presence (red) and absence (black) of PFHA in SSP and PPP polarization combinations. Solid lines are the respective fitted spectra using Eq. 19. Surface pressure of octanolwater and heptanoic acid-water mixtures were 37 ± 2 and 41 ± 2 mN/m, respectively. Adapted with permission from [56]. Copyright 2020 American Chemical Society

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Fig. 11 SFG-intensity spectra in CH-stretch region for hexadecanol-water (a, b) and palmitic acid-water (c, d) interfaces in the presence (red) and absence (black) of PFHA in the SSP and PPP polarization combinations. Solid lines are the respective fitted spectra following Eq. 19. Surface pressure of hexadecanol and palmitic acid monolayer-water interfaces were 38 ± 2 and 25 ± 2 mN/m, respectively. Adapted with permission from [56]. Copyright 2020 American Chemical Society

intensity ratio, (ICH3 SS /ICH2 SS )1/2 , does not change appreciably (∼1.8 and 2.1 in the ‘absence’ and ‘presence’ of PFHA respectively). In addition, the CH3 AS measured in PPP polarization also remains unperturbed in presence of PFHA. Thus, unlike octanol, HA is not expelled from the aqueous surface in the presence of PFHA. At long chain amphiphiles such as for hexadecanol and palmitic acid monolayerwater interfaces, small intensity of the CH2 SS band compare to that of CH3 SS (Fig. 11a, c) indicates that the monolayers are inherently more ordered (absence of ‘gauche defect’) than those of 1-octanol and HA (Fig. 10a, c) [58, 59]. In presence of PFHA, the CH stretch bands of hexadecanol and PA remains unaltered in both SSP and PPP polarizations, suggesting that the surface number density and alkyl chain order remain undisturbed for those long chain surfactant monolayers. Similar experiments in the OH stretch region (results not shown here) corroborate that PFHA is actually expelled from the long chain surfactant (hexadecanol, PA)-water interfaces, while it is retained at the intermediate chain length surfactant (octanol, HA)-water interfaces and affects their alkyl chain order and interfacial water characteristics [56]. The mutual surface prevalence of PFHA and intermediate chain length hydrogenated amphiphiles is not only governed by their relative hydrophobicity, but specific-interaction between their head groups is equally crucial.

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5.5 Osmolyte and Denaturant at Air-Water Interface Osmolyte and denaturant are two opposite class of molecules. Osmolyte protects cellular integrity and native protein structures by maintaining cell volume, against changes of ambient conditions e.g. temperature, pressure, extracellular osmotic concentration etc. [60, 61]. On the other hand, denaturant destabilizes native structure of proteins and destroy cellular integrity. Trimethylamine-N-oxide (TMAO, (CH3 )3 N+ –O− ) is one such well known organic osmolyte, while an alcohol of analogous chemical structure such as tert-butanol (TBA; (CH3 )3 C–OH) is a denaturant [62]. It is largely believed that unfavorable interaction of TMAO with hydrophobic protein surface lead to its expulsion from the protein surface such that the protein surface is exposed to the water that are strongly H-bonded with the expelled-TMAO. The strongly H-bonded water at the protein surface favors the native (folded) structure of a protein, which has lower hydrophobic surface area exposed to water than that of the corresponding denatured (unfolded) state [63–65]. Denaturants, on the other hand, interact favorably with the hydrophobic protein surface and promotes the protein to increase its hydrophobic surface area, leading to the denaturation. As a result, the folding/unfolding of protein by osmolyte/denaturant is a phenomenon at aqueous interface, which requires molecular level understanding. As discussed in the Sect. 5.3, the negative CH stretch bands (CH3 FR = 2935 cm−1 for TBA and CH3 SS = 2975 cm−1 for TMAO; Fig. 12) show that both TBA and TMAO are preferentially oriented as ‘methyl-up’ at the air-water interface. The H-bonded OH stretch region (3000–3600 cm−1 ) remains largely unperturbed in presence of TBA (0.1 M); however, the amplitude of the dangling OH band (∼3710 cm−1 ) decreases from that of the neat air–water interface (compare red and black curves in Fig. 12a). As dangling OH is a characteristic feature of topmost water, a reduction in its amplitude signifies dominant effect of TBA on the topmost water layer due its adsorption on water surface. In the case of TMAO (3.0 M), the amplitude of the dangling OH is largely unperturbed (compare red and black curve in Fig. 12b), suggesting that the topmost water layer is largely unaffected by TMAO, even when present at significantly higher concentration than that of TBA (0.1 M) in the bulk water. Interestingly, in the H-bonded OH stretch region, the Imχ (2) signal is enhanced around 3250 cm−1 . An enhancement in the red region of the OH stretch spectrum is due to strong Hbonding of interfacial water that are associated with TMAO. Altogether, the spectral features in the CH and OH stretch regions suggest preferential accumulation of TBA on water surface while TMAO is depleted from the top surface but do reside in the interfacial region (just below the top surface) while both of them exhibit preference for the methyl-up orientation (Fig. 12c, d). Following the fact that air is hydrophobic, the air-water interface can be considered as a proxy to the hydrophobic protein-water interface. In that sense, preferential accumulation of TBA at the air-water interface suggests the propensity of TBA to be adsorbed at hydrophobic protein surface. TMAO on the other hand, is expected to be repelled from the protein surface. Such expulsion/adsorption of osmolyte/denaturant is fundamental to the folding/unfolding of proteins in water.

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Fig. 12 a air-water-TBA interface ([TBA] = 0.1 mol L−1 ); and b air-water-TMAO interface ([TMAO] = 3.0 mol L−1 ). The spectrum of the neat air-water interface (black curve) is also shown in each panel. c, d depict the schematic of the air-water interface in presence of TBA and TMAO, respectively. Adapted with permission from [31]. Copyright 2016 American Chemical Society

5.6 Osmolyte (TMAO) at Phospholipid Monolayer-Water Interface TMAO not only has beneficial effect as an osmolyte; recent studies have suggested adverse effect of TMAO on human health, specifically it increases the risk of cardiovascular diseases (CVD), presumably by promoting fat deposition at the inner wall of artery [66, 67]. In human, TMAO is produced from its precursor trimethylamine (TMA), which is derived from the bacterial degradation of choline containing food (e.g., egg, red meat). This increases the TMAO level in blood. Although, how does TMAO promote the deposition of fat in the inner wall of artery is largely unknown, it is observed that presence of TMAO creates electrical disturbance at the endothelial membrane forming lipid monolayer-water interface.

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Fig. 13 Imχ (2) spectrum of the DPPC lipid monolayer-water interface in presence (green) and absence (black) of TMAO. Bulk concentration of TMAO was 2.0 M. Surface pressure of the lipid monolayer was 30 ± 2 mN/m. Adapted with permission from [68]. Copyright 2016 American Chemical Society

As can be seen in Fig. 13, zwitterionic phospholipid (1,2-dipalmitoyl-sn-glycero3-phosphocholine; DPPC) monolayer-water interface (black curve) shows a positive OH stretch band with an apparent peak round 3200 cm−1 followed by a dip feature at 3450 cm−1 and a weak positive band around 3600 cm−1 , corresponding to the distinct hydration structures of the anionic phosphate, cationic choline and the hydrophobic glycerol region of the lipid [69–71]. The negative signal in the CH stretch region (2800–3000 cm−1 ) reveals the expected methyl-up orientation of the lipid alkyl chains. Interaction of TMAO (2.0 M) with the DPPC monolayer-water interface increases the H-up oriented interfacial water signal around 3000-3550 cm−1 (green curve) corresponding to the increased population of H-up oriented water by a negative surface field. While, the high frequency region of the OH stretch spectrum (~3600 cm−1 ) remains unchanged, suggesting that the hydrophobic glycerol region of lipid remains largely unaffected in presence of TMAO. This is quite contrasting to the behavior of TMAO at the pristine air-water interface (as discussed in previous section, Fig. 12b), where it is depleted from the interface leading to mild increase in the negative Imχ (2) signal around 3300 cm−1 . The unusual increase in H-up orientation of water in presence of TMAO at the zwitterionic DPPC-water interface is due to preferential interaction of TMAO with the cationic choline moiety of the lipid headgroup. Specifically, the positively charged choline group is screened more effectively by the negatively charged oxygen of TMAO than the corresponding screening of the negatively charged phosphate of DPPC by the cationic trimethyl ammonium group of TMAO. As a result, the DPPC-lipid water interface effectively becomes a net negative from the perspective of the interfacial water. This result suggests that an elevated level of TMAO in circulating blood has the potential to increase the dominance of

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phosphate group of DPPC in endothelial membrane surface, perturbing the subsequent receptor-ligand electrostatic interaction that regulated deposition/removal of fat on the inner wall of artery.

6 Summary Vibrational sum frequency generation (VSFG) spectroscopy is an inherently interface-selective technique applicable at ambient condition to variety of surfaces and interfaces that are accessible to light. VSFG measurements provided a deeper insight into the molecular level structure and orientation of water at the air-water interface - the simplest soft interface, albeit sufficiently complex to capture the molecular information by using conventional methods such as linear optical spectroscopy and scattering based techniques. Despite the interface-selectivity, classical-VSFG spectroscopy as it detects the SFG-intensity i.e. square modulus of χ (2) , does not directly provide the absolute orientation of interfacial molecules. Moreover, for relatively weak signal the spectral band-shape may deform form the true absorption spectrum. Heterodyne-detection of VSFG signal (HD-VSFG) overcomes the shortcomings of classical-VSFG and utilize the full potential of sum frequency generation by independently providing the Im- and Reχ (2) spectra. The Imχ (2) spectrum reveals the accurate absorption characteristics of interfacial molecules and its sign shows the absolute orientation of interfacial molecules or molecular groups. HD-VSFG through its unique advantages of the retention of sign of Imχ (2) and signal amplification reveals the perturbation of aqueous interface by ions and small molecules, especially by the structure making anions and osmolytes which are repelled form the topmost water layer are uniquely captured by HD-VSFG measurement. Narrow band classical-VSFG, because of its readily achievable high spectral resolution (~6 cm−1 ), reveal the alkyl chain conformation of adsorbed surfactant by monitoring its sharp CH-stretch bands. The combined response of CH and OH stretch region from the POP-water, surfactant-water and surfactant-POP-water interfaces shed light into the surface prevalence and mutual interaction of POP at atmospherically relevant interfaces.

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Broadband Terahertz Spectroscopy Sneha Banerjee, Gurivireddy Yettapu, Sohini Sarkar, and Pankaj Mandal

Abstract Terahertz (THz) spectroscopy is a non-contact technique to probe properties and dynamics of molecules and materials in the meV energy range and picosecond timescales. This chapter describes different popular methods of generation and detection of broadband THz pulses, experimental technique, and data analysis procedures of THz time-domain and time-resolved THz spectroscopy. We have also reviewed some recent representative works utilizing these methods. Keywords Terahertz spectroscopy · THz-TDS · TRTS · Picosecond dynamics

1 Introduction Terahertz (THz) radiation occupies the region between the microwave (10 THz) of the electromagnetic spectrum (Fig. 1). The term ‘terahertz’ became popular only in the mid-1970s when it was used by spectroscopists to describe the region below the far-infrared [1, 2]. Today, THz spectroscopy roughly spans from 0.1 to 20 THz, where 1 THz is equivalent to a wavelength of 300 μm, 33.3 cm−1 in wavenumber, and 4.14 meV in energy. The THz spectral range bridges the so-called “THz gap” in the electromagnetic spectrum between the electronics region, which can be described classically, and the photonics, where the quantum nature of light comes into play. Terahertz radiation is abundant in our universe. Most objects emit THz radiation above 10 K as part of black body radiation [3], but the emitted waves from these sources are feeble and go unnoticed. Initially, THz technology was used by chemists, astronomers, earth, planetary, and space scientists to characterize rotational and vibrational resonances, measure, and map the thermal emission lines of S. Banerjee · G. Yettapu · P. Mandal (B) Department of Chemistry, Indian Institute of Science Research and Education, Pune, Maharashtra 411008, India e-mail: [email protected] S. Sarkar Department of Chemistry, University of Southern California, Los Angeles, CA 90089-0482, USA © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 D. K. Singh et al. (eds.), Modern Techniques of Spectroscopy, Progress in Optical Science and Photonics 13, https://doi.org/10.1007/978-981-33-6084-6_5

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Fig. 1 THz range in the electromagnetic spectrum. Possible applications of THz technology

simple molecules [4]. The past 25–30 years have seen an upsurge in the development of THz spectroscopy fueled by the availability of ultrafast lasers. Unlike in other conventional spectroscopy techniques, where the intensity of light is recorded at specific frequencies, THz time-domain spectroscopy (THz-TDS), the most popular form of THz spectroscopy, measures the electric field of the THz pulse as a function of time. A simple Fourier transformation of the time domain data would resolve the amplitude and phase of the spectral components. The amplitude and the relative phase provide the absorption coefficient and the refractive index of the sample in one measurement. Hence one can calculate the complex-valued permittivity without calling for the complex Kramer-Kronig analysis over an extensive frequency range [5]. Terahertz spectroscopy has immense potential of probing numerous physical, chemical, and biological processes occurring in the picosecond timescale and with energies in meV (Fig. 1). Examples of such systems would range from intra and intermolecular vibrations in molecules and molecular assemblies, excitons, bound electrical charges, lattice vibrations in crystalline solids, and charge plasma to relaxation dynamics in liquids and biomolecules [6]. It is also possible to map out the temporal evolution of the optical response function in the sub-picosecond and the picosecond timescales. THz measurements of materials are also possible under extreme conditions of temperature, electric, and magnetic fields. Since many optically opaque materials such as plastic, paper, leather, and wood are transparent to the THz light, non-destructive inspection of mail envelopes at post offices and luggage at airports is possible [7, 8]. THz light’s non-invasive and non-ionizing nature offers potential

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applications in detecting explosives and drugs, biomedical imaging, and as body scanners [9]. Today, the research in THz spectroscopy has grown so diverse that it is not possible to include all aspects in one review article. The major areas of current works can be classified into four categories: (1) developing/improving the THz source, detector, and optics; (2) applying time-domain and time-resolved THz spectroscopy to molecules and materials; (3) THz imaging, and (4) non-linear THz spectroscopy. The scope of this review is limited to pulsed broadband THz spectroscopy using tabletop emitters and detectors based on femtosecond lasers. For other aspects of THz spectroscopy, we recommend the readers to refer to several books and review articles published recently and the reference therein [3, 5, 10–15]. Section 2 discusses the popular and effective ways of ultrafast laser-based generation and detection of broadband THz pulse. Here the focus will be more on using air-plasma-based techniques because of their advantages over the other methods. Next, in Sect. 3, THz time-domain spectroscopy (THz-TDS) and time-resolved THz spectroscopy (TRTS) are presented in detail. In Sect. 4, we discuss some recent representative applications of broadband THz spectroscopy.

2 Generation and Detection of Broadband THz Pulse One of the main research interests of THz science and technology is to improve the ways of generation and detection of broadband THz radiation. The oldest method is the use of photoconductive antennas for generating and detecting THz pulses [16, 17]. Non-resonant processes such as optical rectification (OR) and the linear electro-optic (EO) sampling are also widely used for broadband THz generation and detection [18–20]. Recently, air-photonics have been utilized for generating intense ultra-broadband THz light and its coherent detection [21–23]. The ultrafast laser mediated tabletop methods for the generation and detection of broadband THz pulse discussed in this review are based on a generic scheme shown in Fig. 2. A typical THz spectroscopy setup would consist of an ultrafast laser, a THz source (emitter) and detector (receiver), and elements (THz optics) to modulate the THz radiation from the source to the detector. The ultrafast laser (pulse width 5 μJ per pulse has also been generated using this method [42]. However, an ultrafast amplifier is required as the primary light source for this method of generation and detection of THz. In a typical scheme of THz generation from air plasma (shown in Fig. 5), the fundamental beam (800 nm in the case of Ti–Sapphire based amplifier) from the amplifier and its second harmonic are focused in the air or any other gas, causing it to ionize at the focus. The plasma acts as the non-linear medium that emits THz radiation [43]. Several parameters, such as the gas used for ionization, external dc bias, and the phase, polarization, energy, power, and duration of the optical excitation pulse, have been found to affect the emission. Generation of THz radiation by focusing an intense laser beam into the air was first demonstrated in the early 90s [44, 45]. However, the efficiency using a single frequency light was not significant. An intense THz field was realized using both the fundamental and the second harmonic together to create the plasma [43, 46, 47]. Two models are used to explain the generation of THz radiation from ambient air. The first is called the four-wave mixing (FWM) model [21, 43], according to which the generation is a third-order non-linear process, involving the mixing of the four waves THz, 2ω, − ω, − ω, which is represented as

Fig. 5 Generation and coherent detection of THz radiation from air plasma

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THz = (2ω + THz − ω − ω)

(2)

where THz, ω, and 2ω, represent the angular frequencies of the THz, laser fundamental, and the second harmonic photons, respectively. The difference between a photon of energy (2ω + THz ) and the sum of energies of two photons (ω) yields THz radiation. The THz field amplitude is proportional to the intensity of the fundamental (ω) and the square root of the second harmonic (2ω) beams. The optimal THz output is obtained when the fundamental, the second harmonic, and the THz wave have parallel polarization. According to the asymmetric transient current (ATC) model [42, 48], an intense laser field causes suppression of the Coulomb barrier leading to tunnel ionization of electrons from the atoms or the molecules of gases present in the air. Thus, THz radiation is emitted from a non-diminishing transverse photocurrent via Maxwell’s equations. In the presence of fundamental and second harmonic beam where the laser field symmetry is perturbed, asymmetric current results in THz emission. It was demonstrated that both four-wave mixing and the plasma current contribute to the generation process [49]. Broadband detection of THz radiation was first realized in 2006 [22]. As shown in Fig. 5, the THz field induces a second harmonic (TFISH) of the gate beam through a third-order non-linear process. We can explain the second harmonic generation as (3) ETHz 2ω ∝ χ Eω Eω ETHz

(3)

T Hz where χ (3) is the third-order non-linear susceptibility, E 2ω , E ω and E T H z are the T Hz ∝ electric field amplitude of the 2ω, ω, and THz waves, respectively. Here, E 2ω E T H z , and the intensity of the second harmonic is proportional to the intensity of the T Hz ∝ IT H z ). Hence, the phase information, in this case, is not recovered, THz field (I2ω making the measurement incoherent. The introduction of a local oscillator (LO) at the plasma (overlap) point turns the THz detection to be heterodyned and phasesensitive [50]. An AC external bias applied across a pair of electrodes kept at the plasma point can act as the LO. This heterodyned technique has been termed as the ‘air biased coherent detection’ (ABCD) [51]. Including the local oscillator, the second harmonic electric field is given by the equation

  LO E2ω ∝ χ (3) Eω Eω ETHz 2ω + E2ω

(4)

and the second harmonic intensity has the form,   (3) 2 2  THz 2  LO 2 THz LO ∝ (E2ω ) ∝ χ Iω E2ω + 2E2ω E2ω cosφ + E2ω 

I2ω

2

(5)

where ELO is the electric field supplied by the electrodes and ϕ is the phase T Hz LO and E 2ω . The above equation can be written as difference between the E 2ω

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Fig. 6 THz time-domain waveform and frequency domain amplitude spectrum as obtained from our spectrometer using a ~50 fs NIR (800 nm) pump pulse

2   L O 2 LO E T H z cos ϕ + E 2ω (E 2ω )2 = χ (3) Iω IT H z + 2χ (3) Iω E 2ω

(6)

The first term in the above equation is proportional to the intensity of the THz wave. T Hz LO and E 2ω , is proportional The second term, a cross-correlation term between E 2ω to E T H z , and is the critical component for coherent THz detection. The third term is the DC contribution from LO, and a lock-in amplifier locked to the modulating frequency of the AC bias eliminates that. Introducing a local oscillator enhances the detection sensitivity significantly. Hence, even a low gate beam intensity (insufficient for producing plasma) is adequate for ABCD. Also, using ABCD, a broad THz spectral bandwidth can be detected (Fig. 6).

2.4 Generation of Intense THz Pulses The most standard form of THz spectroscopy utilizes THz radiation to probe the nonperturbed system. High energy THz pulses are not a requirement for these experiments. In recent years, however, the generation of intense THz pulses has also generated much interest in studying the response of these low-energy excitations on non-linear processes in different systems. One of the obvious methods to scale up the generated THz field is to use high fluences using the non-linear media for generation. However, this is limited by the two-photon absorption in the material; the photo-carriers generated screen the THz field generated, reducing the conversion efficiency. A workaround would be to use a bigger spot size of the THz pump so that the effective fluence is reduced while maintaining the total energy [26]. ZnTe crystals of length 75 mm have been used to generate 1.5 μJ energy pulses centered at 0.6 THz, using optical pulses of 48 mJ for excitation [35]. 30 μJ pulses centered at 0.6 THz have also been generated from magnesium-doped lithium niobate (Mg: LiNbO3 ) using a 28 mJ optical excitation pulse [52]. One of the short-comings

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of the THz fields generated using these processes is their reduced focus abilities, making it difficult to reach peak powers above 100 kV/cm [53]. Higher frequencies beams generated using laser-induced plasmas, having a spectrum of 1–7 THz, have shown to reach amplitudes of 400 kV/cm with only 30 nJ pulse energy [47]. Photoconductive switches have also been used to scale up the THz energies by increasing both the excitation fluence and the external bias field. An average power of 20 W of the emitted THz has been reported using freeelectron lasers [53]. Recently, quantum cascade lasers (QCL) have gained popularity; QCLs generally use conduction band transitions in semiconductor heterostructures to achieve laser emission [54]. They offer CW and pulsed mode operations in the mid-infrared regime at room temperature [55], which has been utilized to produce high power, continuous-wave coherent radiation in the THz frequency range [54, 56, 57]. A major limitation of using QCLs is that they operate at low temperatures. The highest cooling temperature used in THz QCL is 178 K. To introduce cryogenic cooling in room temperature QCLs in THz devices is considered to be a major drawback.

3 Time-Domain and Time-Resolved THz Spectroscopy 3.1 Experimental Setup Figure 7 shows the schematic of a THz spectroscopy setup capable of performing both THz-TDS and TRTS [58]. Here, the THz generation and detection are done by creating a laser-induced plasma in ambient air. Other methods of THz generation and detection can also be implemented in this setup with minimum alteration. The primary optical source is a Ti: Sapphire amplified laser with a central wavelength of 800 nm, 50 fs pulse duration, and a repetition rate of 1 KHz. The laser output is split using a beam splitter. One part is used for the generation and detection of THz, and the other part is directed to the optical parametric amplifier (OPA), where it is used to generate optical pulses of different wavelengths for TRTS. The beam directed towards the THz setup is again split into two using a pellicle beam splitter (R: T = 8:92), and the transmitted part is used as the pump pulse for THz generation. The reflected beam is used as the gate beam to map the THz waveform using ABCD. The 800 nm fundamental pump beam (ω) is incident on a type-I BBO (β barium borate) crystal of 100 μm thickness; this generates the second harmonic (2ω) at 400 nm. The fundamental and the second harmonic is focused in the ambient air to create an intense plasma. The plasma generates a wide frequency range of electromagnetic radiation along with THz light. A high resistivity silicon filter is placed just after the plasma, which allows the transmission of only the THz radiation. The THz is then collimated using the first pair of off-axis parabolic mirrors and focused onto the sample; the transmitted beam through the sample is again re-collimated by another couple of parabolic mirrors and focused between a pair of electrodes for detection.

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Fig. 7 Experimental setup for Time-domain and time-resolved THz spectroscopy using airphotonics for THz generation and detection

The gate beam is routed through a delay line (Delay 1) and focuses on the same spot between the electrodes. A high voltage modulator (HVM), modulated at 500 Hz and synchronized with the laser repetition rate, applies an AC bias of 1.5 kV to the electrodes, which acts as the LO for ABCD. The second harmonic of the gate pulse generated is filtered out and detected by a photomultiplier tube (PMT). The second harmonic signal is proportional to the THz electric field. For effective detection, the spatial and the temporal overlap between the THz field and the gate beam are of utmost importance. A current preamplifier (CA in Fig. 7) amplifies the PMT output and converts it into a slowly varying voltage signal. This signal is then detected via a lock-in amplifier (LA in Fig. 7), locked at the frequency of the LO. The amplitude obtained is proportional to the THz electric field. The delay between the pump and the gate beam (Delay 1) is scanned to record the entire THz waveform. Fourier transformation of the time-domain signal gives us the complex frequency domain spectrum. For the time-resolved THz study, an ultrafast pump beam (generated from the OPA in this case) excites the sample before the arrival of a THz probe pulse. The delay between the optical pump and THz probe beam is varied by scanning the Delay 3. The pump beam is chopped using a mechanical chopper. The pump beam passes through a hole in the off-axis parabolic mirror (PM2) and becomes collinear with the THz beam. A black polyethylene (transparent to the THz probe) sheet can be used to block the pump light from entering the PMT. To avoid THz absorption by water vapour, the THz path needs to be enclosed and purged continuously with N2 or dry air.

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3.2 Terahertz Time-Domain Spectroscopy (THz-TDS) In a typical THz-TDS experiment in transmission geometry, a reference signal (E ref , without the sample) is collected first by scanning the Delay 1. If the sample is a freestanding solid sample, the E ref is the THz waveform of the source, detected having nothing at the sample position. On the other hand, if the sample is a liquid or gas, the reference signal is the THz waveform transmitted through the empty sample cell kept at the same position at identical condition. Next, the THz waveform transmitted through the sample (E sam ) is collected. By taking the ratio of the complex Fourier transformation of E sam (t) and Er e f (t), the complex refractive index of the sample can be obtained as [59], f E sam (t) E˜ sam (ω)  = = T (ω) exp(iϕ(ω)), f Er e f (t) E˜ r e f (ω)

(7)

where T (ω) is the power transmittance and ϕ(ω) is the relative phase (Fig. 8).

Fig. 8 a Time-domain THz waveforms transmitted through an empty sample cell and cells filled with alcohol samples exhibit the change in amplitude and phase. b The corresponding amplitude spectra of the waveforms in (b). Optical parameters absorption coefficients (c) and refractive indices (d) determined from the THz waveforms in (a) following the method described in the text. Adapted with permission from Ref. [60]. Copyright 2017 Elsevier

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The Fresnel reflection and transmission losses should be considered while extracting the complex refractive index (n(ω) ˜ = n r e (ω) + in im (ω)) from T (ω) and ϕ(ω) using an iterative method. Further, using k (extinction coefficient) and n (refractive index), other optical constants such as dielectric function, conductivity, etc. can be calculated. For opaque materials or samples with very strong THz absorption, THz-TDS in reflection geometry is generally preferred. Similar to the transmission geometry, the complex reflection coefficient can be measured using the expression [61], r (ω) =

E sample (ω) = r (ω)eiθ(ω) Er e f er ence (ω)

(8)

where  is the change in phase of the reflected electric field of the incident THz beam. A significant problem in the reflection geometry is that the reference reflection (generally from a metal such as aluminum) should not have any offset compared to the sample position. Maintaining this is difficult in this geometry, because even the slightest offset brings about a change in the optical path length, which results in the shift of the THz time delay and leads to erroneous reflection spectrum [62, 63]. Several experimental methods attempted to address the phase determination issues. To avoid the position errors between sample and reference, specific sample properties were utilized [64]. The S and P polarized THz waves reflected from the sample were used to extract the complex functions [65, 66]. Other methods included attaching a slab in front of the sample surface, but this has its problem of contact with the sample [59]. Even with these difficulties, the spectral bandwidth and the signal obtained for materials having a high absorbance is better in the reflection geometry.

3.3 Optical Pump THz Probe Spectroscopy In TRTS, also known as the optical pump-THz probe (OPTP) experiment, an optical pump excites the sample, and the THz radiation probes the temporal evolution of the excited sample. In this case, one needs to record the THz transmissions at pump-on and pump-off conditions. The pump-induced change in THz transmission (− E(t p )) can be collected directly by modulating the pump beam and using a lock-in amplifier referenced to the pump-modulation rate. However, the effect of laser fluctuation is minimum if a double lock-in technique is used where the pump-induced change (pump-on) in THz transmission (− E(t p )) and the corresponding THz transmission through the non-photoexcited (pump-off) sample (E 0 (t p )) are recorded simultaneously [67]. The signal from the current preamplifier is split and sent to two separate lock-in amplifiers. The THz probe beam is modulated at 500 Hz, and this frequency acts as the reference for one lock-in amplifier. The reference frequency for the second lock-in amplifier is the frequency at which the optical pump beam is modulated. To minimize the crosstalk between the signals, the modulation frequencies are chosen such that they are not harmonic to each other.

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The TRTS experiment is performed in two ways. (a) Frequency averaged experiments are carried out by varying the delay between the optical pump and THz probe by scanning Delay 3 after fixing Delay 1 or Delay 2at the peak  THz field (tmax ). The photoinduced change in THz field amplitude, E tmax , t p , is recorded as a function of delay between the THz probe and the optical pump pulses, t p . This method provides a frequency averaged response of the photoexcited sample as a function of pump-probe delay. (b) In the frequency-resolved scan, Delay 2 is scanned at a fixed pump-probe delay (Delay 3). The recorded  signal in this scan is the pump induced  changes in the THz waveform, E t, t p , at a fixed pump-probe delay (t p ). A repeat of this step at several different pump-probe delays will provide the temporal evolution of the entire THz spectrum of the photoexcited sample. Use of Delay 3 ensures that the whole THz probe pulse experience the same pump-probe delay (Fig. 9). The transient photoconductivity ( σ) of liquid samples placed between the two windows, under the condition that n 1). However, one must be careful in deciding if the thin-film approximation is applicable or not for a specific experiment. The complex transmittance is given by equation [70]: T ∗ (ω) = |T (ω)|eiϕ(ω) =

1 + ns 1 + n s + Z 0 σ ∗ (ω)d f

(11)

where Z0 = 376.7 is the impedance of free space. From the above equation, complex conductivity (σ*) can be analytically solved: 1 + ns σ (ω) = Z0d ∗



sinϕ(ω) cosϕ(ω) −1−i |T (ω)| |T (ω)|

 (12)

3.4 Effective Medium Theory Often THz-TDS and OPTP experiments measure macroscopic permittivity/conductivity of composite systems, particles embedded in host materials, as shown in Fig. 10. In such a scenario, one needs to use an appropriate effective medium theory (EMT) approach to determine the intrinsic dielectric response of the particles of interest. We often encounter this issue especially for studying nanocrystals (NCs) in colloidal dispersion [58]. The scattering effects can be neglected if the particle size is significantly smaller than the wavelength of THz probe light (1 THz = 300 μm) [71]. There are several EMTs in the literature [71, 72]. The most commonly used ones are the Maxwell–Garnett (MG) and the Bruggemann approximation [73, 74]. In MG theory, the experimentally observed effective dielectric function (ε) is related to the dielectric functions of the host (εh ) and the inclusion (ε P ) materials as: ε − εh ε P − εh = f ε + K εh ε P + K εh

(13)

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Fig. 10 The effective dielectric function of NC composite (ε), scattering effects are negligible as the wavelength of THz is larger than the size of NC. The right panel shows the impact of the geometry factor in Maxwell–Garnett EMT

where f is the volume fraction of the inclusions, and K is the geometry factor. This also can be rearranged for inclusions as  εP =

f εh ε + K εεh − K εh2 + f K εh2 ( f K ε + f ε + εh − ε)

 (14)

MG theory has its own limitations. It is not applicable when there is charge transport occurring between the NCs. However, it is unlikely that charge transport will take place in NCs because the NCs are capped with insulating capping ligands and are surrounded by non-conducting host medium. It is also not applicable for concentrated solutions. In case of large filling fractions and large dielectric contrast between the constituents, the Bruggemann theory is applicable. In Bruggeman theory, the experimentally observed effective dielectric function (ε) is related to the dielectric functions of the host (εh ) and the inclusion (ε P ) materials as f

εh − ε εP − ε = ( f − 1) εP + K ε εh + K ε

(15)

The above Eq. (15) can be rearranged for nanoparticles as  εP =

f εh ε + K f εεh − K εεh + K ε2 ( f K ε + f ε + εh − ε)

 (16)

Here f is the volume fraction, and K is the geometric factor. For spherical particles, K = 2 and K = 1 for long cylinders whose axis is perpendicular to the THz electric field.

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4 Applications of Broadband Terahertz Spectroscopy 4.1 Application of THz-TDS THz spectroscopy gained popularity because of its broad bandwidth, sub-ps temporal resolution, and the ability to simultaneously obtain the amplitude and phase information, making it a better alternative to FTIR. The complex dielectric spectrum can be obtained in all three phases, solids, liquids, and gases utilizing either transmission or reflection geometries. One of the most studied systems using THz-TDS is water vapor [31] and liquid water, along with water mixtures at these low frequencies [75, 76]. Formation and melting of solid phases of water like ice and clathrate hydrates have also been studied using THz absorption spectroscopy. The solid phases have a distinct hydrogen bond network, which might also be present in supercooled water and the hydration shell around a hydrophobe. A temperature-dependent THz/far-infrared absorption spectroscopy reveals the similarity of the hydrophobic hydration of the aqueous alcohol solution with the structure of both supercooled water and ice [77]. THz spectroscopic studies of ices in molecular clouds are essential to understand the mechanism behind the evolution of stars and planetary systems. One of the significant difficulties in probing molecular clouds is to estimate the amount of gases present in the cloud. THz-TDS provides accurate measurements of the dielectric properties of ice without any edge distortions that come with the Kramer-Kronig analysis in the THz frequencies. The dielectric spectrum of ice has information about its intermolecular modes, which gives unique information about the structure of ice. THz-TDS also makes possible the study of other ice analogs, like CO, CO2 , and CH3 OH, present in these atmospheric clouds [78, 79]. THz-TDS has been used to measure the complex dielectric properties of atmospheric pressure plasma jets. Plasma can be envisioned as a quasi-neutral gas made up of charged and neutral particles that exhibit collective properties. The electron density of the plasma has been calculated from the complex refractive index. A comparison of the results shows a linear increase of electron density with the increase of discharge voltage. Helium has been found to perform lower than neon at the same discharge voltage due to its higher ionization potential [80]. THz time-domain spectroscopy has been used as a powerful probe to understand the ionic conduction mechanism in solids. It has been used to characterize solid electrolytes, like zirconia, to improve the fuel cells and solid-state batteries. The THz response obtained has been identified as vacancy hopping to adjacent sites. A temperature-dependent steady-state study gives information on the intrinsic energy of migration of these vacancies [81]. Metal–organic frameworks (MOFs) have emerged as an important class of functional materials due to their intrinsic structural and mechanical properties, microporosity, and large surface areas. Identifying the low-frequency vibrational modes of these MOFs is essential as these modes play a crucial role in enzymatic catalysis and ligand binding. THz-TDS has been used extensively to study the low-frequency

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spectra of these materials [82–84]. Along with THz-TDS, time-resolved terahertz spectroscopy (TRTS) has also been utilized to measure the photoconductivity of MOFs accurately [85]. Phase transitions in molecular crystals can also be explored using THz-TDS. Generally, phase transitions in molecular crystals occur via a nucleation and growth mechanism. However, in DL-norvaline, this mechanism is Martensitic, where the lattice does not undergo random reorganization but shows a memory effect. Temperature-dependent THz spectroscopy of DL- norvaline shows a remarkable change in the absorption spectra at low temperatures, suggesting a phase transition. More information on the dynamics of this transition is obtained by changing the grain size and doping the crystal with similar amino acids [86] (Fig. 11). Water solvation of gold nanoparticles has also been probed using THz spectroscopy. Gold nanoparticles have been used as electrocatalysts for oxygen reduction, carbon dioxide reduction, and alcohol oxidation. Mid IR (MIR) and THz spectroscopy have been used to explore the entire range of intramolecular and intermolecular dynamics of liquid water in the presence of charged and neutral gold nanoparticles [87]. A large number of aqueous salt solutions have also been studied using THz-TDS. In a recent study utilizing THz-FTIR spectroscopy, the resonances for anions have been identified as the rattling motion of the ions inside their hydration

Fig. 11 The measured absorption coefficient of DL-norvaline. The absorption spectrum changes dramatically between 190 and 180 K, indicating that the crystal lattice changes at this temperature. The absorption increases with decreasing temperature and reaches a maximum at 140 K. The absorption then decreases as the temperature is further decreased. Adapted with permission from Ref. [86]. Copyright 2018 American Chemical Society

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water cages. The response obtained for hydration water was also found to correlate well with the Hofmeister series for both the cations and anions [88]. Steady-state THz spectroscopy has also been utilized to explore the low-frequency dynamics of various peptides and proteins. Due to the low energy of THz radiation, THz-TDS is sensitive to the low-frequency motions of these molecules. Also, since most local conformational changes in proteins occur in picosecond timescales, THz-TDS becomes an important tool to probe these dynamics. THz-TDS, along with DFT calculations, have successfully identified various modes in the absorption spectra of L-carnosine, a dipeptide naturally occurring in muscles [89]. The THz-TDS spectra of six different tetrameric peptides show that this technique is sensitive to the differences in the primary and secondary structures of these peptides [90]. Terahertz absorption spectroscopy, in combination with molecular dynamics simulations, has also been used to understand the molecular mechanism behind the antifreeze activity of antifreeze protein III [91]. THz spectroscopy has been used extensively to develop new medical techniques to examine different biological tissues and organs. It has been used to determine the water content in various biological samples. A 2018 study of the THz absorption coefficient spectra of 70 human blood samples shows that the spectra are sensitive to the blood glucose level and follow a linear relationship [92].

4.2 Application of Time-Resolved THz Spectroscopy Time-resolved THz spectroscopy offers the unique advantages of monitoring dynamics from sub-picosecond to nanosecond timescales of excited states of matter that respond to optical field oscillating in THz frequency with photon energies in the range of 10–400 cm−1 . Molecular rotations, intra, and intermolecular vibrations, charge carriers in metals and semiconductors, lattice vibrations in any single crystal or crystallites are some of the entities that can interact with THz light. Hence, in principle, time-resolved THz spectroscopy can monitor the properties of matter and their temporal evolution at the ultrafast timescale. There is no limitation in terms of the physical state of the system to be evaluated. This technique has already been established as a non-contact probe for ac-conductivity and has been utilized to study semiconductors (bulk and nanoparticles), superconductors, and many other composite materials. Here, we discuss some recent representative applications of TRTS in semiconductors and 2D materials. Lead halide perovskites (LHP) have garnered unprecedented attention in the last six years due to their incredible improvement in solar cell efficiency (~25.5%) [93] within a very short span. LHPs are synthesized using simple wet chemistry, yet they exhibit properties that can be expected only from extremely pure expensive semiconductors such as silicon and GaAs. Time-resolved THz spectroscopy has played a significant role in understanding the fundamental physics responsible for its remarkable showcase of properties [94–96]. The charge carrier dynamics and important semiconductor properties, such as carrier mobility, diffusion length, have

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been evaluated using TRTS in several LHPs in the polycrystalline thin film and nanocrystalline states [58, 97–99]. A recent TRTS result on a mixed cation and mixed halide perovskite FA0.85 Cs0.15 Pb(I0.97 Br0.03 ) at low temperature is shown in Fig. 12 [100]. Here TRTS could resolve the interaction between charge carriers and the low energy phonons in real-time and their contribution in modulating carrier

Fig. 12 a THz-TDS data of the photo-excited perovskite thin-film. b Real photoconductivity as obtained from the difference between the excited and the equilibrium THz conductivity. The c real and d imaginary part of THz photoconductivity at different pump-probe delays, showing the contributions from photogenerated charge carriers and phonons. Adapted with permission from Ref. [101]. Copyright 2018 American Chemical Society

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transport properties. Also, TRTS played a significant role in understanding the large polarons formation that determines the modest mobility values in LHPs [101, 102]. Graphene is the most celebrated two-dimensional (2-D) material, a single layer of hexagonally arranged covalently bonded carbon atoms. Graphene sheets are stacked together by van der Waals forces to form graphite. The exotic properties of graphene are dramatically different from that of graphite. There are several other 2D materials, which are mostly few atomic layer sheets, such as transition metal dichalcogenides (TMD), and MXenes that shows properties ranging from insulator to semiconductor to metal. The properties of these 2D materials depend on several factors, including the number of layers and level of defects. TRTS has been extensively used to study the carrier dynamics and transport properties of graphene [103] and many TMDs [104]. With many new 2D materials being designed and synthesized recently, TRTS will play an essential role in understanding them [104].

4.3 Non-linear THz Spectroscopy Recent advances in the generation of intense THz pulses have made it possible to explore the light-matter interaction in the THz regime. With pulse duration corresponding to 1 ps, intense THz pulses can significantly distort the electronic potential bringing about fundamental changes in the material properties. Because of its low photon energies of the order of kB T or below (at room temperature), THz radiation can access the molecular vibrational modes, which generally defines the equilibrium physical and chemical properties of a system. Excitation using THz pulses have now been utilized to study various systems using several experimental techniques like the THz Kerr effect, THz-pump-X-ray-probe, THz-pump-optical probe, and THz pump-THz-probe spectroscopy [105]. Understanding the intermolecular modes of vibrations in proteins and its local rearrangement dynamics, which typically fall in the THz regime, is of utmost importance because they strongly influence the protein structure. Whether quantum mechanics play any part in the functioning of these biological systems has been a matter of debate for a long time. Froehlich had proposed that within a protein, the vibrational modes are ordered and can condense into a lowest-frequency vibrational mode in a process similar to the Bose–Einstein condensation and thus, quantum mechanical coherence could be observed in biological samples in the THz regime. Structural changes induced by irradiation with 0.4 THz on a lysozyme crystal was probed by X-ray diffraction showed non-thermal changes in the electron density. These changes occurring in micro to millisecond timescale is much slower than expected and can be explained only by Froehlich condensation [106]. Another non-linear experimental technique that is gaining popularity is the THz Kerr effect (TKE). The advantage TKE has over the optical Kerr effect (OKE) is that it can successfully determine α, where αT K E = α Z Z − ( α X X + αY Y )/2, with α Z Z being the polarizability along the permanent dipole moment [107]. The sign of α is projected on the TKE signal, which gives new information on the

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polarizability tensor of the system. TKE has been successfully used to study water, methanol, and various other simple liquids [108–110]. Excitonic interactions in ZnSe/ZnMgSSe multiple quantum wells with THz radiation have been explored using the THz-pump-optical probe spectroscopy. At low THz fields, the dependence of the excitonic absorption on the THz field was in accordance to the perturbation theory (the Stark shift). In contrast, there was an apparent deviation of the experimental results from this theory at high THz fields, implying that the interactions are now in the non-perturbative regime [111]. THz-pump-THz-probe experiments have been utilized to explore the field of semiconductor nonlinearities in the THz frequency range. Single-cycle intense THz pulses accelerate the free carriers in doped semiconductors to high energies, while the THz probe beam can probe the free carrier dynamics in picosecond timescales. THz pump/THz probe experiments have been done to monitor the intervalley/intravalley scattering of hot electrons in GaAs [112] and the impact ionization in InSb and InAs [112, 113].

5 Conclusion Today THz spectroscopy has come a long way, starting from the first successful generation and detection of pulsed THz radiation. Now, it is possible to perform THz spectroscopy with intense broadband THz light on a tabletop. THz spectrometers capable of performing THz-TDS and TRTS with reasonably large spectral bandwidth are commercially available for several years. In the coming years, with the advances in laser technology and electronics, the performance and applicability of such instruments will undoubtedly improve. This will encourage more and more researchers to utilize THz spectroscopy in various fields that are not explored yet. THz-TDS is a steady-state absorption spectroscopy in the far-far Infrared range. For a sample to absorb THz radiation, it should have dipole moment oscillating in THz frequency. So, this technique is useful for most polar systems. On the other hand, for the centrosymmetric system, THz-TDS can probe only the IR-active (THz-active) vibrational modes. The Raman active modes will remain silent while interacting with THz photons. The use of optical Kerr effect (OKE) spectroscopy can conveniently solve this issue [114, 115]. The Optical Kerr effect is a third-order non-linear process. It measures the derivative of the time-correlation function of the anisotropic part of the polarizability tensor, unlike THz-TDS, which measures the two-point correlation function of the dipole moment. The advantage of doing an OKE and THz-TDS study together is that it is possible to get complementary information on the same dynamics of a system [116]. The experimental setup for OKE is relatively straight-forward compared to other forms of pump-probe spectroscopies. This era is the era of exotic materials for innovative applications. The non-contact nature of THz spectroscopy to probe materials is one of the many advantages that will be handy to study properties and dynamics in nanomaterials and sensitive materials of a different kind. Because of its unique way of probing materials that are not possible by other methods, THz spectroscopy will find more applications in materials science

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research involving photovoltaics, thermoelectrics, photocatalysts, plasmonics, spintronics, and magnetic materials. More work will soon be done where THz pulse will be used as a sub-ps perturbation to perform non-linear THz spectroscopy and used as a trigger for real-life applications. However, more will be achieved if THz spectroscopy and other well-established spectroscopies are used to study a new phenomenon.

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Recent Advances in Raman Spectroscopy and Applications

Overview of Raman Spectroscopy: Fundamental to Applications Deepak K. Pandey, Hardik L. Kagdada, Paridhi Sanchora, and Dheeraj K. Singh

Abstract Raman spectroscopy is the versatile technique for the characterization of materials in numerous fields of research, not only limited to the science and technology but also expanded towards the archeological, food, forensic analysis, and biomedical applications. The present chapter depicts the journey of the discovery of the Raman effect for growing the interest of the reader for Raman spectroscopy. Further, we discussed the fundamentals including the classical and quantum theory, which was followed by the instrumentation of Raman spectroscopy and an overview of the applications in diverse fields of research. We believe that this chapter leads to an increase in the knowledge and recent advances in the application of Raman spectroscopy. Keywords Raman spectroscopy · Classical and quantum theory · Instrumentation · Materials characterization · Biomedical applications

1 Historical Overview Sir Chandrasekhara Venkata Raman, the scientist behind the novel discovery of secondary scattering of radiation, later known as the “Raman effect” [1]. Raman was inspired by the work of British Physicist Lord Rayleigh and his exploring mind and spontaneous approach towards unraveling secrets of nature. Sir C. V. Raman started his research journey with an understanding of the Doppler effect in molecular systems, and later he investigated the light scattering from Sulphur suspension. The turning point of his research carrier arrived when he was selected as a delegate at the University of Congresses organized at Oxford University. During his journey to Europe, passing from the Mediterranean Sea, Raman was speculated by the deep blue sparkle of the seawater. However, Lord Rayleigh already explained the blue color of the sky by the scattering of sunlight by the presence of gaseous molecules in the atmosphere, and also mentioned that the blue color of the sea has nothing D. K. Pandey · H. L. Kagdada · P. Sanchora · D. K. Singh (B) Department of Physics, Institute of Infrastructure Technology Research And Management, Ahmedabad 380026, Gujarat, India e-mail: [email protected]; [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 D. K. Singh et al. (eds.), Modern Techniques of Spectroscopy, Progress in Optical Science and Photonics 13, https://doi.org/10.1007/978-981-33-6084-6_6

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to do with that, it is just the reflection of the sky. After astonishing about the blue color of deep seawater, onboard Raman carried out few experiments of viewing the surface of seawater through Brewsterian angle and observed that the blue color of deep seawater is far from the reflection of the sky and depicts that the phenomena have nothing related to the reflection of the sky. After his return, Raman immediately started the series of experiments on the scattering of light at the Indian Association for the Cultivation of Science (IACS), Calcutta, India. During this period (at least 6 years) Raman and his brilliant students have published almost 56 research papers in various prestigious scientific journals and proceedings on different laws of molecular scattering for the structure of molecules, pressure and temperature-dependent phase transition, etc. [1]. During the experiments of scattering phenomena, Sir Raman and his students obtained the frequency shift after the scattering of the incident light. However, this observation is not in agreement with the Rayleigh scattering, therefore, they assumed that the shift in frequency may be due to the fluorescence. To get rid of this, they purified the samples too many times and performed the same experiments, nevertheless they obtained the frequency shift, which leads to the belief that the fluorescence is not present. Therefore, Raman told his student, K. S. Krishnan to focus only on the undefined scattering in the liquids. For this experiment, the source of light is the sunlight which was focused by telescope objective lens and shortfocus lens. Then the light was passed through the blue-violet filter and incident on the flask, which contained the liquid sample. When the secondary green filter was along the incident beam, scattered light was not observed in the transverse direction. However, when they used the mercury bulb instead of sunlight, the opalescent of the track of scattered light observed for over 80 liquids and gas samples and all shows ubiquitous nature phenomena. Further, it was different from the fluorescence and exhibits strong polarization same as the unmodified scattered light. They named the effect as ‘modified scattering’ and realize that the phenomenon is a new fundamental observation. Then after Raman communicated the letter to Nature journal entitled “A new type of secondary radiation” in February 1928 [2]. In the notes of K. S. Krishnan, an interesting point is noted that modified radiation is originated from the molecular vibrations from the normal state. Further, Raman and his student started to understand the effect of the incident light having a different wavelength. In that experiment, they surprisingly observed the modified scattering was distinct from the incident unmodified light. These results were further sent to the Nature journal for the publication and after the rejection of the referee, the editor Sir Richard Gregory has taken a responsibility for the publication. In that paper, Sir Raman and his students have determined that incident lines generate its modified line having strong polarizability. Further, they also acquired that most of the modified lines consisting of the lower frequency than that of the incident one, while few shows the higher than the incident. Such frequency lines were observed for almost all the type of matters such as liquids, gases, crystalline materials as well as the optical glasses, which indicates the universal behavior of the phenomenon. Instead of relying on the costly equipment, the fine and final form of the discovery of modified scattering depends on the caliber of the scientists. For such great achievement, Professor Raman was awarded the Noble Prize for physics in the year of 1930.

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Note: The above historical overview is based on the article cited [2] by R. S. Krishnan and R. K. Shankar. After the discovery of the Raman effect, physicists and chemists started working on a wide range of problems and contributed more knowledge in both chemistry and physics field. Additionally, the development of laser boosts the development of the Raman spectroscopy for the characterization of materials in its different forms. The present chapter briefly explains the theory of Raman spectroscopy, instrumentation and the applications in the diverse fields of research.

2 Theory of Raman Spectroscopy Raman spectroscopy is the inelastic scattering of light by the object. The molecule gets excited while interacting with the incident light which results in the distortion of the electron cloud. If the distorted electron cloud acquired its original state by emitting the photons having the same frequency of the incident radiation, the phenomenon is called the elastic scattering, which is also admitted as the Rayleigh scattering [3]. However, when the incident light causes nuclear motion, during the scattering process, the energy transfer takes place between the molecule and the photon (either molecule to the photon or photon to the molecule). This process is recognized as the inelastic scattering of the radiation or the Raman scattering. This process is very weak, as the one photon is scattered from every 106 –108 incident photons in inelastic scattering [3]. Therefore, it is quite difficult to detect the scattered photons, which further leads to the slow development of Raman spectroscopy. However, the discovery of laser in 1969, raises the growth of the Raman effect and now a day it is widely used in almost all fields of research in science and technology. The theory of Raman scattering is explained through both the classical and quantum mechanical approach. The classical theory treated the electromagnetic waves and the scattering materials classically. However, the classical theory does not rule out some of the aspects of frequency dependence and the intensity of the Raman scattering. While the quantum mechanical approach to the Raman scattering provides the insights of intensity and the selection rules as discussed in the following sections.

2.1 Classical Theory The classical theory is based on the induced electric dipole, which relies on the vibrational frequency and the electric field of the incident radiation. When incident radiation is passed through the material, the scattering of light occurs. Further, the interaction between the incident light of the electric field and the molecules generates electric dipoles, having permanent dipole moments. The induced electric dipole ( p) in the presence of incident radiation is given by

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p=α·E

(1)

Here, the α is the polarization tensor, depending on the vibration of the atoms in molecules [4], while E is the electric field of the incident radiation at time t having a frequency v, expressed as, E = E 0 cos(2π vt)

(2)

Further, during vibration of the molecule, the atomic position of the nuclei changes from their equilibrium results in a variation of the polarizability, which can be expressed as the Taylor series in the vicinity of the position of the vibration, α = α0 +

 ∂α 1  ∂ 2α Qi + Qi Q j + · · · ∂ Qi 2 i, j ∂ Q i ∂ Q j i

(3)

Here, α0 stands for the polarizability in the equilibrium position, while Q i is associated with normal coordinates and having dependences on the vibrational frequencies of molecules. In the above equation, neglecting the higher-order terms, the change in polarizability can be written as follows: α = α0 + α1 Q i where α1 =

∂α ∂ Qi

(4)

.

Furthermore, in ideal condition, the vibration of molecules can be considered as simple harmonic oscillations, therefore, Q i is expressed as Q i = Q 0 cos(2π vi t)

(5)

Here, Q 0 is the amplitude of the Q i . Substituting the Q i in Eq. (4), and then combining Eqs. (2) and (1), α = α0 + α1 Q 0 cos(2π vi t)

(6)

p = α 0 E 0 cos(2π vt) + α 1 Q 0 E 0 cos(2π vi t) cos(2π vt)

(7)

This further can be simplified as the following expression, 1 p = α 0 E 0 cos(2π vt) + α 1 Q 0 E 0 [cos 2π (v − vi )t + cos 2π (v + vi )t] 2

(8)

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From the Eq. (8), it is obvious that p consisting the three parts of the frequencydependent induced dipole moment, p(v); p(v − vi ) and p(v + vi ). The first term on the right-hand side of the Eq. (8) determines the Rayleigh scattering which arises from the oscillation of the electric field with frequency v, causes the vibration of the electric dipole of the molecule at the same frequency. However, in the rest of the term, the induced electric dipoles of molecules oscillate with different frequency (v ± vi ) then the incident radiation. This phenomenon is determined as the Raman scattering and Eq. (8) gives the qualitative picture of the Raman scattering mechanism from the classical theory. The molecules oscillate with a lower frequency than the incident (v − vi ), then the scattering is known as the Stokes scattering, while viceversa (v + vi ) reveals the anti-Stokes scattering [5]. Rayleigh scattering is presented in every molecule as the classical definition of the α 0 exhibits always a non-zero component. Further, in a similar case, the Raman scattering exists only when at least one of the components of α1 must be a non-zero. Therefore, the salient feature to obtain the Raman scattering is the change in the polarizability must be non-zero [6]. The above discussion suggests that classical theory well explained the Rayleigh scattering and its frequency dependence on the polarizability tensor. However, for the case of Raman spectroscopy, the classical theory has several pitfalls. Classically, from Eq. (8), it is to be noted that the amplitude of the Stokes and anti-Stokes Raman scattering is the same. However, in experiments, the Stokes lines are stronger in intensity than the anti-Stokes scattering, which is unclear from classical theory. Further, the explanation of factors that govern the Raman intensity is also lacking. Therefore, quantum physics is needed and discussed in the following section.

2.2 Quantum Theory of Raman Scattering Quantum mechanics deals with the discrete energy levels, where the transition from one energy level to another results in the emission or absorption of the radiation. Further, the incident radiation is also quantized having the photons with the discrete energy. Therefore, the Raman effect may be explained as the collision between the molecule and photon of the incident radiation. Let us consider that the incident photon has the frequency v and the energy hv, which incident on the molecule having the energy E. According to energy conservation, 

E + hv = E + hv



(9)

Here, the left-hand side of the equation depicts the total energy of the system before the collision and the right-hand side reveals the energy of the system after the   collision. Further, E and v is the energy of the molecule and frequency of photon after the collision. From Eq. (9)   hv + E − E v = h 

(10)

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The above equation reveals the three cases: (i) The total energy of the molecule  before and after the collision is the same (E = E  ), further gives v = v. This suggests that the photon energy is not changed after the collision, which gives the   Rayleigh scattering of the molecule. (ii) E > E and v = v + v. This might happen when the molecule is already in the excited state due to the thermal energy and transfers the energy to the photon. This phenomenon is known as the anti-Stokes Raman scattering. (iii) The molecule absorbs the energy from an incident photon and  the final energy of molecule increases after the collision, i.e. E > E and further,  v = v − v. This is determined as the Stokes Raman scattering. Here, v is the frequency difference of incident and scattered photon, which is the characteristic shift in the vibration of the molecule, noted as the Raman shift (see Fig. 1). Further, at room temperature, most of the molecules are in the ground state, which results in the more intensify Stokes Raman line. Further, the rise in temperature causes an increase in the intensity of the anti-Stokes scattering relative to the Stokes lines [7]. The intensity of scattered light in the Raman scattering is further given by the following relation, while the transition from ith state to the jth state [8], I ji =

    ∗ π2 4 αρσ ji αρσ ji K ± v) (v 0 ρ,σ ε02

(11)

Fig. 1 Schematic presentation of the quantum description of the Rayleigh, Stokes, and anti-Stokes scattering in terms of energy (upper) and intensity (lower) difference

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Here, αρσ is the polarizability tensor, during the transition from ith state to the jth state, K 0 defines the irradiance of the incident radiation and ε0 is the permittivity of vacuum. In general, the Raman scattering is recorded for the Stokes scattering and in some special cases, the anti-Stokes scattering is also analyzed.

2.3 Selection Rules for Raman Scattering The necessary condition for the Raman active transition is the non-zero change in polarizability of the molecule, during the transition from one state to another. Moreover, the selection rules also reveal whether the transition takes place or forbidden. 0 ), which Let us consider the total wavefunction of the ground state vibration (ψvib j=1 exhibits the fundamental transition to the first excited state (ψvib ). The transition can be written as j=1

0 → ψvib ψvib

(12)

The polarizability (α) already determined in the Eq. (4), and during the transition using Eq. (12), the α can be expressed as [9], j

j

0 0  + α1 · ψvib |Q i |ψvib  [α] j←0 = α0 · ψvib |ψvib

(13)

The above equation reveals the concept of orthogonality, which solely provides the quantum mechanical understanding of the Raman scattering. Therefore, the selection rules for Rayleigh scattering is j

j

0 0  = 0, if j = 0; ψvib |ψvib  = 1, if j = 0 ψvib |ψvib

(14)

As discussed above, the second term of Eq. (13) determines the Raman scattering. The Raman effect will have observed only in the case when both terms in the second part of Eq. (13) are non-zero. i.e. j

0 = α1 · ψvib |Q i |ψvib

∂α j 0 · ψvib |Q i |ψvib  = 0 ∂ Qi

(15)

Here, the first term defines the derivative of the polarizability concerning the normal coordinates, which depicts the change in polarizability. Further, if the normal coordinate and wave function of vibration is revealed by the same quantum number i.e. j = i, then the second term of Eq. (15) is non-zero. In terms of vibrational frequency v = ±1 must require for the fundamental transition to be Raman active. The transition from v = 0 to v = ±1 is the fundamental transition for harmonic approximation. However, the anharmonic corrections in the vibrational energy levels revealed the overtones in the Raman bands, therefore the selection rule v = ±1 might not be applicable to the anharmonic vibrations. The overtones occurred in

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Raman spectra when the change in quanta is more than one. The transition from v = 0 to v = 2 is determined as the first overtone, while the transition from v = 0 to v = 3 is defined for the second overtone. Therefore, the selection rules for overtones is v = ±2, ±3, · · · ± n, which are observed at the higher wavenumbers. However, rapid decrease in the probability of overtones occurs toward the higher order quanta. Further, the combined transition from one quanta of vibration and another quanta of different vibration leads to the new vibrational states, known as the combination bands.

3 Raman Instrumentation The first Raman microspectrophotometer was developed in the year of 1976 in France, Europe [10]. The Raman spectrophotometer design has difficulty to enable imaging measurement along with spectroscopic investigation. The former spectrophotometer was interfaced with an optical microscope which affected the instrument working on both single point examination and mapping. Raman spectrophotometer microscope is used for focusing a laser beam onto the surface of any compound with the incorporation of short-wavelength lasers for the investigation of large fragile objects or artifacts [11]. However, it can be dispersive or non-dispersive depends upon the grating or prism generally used for the recorded spectra. Raman spectra is presented as Raman intensity (arbitrary units)-vs-wavenumber shift (cm−1 ) [12], recorded in the range of 4000–10 cm−1 [13]. Although, it is well known that the 4000–400 cm−1 range is significant for Raman analysis due to the active normal modes of vibration of organic molecules occur in this range. The quality of the Raman spectrum undeniably depending upon the design of the Raman spectrophotometer along with the optical components. Therefore, it is important to understand the role of components used in the Raman spectrometer (See schematic Fig. 2).

3.1 Source Before the discovery of the laser, the Raman effect was studied using the mercury arc lamp as a light source (see Fig. 3a) having the 435.8 nm line of coiled low pressure [12, 14, 15]. Then the invention of laser replaced mercury lamps as a source of the incident radiation [12]. The availably of these laser sources with the range of wavelengths along with stable and intense beam of radiation made them appropriate for Raman scattering experiments. Later on, a wide range of lasers become very popular like the argon-ion (488 and 514.5 nm), krypton ion (413.1 and 647.1 nm), helium–neon (632.8 nm), near IR diode lasers (660–880 nm), neodymium–yttrium aluminum garnet (Nd: YAG) and neodymium–yttrium ortho-vanadate (Nd: YVO4 ) (1064 nm) and frequency-doubled Nd: YAG and Nd: YVO4 diode lasers (532 nm)

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Fig. 2 A schematic diagram of instrumentation for Raman spectroscopy

Fig. 3 a Water-cooled mercury arc lamp for excitation of Raman spectra [6]. b ND: YAG solid-state laser

have been utilized in the Raman spectrophotometers [9, 11]. The argon-ion and krypton ion light sources are short-wavelength sources that can produce significant fluorescence. However, diode or Nd: YAG lasers are long-wavelength light sources and can be operated at much higher power without causing photodecomposition of sample and eliminates or reduces fluorescence in most cases [14].

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Raman lasers are optically pumped. However, this pumping does not produce a population inversion as in conventional lasers. Rather, pump photons are absorbed and “immediately” re-emitted as lower-frequency laser-light photons (“Stokes” photons) by stimulated Raman scattering. Thus, these pump sources are employed onto a sample by probing an intense beam or Stoke’s beam (copropagating or counterpropagating). Indeed, this energy exchange occurs only if the frequency difference between the pump and the Stokes laser beam frequencies matched to a molecular vibrational frequency of the sample. Moreover, silicon and optical fiber lasers are also being used as a source of Raman spectroscopy. The optical fiber lasers are based on Bragg’s fiber gratings which amplify easily and provide the continuous-wave operation for the sample interaction. Furthermore, silicon laser sources came into light in 2005 which was based upon the photonics of semiconductors due to its indirect bandgap. They emit the light as a waveguide and interact with a sample [16]. While using the lasers as a source, the following criteria must be satisfied to obtain good quality and accurate Raman spectra. The frequency of the lasers should be highly stable within the two or more frequent measurements. The lasers with narrow bandwidth are highly recommended as they directly influence the resolution of the obtained spectra.

3.2 Filters The spectrophotometer consists of the various types of filters, which have various application at different stages of the Raman scattering process. For the isolation of a single beam of laser light having a certain wavelength, the bandpass filters are used. Further, the low pass filter allows the shorter wavelength radiation than certain limiting value, while some of the filters are used to block the unwanted band of wavelengths, known as band block filters. As the spectrophotometer can be dispersive, it is needed a combined setup of notch filters and a high-quality grating monochromator. Moreover, the monochromator can be used having double or even triple gratings with super notch filters (long-wave pass-LWP) and rejection filters (short wave passSWP). These filters are optical filters, attached with a spectrophotometer and help to differentiate the scattering radiations. For example, to separate relatively weak Raman lines from intense Rayleigh scattered radiations the edge filters and holographic filters are used [3, 9, 12, 14, 17]. The filter cut off of all the filters is based upon the optical density of filters. Further, the greater optical density of filters blocks the larger amount of light [9].

3.3 Raman Spectrophotometer A conventional spectrophotometer is designed to receive light with several wavelengths and separate these wavelengths then ‘detect’ each wavelength by the detector,

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which converts a signal into a spectrum (see Fig. 2). Figure 4 shows a typical Raman spectrophotometer (double monochromatic). The separations of two wavelengths are used to determine the physical quantities of a sample. The collected beam from the Raman microscope is focused on the entrance front slit. After the passing of light through the slit, it diverges until it reaches a focusing mirror (FM) whose focal length corresponds to the distance between the mirror and slit. Afterward, the reflected light collides with the mirror and the light becomes “collimated.” Moreover, the light hits the grating (G1), the grating has an array of finely spaced lines on a reflective surface which produces constructive and destructive interference. Similarly, there is a constructive and destructive inference in the exit front slit, collide with the grating (G2). These two phenomena are dependent on the wavelength and angle of the incident light through the following equation [9], nλ = d(sin i − sin α) Here, λ and i determine the wavelength and angle of incident radiation (see Fig. 5), while α and n is the diffraction angle and order of diffraction. Thus, at a different angle, each wavelength is reflected and reaches the mirrors and finally towards the array of detectors [9]. Further, the resolution of the spectra is also dependent on the spacing d of the groove and the selected grating. Moreover, the Raman spectrophotometer is intended

Fig. 4 The optical arrangement of the monochromator in Raman spectrophotometer

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Fig. 5 Diffraction of incident light from the grating elements

to obtain the desired spectrum by using the spectral dispersion at a selected wavelength, then the angles and the focal length of the focusing element. These are kept images as tight as possible on the surface of the flat detector. The Raman spectrum becomes meaningful when the given wavelength values are converted to the Raman shifts using the following relation, v Raman = v Laser ± vV ibration = v

(16)

Here v is derived from λ(wavelength) by using v(cm−1 ) = 1/λ nm

(17)

By using the above equations, the spectrum can be obtained in the wavenumber shifts consisting of widths of lines (full width at half maxima- FWHM) between 1 and 10 cm−1 . Therefore, to enhance the pixels or resolution of the spectrum the selection of grating would be straightforward and significant. Moreover, the dispersion obtained in the Raman scattering is also depending on the groove density, and focal length [18]. Further, the reflectivity is a complex function of the groove shape, density, polarizability of the light on grating and spacing. It is also dependent on the groove profile along with the angle of grating tilt, and the metallic coating applied to the surfaces [14, 18, 19].

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3.4 Detectors The earlier models of dispersive Raman spectrophotometer consist of thermoelectrically cooled photomultiplier tubes (PMTs) and photodiode array detectors (PADs) as a detector segment. For better detection, the quality of the response curve and the sensitivity should be high. Further, the number of channels and the reduction in noise signal is also the important parameters for high-quality detection of the Raman scattered photons. By the time evolution, these detectors were replaced by charge transfer devices (CTDs) and charge injection devices (CIDs) which are more sensitive than previous detectors. Moreover, the single-channel and multichannel detectors are also used for Raman spectroscopy. However, these have certain advantages such as linear multichannel detections which exhibit the low signal to noise ratio [9]. Therefore, the problem of the detector is overcome by advances in instrumentation and technology leads to extremely suitable devices for the detection of Raman spectrograph such as charged coupled device (CCD) detector. The difference between CTD and CCD is the processing of forming arrays. CCD act as a detector by forming arrays of scattering light optical signals. In CTD the incoming optical signal transforms into charge at the photo site which is integrated and transferred to readout devices [1, 3] and the CCD detectors mostly have multichannel made up of silica, on which the incident photon will generate the electron–hole pairs. Further, the silica chip of the CCD detector contains the matrix of the contact point, placed at the positive potential, where each point consisting the small area having 104 –106 electrons, ahead of the saturation capacity. The CCD detectors are used for laser wavelengths of less than 1 μm. However, the low band-gap semiconductor (single element) such as Germanium (Ge) or Indium–Gallium–Arsenic (InGaAs) detectors are utilized for laser wavelengths of greater than 1 μm [3, 14–16].

4 Applications of Raman Spectroscopy After 90 years, since its discovery, Raman spectroscopy is still an established, versatile and quasi-indispensable tool in the arsenal of scientists in the various fields across the globe for the molecular investigation, characterizing the materials, finding the crystallographic orientation and others. It is one of the rapid and non-destructive characterization techniques with the high spatial and spectral resolution that is applicable at both the laboratory and industrial manufacturing scales. Over the past two decades, it has gained broader acceptance as a mature analytical tool for the non-invasive and rapid characterization and detection of various new molecular species and microbes. This technique has applications in many fields such as material science, mineralogy, surface analysis, biotechnology, food and beverages, environmental monitoring, forensic science, diagnostics, medical and clinical chemistry, pharmaceutical, etc. As it is not possible to list all of its applications in this chapter, we are primarily focusing its application in the study of hydrogen bonding, material sciences (allotropes of

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Fig. 6 Various applications of Raman spectroscopy

carbon, two-dimensional dichalcogenides, archaeological materials, etc.), biological applications, food and agriculture, and in forensic science. Figure 6 portrays a schematic of the summary of numerous applications of Raman spectroscopy. The following subsection distinctly discusses briefly the novel applications of Raman spectroscopy.

4.1 Study of Hydrogen Bonding (HB) Intermolecular interactions play a pivotal role in understanding the mechanism of various chemical, biological and physical processes [20–24]. Hydrogen bonding (HB), which is of crucial importance for chemical structures and reactivity, is one of the most important intermolecular interactions, especially in chemical and biological systems, and has been investigated extensively [25, 26]. In particular, it serves an important role in intermolecular identification, supramolecular design, crystal engineering and biological processes [27, 28]. It also plays a key role in determining the different properties of certain molecular systems, such as binary mixtures of the various solvents, ionic liquids (ILs), etc., which are significant for the myriad applications. Since most biological processes occur in the liquid state (in the water environment) and above described molecular systems often remain in liquid condition. Therefore, in the liquid phase, where even the formation and cleavage of these compounds occur rapidly, Raman spectroscopy is perfectly suited for the study of HB as this may expose both structural and dynamic aspects in the liquid phase [29, 30]. The line width in Raman spectra provides knowledge about molecular dynamics, while a vibrational band’s wave number position is related to the corresponding force constant, which depends on the electronic structure and bonding. HB analysis in different systems like water may provide a wealth of knowledge about their

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structure in pristine and mixture form, as well as in bio-molecules. The simplest and fundamental structure of the water molecule reveals the significant interesting physical properties which have been studied most extensively among all known liquids [20, 31, 32]. Raman spectroscopy among the spectroscopic techniques is suitable to study the vibrationally-averaged structure (V-structure) of water and its dynamics (both in bulk and confined state) which reflects the orientation of water molecule due to its smaller observation time τ = 10−13 − 10−14 s compared to the relaxation time of rotational rearrangement τr of water molecules in the liquid phase τr = 10−11 − 10−12 s [31, 33–36]. The liquid water consists of a random, three-dimensional HB network spanning a broad range of O–H•••O HB angles and distances [37]. Raman scattering from liquid water consists of intermolecular fluctuation bands in low-frequency regions due to the interaction of water molecules via hydrogen bonds, a ν2 (bending) band close to 1645 cm−1 , a combination of bending and libration bands (ν2 + νL ) are close to 2100 cm−1 , and a broad band of OH stretching vibration was obtained between 2800 and 3400 cm−1 [38–43]. The OH/OD stretch band at 2800–3800 cm−1 is especially insightful on the Raman spectra of water regarding the structure since it is prone to water molecules in local environments [44, 45]. The OH/OD stretch bands act as a marker band for the phase transition of the water from solid to liquid and gas phase, where the main peak shifts from ~3150 cm−1 (normal ice) to ~3420 cm−1 (liquid water) and then the transition to gas vapor leads the shifting to ~3650 cm−1 . Carey and Korenowski have deconvoluted this broad band into five Gaussian fitted center frequencies [46]. Whereas Zhelyaskov et al. using Fourier deconvolution divided the OH spectra into three anisotropic and four isotropic components and Li et al. decomposed the contour into four components [47, 48]. Li and colleagues revealed that the Raman spectra of water could be deconvoluted into five Gaussian components attributed to water molecules coupled entirely or partially with a hydrogen bond [49]. Sun categorized local water molecule HB by deconvolving the OH stretch band into five Gaussian subbands of Raman peak attributed to DA (single donor-single acceptor), DAA (single donor-double acceptor), DDA (double donor-single acceptor), and DDAA (double donor-double accepter) and free OH [50]. On the contrary, Djuriˇckovi´c et al. argued that matching the spectrum was futile and studied the actual spectra clearly without any deconvolution [51]. The perception that liquid water has partially hydrogenbonded (distorted) and absolute hydrogen-bonded (tetrahedral) configurations, that are strengthened despite confounding studies to compellingly illustrate the details derived through spectral analysis. Overall, liquid water is categorized as a tetrahedral liquid according to its coordination number, as the following equation: rmin

NC = 4πρ

r 2 drgoo (r )

(18)

0

where r min is the location of the first minimum in goo (r ), goo (r ) is oxygen–oxygen radial distribution function, and r is the number of the number density of water

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[52, 53]. In addition, the influence of temperature and pressure contributes to major changes in the structure and physical properties, which will reflect in the Raman spectra. Hu et al. recorded in situ Raman spectra of water at a constant pressure of 30 MPa and over a large temperature range from 253 to 753 K [54]. The measurement revealed two significant findings, one is the central frequency blue-shifted by 200 cm−1 indicates HB network collapsed above 673 K where water enters into the supercritical state due to which widespread HB tetrahedron disappeared. Another is that the Gaussian deconvolution of the Raman contour gives five components below 533 K and four components above 533 K shows that OH groups in water are involved in stretching vibrations with various energetic states showing high-temperature dependence. Intermolecular vibrational couplings (IVC) and Femi resonance (FR) may be the crucial factors in influencing the Raman water spectra other than HB interactions [55]. The key to spectra interpretation is to consider the relationship between the FR, inhomogeneous HB structure, couplings, and spectral characteristics, but it is a difficult task. Many studies suggested that the OH/OD stretch band represents not just the involvement of HB interactions [44, 55–59], but it is the participation of IVC/FR whereas others have recognized the isolated water molecule’s symmetric and antisymmetric modes [59]. In this regard, most recently, the Hu group discussed the water structure using Raman spectroscopy at a temperature range from 303 to 573 K based on the isotopic substitution (IS) effect [60]. Five dominant HB configurations are reported in water: two types of tetrahedral, single donor (SD) HB configuration, single hydrogen-bonded water (SHW), and free water (FW) without any HBs, represented by five sub-bands. The rise in temperature splits the HB arrangement and IS further favors the transfer of structure from tetrahedral to SD, SHW, and FW. Then, temperature and IS considerably reduce the number of HBs in water. To the current status, this fundamental field still attracts researchers around the globe, and with the aid of Raman spectroscopy, they are studying the structure of water not only in pristine form but in various confinement states [45, 61, 62], on polymer surfaces [38, 43, 63, 64], different interfaces [56, 65, 66] and in their binary mixtures with other solvents or molecular systems [67–74]. Several reviews articles summarized the different studies that address basic questions on the theory that governs the interaction between different surfaces and water. Most recently, the Tian group has studied the influence of 11 hydrated ions on the OH stretching vibration of water using Raman spectroscopy and revealed that ions primarily break the tetrahedral HB and promote the formation of partly hydrogen-bonded and free water molecules [75]. Understanding the structure of HB networks in binary solutions such as ethanol–water is essential for elucidating the role of water molecules in many biological and chemical processes that occur in aqueous solutions, and the anomalous properties of water itself [76, 77]. Most recently, the Men group studied formic acid–water binary mixtures with the aid of Raman spectroscopy and revealed the effect of formic acid (FA) on the OH stretching band indicating that the structure of FA-water undergoes two phase transitions [78]. Similar to water, a new class of solvent is ionic liquids (ILs), which have emerged since the last two decades as a revolutionary material and have many applications in energy, medical, and advanced material design which makes it attractive between

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academia and industrial communities [79, 80]. ILs attracted many researchers owing to their structural diversity, distinctive, intriguing characteristics such as low vapor pressure, low melting points, non-volatility, non-flammability, and excellent heat and chemical stability as well as the unique physicochemical properties [81–86]. The structure of ILs consist of bulky cations and various anions, reveals them as task-specific configurable, customizable, and designer solvents. Due to the cationanion complex in ILs, various forces such as weak forces (dispersion forces, van der Walls), specific (charges, dipole and hydrogen bonding (HB), etc.) and strong (Coulomb) forces are responsible for the novel physical properties. Coulomb force is only 70% of total energy, suggesting that the weaker interactions cannot be overlooked [87]. In particular, HB is deemed important for ILs structure and interaction modes [88–94]. Raman spectroscopy is useful not only in HB study in ILs but also in the identification of the various conformers in crystals and liquid forms of ILs [95– 100]. Alkylammonium nitrate ILs are perhaps the most investigated ILs because they are the first synthesized ILs in the laboratory [101]. Bodo et al. reported methylammonium nitrate (MAN) crystal structure and revealed the presence of HB interaction using Raman spectroscopy, where nitrate anion asymmetrically coordinated with MA cation [95]. Raman spectra revealed in longer alkyl chain ILs that propylammonium cations exhibit trans-conformations in a crystalline low-temperature state and undergo a crystal polymorphism transition with increasing temperature, where propylammonium cations exhibited in gauche conformation [96, 102]. Further, in the case of Imidazolium cation based ILs, Raman spectroscopy revealed the existence of trans-trans and gauche-gauche conformations in monoclinic and orthorhombic crystal structures of 1-butyl-3-methylimidazolium (C4 mim) cation in combination with Cl, Br, I, BF4 , and PF6 anions [97, 99, 100]. Recently, our group has extensively studied the HB interactions in Cn mim X ILs where n = 2, 4 and X = Cl, Br, I, and BF4 and revealed the presence of gauche-trans conformations [103–105]. In C4 mim X ion-pairs, two characteristic Raman bands were identified at 600 and 624 cm−1 for two isomers (gauche and trans) of the butyl chain in imidazolium cation. Also, significant changes in intensities of these bands were observed with different anions (Cl, Br, I, and BF4 ) [103]. In the Raman spectra of C2 mim X ILs, the vibrational bands at 3061, 3074, 3090, and 3125 cm−1 for the C2 mim X (X = Cl, Br, I, and BF4 ) ion-pairs, respectively, are attributed to the C2 − H9 stretch vibrational band which is a marker band for structural changes of ion-pair interactions [105]. Here, the noticeable red shift for C2 mim Cl indicates a stronger HB between cation and anion compared to ILs composed of Br, I, and BF4 anions [105]. Additionally, 598 and 620 cm−1 Raman vibrational bands represent the gauche (non-planar) and trans (planar) conformation, respectively [105]. Increasing the complexity of anions, from single atom halide anions and highly symmetric multiatom (BF4 ) to less symmetric CF3 SO3 (trifluoromethanesulfonate) and NTf2 (bis(trifluoromethylsulfonyl)imide) anions, greatly affect the strength of HBs in ion-pairs. In C2 mim TfO (1-ethyl3-methylimidazolium trifluoromethanesulfonate) IL, the C2-H9 vibrational bands occurs at 3116 cm−1 which is at higher wavenumber side in comparison to C2 mim X ILs indicates multiple HBs are forming in this ion-pair and exhibits the bifurcated and chelated structure [104].

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Adding the water to ILs greatly changes the cation–anion interactions and modifies the related physicochemical properties such as viscosity, polarity, electrical conductivity for various applications [106, 107]. Therefore, we must gain a better understanding of the interactions between the ILs and water molecules. Extensive studies were done by researchers using Raman spectroscopic techniques to study HB interactions and conformation equilibria [103, 109–111]. In this regard, our group recently examined the impact of water on the C2 mim X ILs, where all the ILs identified in gauche conformation, and also reported the weakening of the interaction between ion-pair due to the interaction with water molecules [112, 113]. The C4/5 -H Raman bands greatly blue-shifted by +33, +41, and +42 cm−1 going from neat ILs having Cl, Br, and I anions to the corresponding water mixtures, respectively, indicates weakening of ion-pair interaction [113]. In a water-rich environment, the change in relative intensities of Raman bands at ~2946 and ~2950 cm−1 corresponding to νs (CH2 ) and νas (CH3 ), respectively, confirm the conformational change of the alkyl chain [113]. Since this area is so vast that we cannot list all the studies and aspects here, these review articles can be very useful for interested readers from which they can obtain more basic knowledge of these interesting materials and their applications [114–116].

4.2 Material Science In material science, Raman spectroscopy emerges as privileged research equipment that offers a characterization of a diverse and demanding selection of specimens from carbonaceous materials to Archaeological materials. This technique is extremely applicable as an important method of characterization to many classes of materials due to its fast and non-destructive nature. Thus, this segment discusses the immense signature applications of Raman spectroscopy in various material science fields.

4.2.1

Carbonaceous Materials.

Natural carbon demonstrates its unique characteristics and novel properties such as tremendous mechanical strength, range of conductivity in different phases and dimensions. The Raman spectroscopy is being a versatile tool for understanding the change in structure and vibrational properties. Ferrari et al. have recorded the Raman spectra of graphene (defect-free) and bulk graphite shown in Fig. 7a, which clearly differentiate both the structures [117]. Also, the G peaks slightly shifted to low wavenumbers. Figure 7b shows that the D peak is changed in shape, width, and position as the number of layers increases [117]. Figure 7c reveals the disorderinduced Raman spectra of graphene portray an additional D and D’ peak at 1350 and 1620 cm−1 , respectively, where the D band is located at half of the position of the 2D peaks [118]. Sahoo et al. evaluated the field-emission properties of reduced graphene oxide (rGO) by studying the in-situ Raman spectra [119]. Figure 7d shows

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Fig. 7 Raman spectra of a comparison of bulk graphite and graphene (at 514 nm), b with an increasing number of layers (at 514 nm), a, b Adapted with permission from Ref. [117]. Copyright 2006 The American Physical Society, c a graphene edge, Adapted with permission from Ref. [118]. Copyright 2009 Elsevier B.V., d comparison of monolayer graphene and graphene oxide, Adapted with permission from Ref. [119]. Copyright 2013 American Chemical Society, and e graphite, graphene oxide, and reduced graphene oxide, and Adapted with permission from Ref. [120]. Copyright 2015 AIP Publishing LLC

the comparison between the graphene oxide and monolayer graphene where no D peak in the monolayer graphene observed indicates a defect-free sample and breakdown in hexagonal honeycomb lattice in graphene oxide (GO), leads to the broader G peak compared to monolayer graphene [119]. The difference between graphite, GO and rGO were studied by Perumbilavil et al. and found that G band in GO is shifted to a higher wavenumber due to the oxygenation of graphene (see Fig. 7e) [120]. Whereas, in the case of rGO, the G band is shifted toward the low-frequency region due to the enlarged number of sp2 carbon atoms. Graphene quantum dots (GQDs) have versatile optoelectronic properties [121, 122] which make them potential materials for a variety of applications including photovoltaics [123–125], light emission [126–130], electrochromic [131], memory devices [132] and bio-sensing [133, 134]. Doorn and colleagues conducted a comprehensive analysis of the impact of size on Raman spectra for a collection of bottom-up synthesized GQDs, with different sizes (0.89–1.62 nm), and compared the results with the spectra of larger graphene nanoplatelets [135]. It was concluded that the Raman spectra exhibit graphene characteristics and substantial variability in intensity in D and G bands with the dimensions

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of GQDs. Raman spectroscopy was most recently implemented to study its electrical conductivity and identify GO in an aqueous environment [136, 137].

4.2.2

Chalcogenides Materials

The transition metal dichalcogenides (TMDs), described by molecular formula MX2 (M = Mo, W; X = S, Se, Te), gained considerable attention due to their exceptional electronic and optical properties and wide-ranging applications of devices [138– 143]. Raman Spectroscopy is used extensively for characterizing these materials [144, 145]. Tripathi and Mishra developed an analytical tool that was based on open-source Python modules to investigate the number of layers in two-dimensional materials [146]. The coefficient of thermal expansion which is a fundamental property of these 2D materials must be well defined as it is essential to the dry transfer process and thermal management of 2D material-based devices. Hence, Zhang et al. presented the three substrates strategy to measure the coefficient of thermal expansion of monolayer molybdenum disulfide (MoS2 ) using micro-Raman spectroscopy [147]. Based on the symmetry analysis, they showed that owing to the influence of thermal mismatch and free expansion, the coefficient of thermal expansion would disturb the point optical phonon frequency via both the temperature coefficients and in-plane thermal stress [147]. Wang et al. studied multiple orders of Raman scattering in monolayer transition metal chalcogenides (TMDC) based on the intrinsic excitons coupled with different phonon modes [148]. Here, researchers concluded that multi phonon systems demonstrate multiple order overtones at the same interval of energies of both ωLO (longitudinal optical) and ωSO (surface optical) phonon modes [148]. Also, the overtone intensities rely on the gained Huang-Rhys factor values, which can be modulated by the intensity of the pairing of exciton-optical phonons, the cutoff wave vector of the optical phonon modes, and the large Bohr radius exciton [148]. To study the hydrogen evolution photocatalytic property of these TMDs, Guo et al. employed Operando Raman spectroscopy [149]. The spectroscopic studies indicate that the hydrogen atoms can be adsorbed by intermediate species produced during the photocatalytic phase to active sulfur and selenium atoms. Therefore, it can be concluded that the developed Operando Raman spectroscopy approach provides a new tool to elucidate catalytic reaction mechanisms in a practical and complex environment as a fast, insightful, and general analytical method.

4.2.3

Archaeological Materials

Cultural heritage stewardship includes preserving, maintaining, and restoring tangible artwork, archaeological objects, and collections of museums. The need to restore and preserve important works of art is obvious and has been a focus of museum activity for centuries. Raman spectroscopy is a significant resource in art conservators and archaeologists’ arsenals and has been extensively utilized in the study of ceramics [150, 151], gemstones [152], wall paintings [153], complex mixtures of

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sculpture [154, 155], glass [156], manuscripts [157], and rock art [158]. Raman spectroscopy is now so effective in art and archaeology that there have been recent reviews of available instruments applied to measurements outside the laboratory by prominent researchers in this area [155, 159, 160]. Archaeological soils are causally correlated with the presence of biochar with a high carbon accumulation potential and nutrient adsorption. To obtain structural details and distinguish alterations in biochar particles, the Sousa group implemented Raman spectroscopy and verified that biochar persistence in the atmosphere is due to its graphite composition [161]. Estalayo et al. carried out a study on a shipwreck belonging to the second half of the fifteenth century, which was found by chance in 1998 in the sediments of the Urdaibai estuary, in Urbieta (Gernika, Basque Country), 4 m underground [162]. Researchers conducted Raman spectroscopy here to ascertain the origin of the shipwreck and found that the presence of zinc in the pieces suggests a major effect of the polluted sediments deposited over the last 80 years on the upper part of the burial in which the shipwreck was situated. Lately, Raman spectroscopy was used in combination with the scanning electron microscopy (SEM) to study the pigments from the Ptolemaic period on an Egyptian cartonnage (305–30 BC) by Guard et al. [163]. The authors found that Raman spectroscopy classified the minerals associated with each pigment and reported the presence of cinnabar (α-HgS) in the red part of the sample, while a mixture of orpiment (As2 S3 ) and probably bonazziite (β-As4 S4 ) and/or alacránite (As8 S9 ) were detected in the fragments’ yellow-colored regions. Ozán et al. also used the utility of Raman spectroscopy to improve the study of ancient paintings in experimental rock art [164].

4.3 Forensic Science A justification for exploring its use in forensic science has been the recent technical developments in Raman spectroscopy. Hence, this segment summarizes recent findings in Raman spectroscopy concerning forensic science. The areas discussed in this section are involved among other topics: explosives, drug abuse, body fluids, and document examination.

4.3.1

Raman Spectroscopy of Body Fluids for Forensic Applications

Advances in forensic science have contributed significantly to better criminal investigation in general, and in particular to the identification of criminals and evidence. Effective detection of bodily fluids in crime scenes is crucial to forensic investigations, as they provide critical pieces of DNA evidence leading to the eventual prosecution and criminal justice resolution. Raman spectroscopy is, therefore, an effective tool for detecting and differentiating body fluid, among other possible environmental inferences. Boyd et al. investigated Raman scattering from human blood as a function of parameters important for forensic field research, such as substrate,

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the person from the sample was taken, sample dilution, and sample age [165]. In this analysis, they found that after drying the blood, the scattering peaks at 1155 and 1511 cm−1 vanishes, and this band was not present in a sample that was stored for more than a week. Certainly, this research demonstrates the ability of Raman spectroscopy for blood analysis, both in the laboratory and at the crime scene. Zou et al. showed the ability of Raman spectroscopy to classify an unknown material that may be human blood or semen without the use of chemical reagents [166]. In this study, they found that Raman scattering peaks at 2907 and 2968 cm−1 is due to the presence of semen, while peaks at 1057 cm−1 are only found in human semen and peaks at 1082 and 1165 cm−1 are unique to blood spectra. In combination with the multivariate data analysis, Lednev and the group used Raman spectroscopy to construct a statistical method that accurately identified the race of all 18 semen donors [167]. Results from this research indicate that Raman spectroscopy may be a valuable resource for forensic investigators. Al-Hetlani et al. recorded Raman spectra of the dry traces of oral fluid with advanced statistical tools to distinguish between smoker and non-smoker donors (differing gender, age, and race) [168]. Casey et al. recently showed the results of Raman spectroscopy, combined with chemometric analysis, which specifically discriminates between body fluids and a range of environmental interfaces (EIs) [169]. Shaine et al. reported 785 nm of excited SERS spectra of dried bloodstains and the analysis of these spectra indicates that this blood signature can be classified with 100% specificity and sensitivity concerning other body fluid’s SERS spectra [170]. All these latest studies indicate that Raman spectroscopy can be very useful in the scene of crime investigations where identification of body fluids along with the various EIs is most relevant.

4.3.2

Document Examination

Forensic inspection of the questioned documents may include the examination of many factors including signatures, the handwriting of individuals, dating and changes made in a document, determining the sources, and counterfeit documents [171]. Buzzini and Suzuki presented a comprehensive overview of forensic applications of Raman spectroscopy and looked specifically at the study of ink and paint evidence of pigments and dyes [172]. Using Raman spectroscopy, Lee et al. identified ballpoint pen inks (black and blue) based on a visual analysis of their spectra and identified major marker bands [173]. Such bands have been matched for the spectra of various types of pen inks. This technique enabled blue ink to be segregated with 94% accuracy and 95% for the black ink. Further, Mohamad Asri et al. reported that the integration of chemometrics with Raman spectroscopy could enhance the descriptions of the various pen inks. [174, 175]. The methodology of principal component analysis (PCA) enables the grouping of blue and red ballpoint pen inks by type [175]. In another piece of work, Raman and Fourier transform infrared (FTIR) spectroscopy, together with PCA and Pearson’s product-moment correlation coefficients (PPMC), enabled the classification of unidentified ink [174]. Borba et al. employed another chemometric approach where multivariate curve resolution alternating least squares

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(MCR-ALS) to show the order of crossed lines drawn and by utilizing confocal Raman spectroscopic imaging obliteration in writing samples [176]. Recently, Zi˛ebaPalus and the group examined the causes responsible for paper degradation and their ageing [177]. Further, Gorshkova et al. used the method of spectral analysis to determine the age of the writing inks, and in this process found that the narrow section of Raman spectra contains characteristic lines and the analysis of this narrow section using PCA chemometric approach enabled the separation of writing inks into groups (clusters) corresponding to different creation interval [178]. Ultimately, it is evident that Raman spectroscopy can be used to investigate several specific features relating to the study of questioned documents for forensic casework records.

4.3.3

Identification of Drugs of Abuse

The widespread use of illicit drugs within contemporary society is an unprecedented global crisis in human history. Certain drugs (e.g., cocaine) are closely associated with a rise in crime and abuse. Moreover, the widespread use of illegal drugs poses a serious threat to the protection and welfare of humanity, public health particularly youth; as well as to the safety and sovereignty of countries [179]. Lednev group and Khandasammy et al. have presented comprehensive reviews on applications of Raman spectroscopy in the detection and estimation of cocaine and other illegal drugs of abuse [180, 181]. Silveira and the group observed that Raman spectroscopy (near-IR) may be utilized to differentiate the various types of cocaine (crack cocaine, cocaine hydrochloride powder, freebase powder, and paste) [182]. Penido et al. combined the FT-IR spectroscopy with the utility of Raman spectroscopy to quantify cocaine in ternary mixtures in another piece of work [183]. Investigators noticed that Raman spectroscopy could be utilized in combination with PLS regression models to estimate the quantity of cocaine in the samples, even at low content of cocaine (as low as 30%) [183]. Jones et al. have used a combined approach of IR and Raman spectroscopy and reported that 76% of the 221 psychoactive drugs they analyzed could be correctly classified after expanding the instruments’ libraries [184]. Rebiere and colleagues used Raman spectroscopy in combination with nearIR spectroscopy to examine eight specimens of anabolic drugs, which are illegally utilized by several athletes for performance-enhancing purposes [185]. Some of the samples they found were containing steroids and many of the remaining ingredients listed were lactose, talc, starch, and sucrose [185]. In the last few years, researchers have been working on developing small, portable Raman spectrometers that can be taken to the crime scenes. Ali and group illustrated the capability of portable Raman spectrometers to recognize flunitrazepam, known as a date rape drug also recognized as Rohypnol or “roofies”, in different spiked alcoholic drinks at low concentrations such as 0.01% and for that, no sample extraction required [186]. The use of surfaceenhanced Raman spectroscopy (SERS) and the evolution of new SERS substrates is a major subject of extensive studies in recent years investigating drug analysis and tracking. The comprehensive details of the fundamentals and working of SERS are discussed in the next chapter. Most recently, the Fu group using commercial SERS

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substrate has analyzed cocaine standards down to a concentration of 1 ng/mL and the outcomes of this study indicate the possible future usefulness of the technique for onsite testing [187].

4.3.4

Detection of Explosive Materials

Explosives represent a field of forensic chemistry in which the material may present a deadly danger to the public at large [188]. Research of explosives is conducted by both national security departments engaged in pre-explosion identification and law enforcement agencies dealing with post-blast investigations [189]. Among various explosives, trinitrotoluene (TNT) is an extremely dangerous and explosive nitroaromatic compound and a source of concern internationally. TNT is most widely used as an explosive substance for guided explosions [190] but is being employed for several years in military and terrorist activities such as improvised explosive devices (IEDs) and landmines production [191]. Wu et al. recently presented a review concentrating primarily on techniques for enhancing the Raman spectroscopic technique including alteration of the SERS substrate and surface efficiency of explosive detection [192]. Elbasuney and group employed laser-induced Raman spectroscopy to detect spectra from the three main explosive categories: nitric esters, nitro-compounds, and nitramines. The result was that explosives could be separated from all three explosive groups except for nitrocellulose [193]. Elbasuney and colleagues used Raman spectroscopy for the identification of IEDs-related explosive substances and obtained more detailed profiles for the compounds used (ammonium nitrate, urea nitrate, fuel oil, ammonium perchlorate, and nitroguanidine) [194]. Most recently, Diaz, and Hahn used Raman spectroscopy as a sensor for the detection of ammonium nitrate (as an explosive precursor often used in IEDs) [195]. Using the utility of Raman spectroscopy, authors have achieved a relative limit of detection (LOD) of ammonium nitrate in water 0.1% (1 mg/g), and the absolute limit of detection was 1.0 μg. The interest in using SERS for explosive detection has also reappeared, as illustrated by a review article by Wu et al. that deals only with SERS for explosive analysis [192]. In this regard, the Faulds group used SERS to detect TNT, hexanitrostillbene (HNS), and 2,4,6-trinitrophenyl methylnitramine (tetryl), with an outstanding LOD of 135.1 ng mL−1 for HNS, 17.2 ng mL−1 for tetryl, and 6.81 ng mL−1 for TNT as a fast, sensitive and selective technique [196]. These recent studies show potential for the production of Raman spectroscopy based rapid and portable assays that can be used in the field to achieve accurate and quantitative identification.

4.4 Food Analysis With the globalization of food and its complex networking mechanism, a wide variety of food pollutants is introduced into the food system that can occur inadvertently,

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deliberately, or naturally. This situation has made food safety a major global concern today and urged the need for innovative technologies that can deal with food contaminant detections as effectively as possible. Due to their importance as food resources, the quality and sustainability of dairy products have always been major concerns of ordinary people, authorities, and researchers. Raman spectroscopy is, therefore, an efficient, accurate, simple, practical technique for the detection of different contaminants in food products. Recently, in combination with multivariate analysis, Genis et al. recorded Raman spectra to determine the origins of fats in margarine, corn, and, palm oils found in white and ultra-filtered cheese [197]. Samples of milk fat and non-milk fat/oils showed significant variation in intensities of Raman spectra (see Fig. 8a). Most recently, Zhang et al. demonstrated a new fusion strategy in combination with the Raman spectroscopy and a support vector machine (SVM) algorithm to classify various dairy products [198]. In this study, three pasteurized milk samples were taken from different brands, and Raman spectra were recorded which was found to be identical (see Fig. 8b). Using Raman and SVM algorithm techniques, the best spectral feature (Fig. 8c) intervals were determined by an identification accuracy rate of 93% [198]. Taylan et al. have performed the first study using Raman spectroscopy with chemometrics and were able to distinguish the butterfat from Lard fat and their recorded spectra are shown in Fig. 8d, where the marker band for lard at 1271 cm−1

Fig. 8 Raman spectra of a margarine, corn oil, palm oil, and milk fat, Adapted with permission from Ref. [197]. Copyright 2020 Elsevier Ltd. b Three dairy products (CZ: Chun Zhen-pasteurized heattreated flavored yogurt products from Mengniu Dairy Group Co. Ltd., AM: ambrosial-pasteurized heat-treated flavored yogurt products from Inner Mongolia Yili Industrial Group Co., Ltd., and MS: momchilovtsi-pasteurized heat-treated flavored yogurt products from Bright Dairy & Food Co., Ltd. c Extracted spectral feature intervals of MS, AM, CZ, B, and C adapted with permission from Ref. [198]. Copyright 2020 The Royal Society of Chemistry. d Lard and butter samples. e Adulterated butter samples at the concentration range of 0–100% lard fat (w/w), Adapted with permission from Ref. [199]. Copyright 2020 Elsevier Ltd.

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can be seen [199]. Further, the intensity of this band slightly increased with rising lard fat content in mixture samples (0, 3, 5, 10, 20, 40, and 100% lard w/w) that appears in the Raman spectra shown in Fig. 8e [199]. Reiner group recently looked into the applicability of Raman spectroscopy as an online process control method during the processing of consumer milk [200]. Similar to dairy products, Honey, as a natural sweet substance is also an important part of diet due to its miraculous properties such as antimicrobial, antioxidant, antiproliferative, anticancer, anti-inflammatory, and antimetastatic effects. In this regard, Samarghandian et al. recently reported a study that emphasizes the therapeutic aspects of honey’s ability and its multitude [201]. Most recently, Molnar et al. used the utility of Raman spectroscopy in honey authentications where authors developed a new green sample preparation method and analyzed distinct honey varieties from different regions of Romania using this approach [202]. It was concluded that using this technique, no spectral changes were observed in different honey varieties even after three days, indicates that this method can be successfully applied to the investigation of different types of honey with different degrees of crystallization and fluorescence [202]. Coffee is another product consumed as a beverage in the world largely and also a major source of caffeine [203]. It has great economic significance and the worldwide consumption of coffee is about 155 million bags per year [204]. The quality of coffee is an important issue in this way which can affect human health [205, 206] and therefore a fast-reliable tool is needed to investigate coffee quality. Most recently, various strategies were developed using Raman spectroscopic to investigate the quality of coffee beans [207–209]. With the aid of chemometrics, Santos and colleagues used the Raman spectroscopy to classify four Arabic coffee genotypes: one Mundo Novo line (G1) and three Bourbon lines (G2, G3, and G4) [209]. Using Raman spectroscopy, most contributed bands were identified at 1567, 1479 cm−1 and 1442, 1302 cm−1 for kahweol and fatty acids, respectively. The partial least square analysis of the Raman data was carried out which effectively discriminates the coffee genotypes [209]. Various investigations reveal the potential of Raman spectroscopy and its advanced variants for food analysis as it is a fast, reliable, non-destructive, and realtime analysis tool [210–214]. Further, Nache and colleagues investigated the quality of porcine meat using Raman spectroscopy with the ant colony optimization (ACO) metaheuristics using pH as an indicator and identified the quality markers pH45 and pH24 for the meat that helps to assess the quality of the meat [215]. The Logan group used Raman spectroscopy to verify the quality of the beef and suggested with their analysis that Raman spectroscopy is a viable alternative tool for the authentication of beef carcasses from grass and grain-fed production systems [216]. Most recently, Kang et al. used deep Raman spectroscopy with a pair of fiber optic probes to analyze subcutaneous swine fat and revealed that this technique could be used to measure the local distribution of subcutaneous fat [217]. The use of Raman spectroscopy in food analysis is still in its initial stages, as most Raman systems were developed in laboratories for research purposes. Therefore, portable Raman systems have great potentials as they composed advanced light source and detector technologies. We believe that the Raman spectroscopy will be one of the most important methods to be used soon in food analysis.

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4.5 Biological Applications As non-invasive, fast, and inexpensive methods for obtaining information on the content of biological samples, Raman spectroscopic techniques have recently gained growing clinical importance. We must be able to evaluate regenerative processes in cells, tissues, organs, and patients at a biochemical level to ensure that work is transferable from bench to bedside. Raman technique gives the analyte’s vibrational frequency, which can be viewed as their “fingerprint” allowing for easy analysis and identification. During the past few years, Raman spectroscopy, has undergone significant technological advances, as resolved by issues such as fluorescence, low sensitivity, or reproducibility. Raman spectroscopy has tremendous biological applications such as bio-molecules recognition [218–227], cancer [228–231], immunology [232], microorganism detection [233], cell therapy [234], live-cell studies [235], diabetes [236], virus capture and recognition [237–240]. This section discusses the specific use of this technique in the study of recognition of bio-molecules, cancer, diabetes and viruses.

4.5.1

Raman Spectroscopy of Biomolecules

Biomolecules are basic building blocks of life and core of all processes of life, involved in conducting important metabolic reactions and preserving living organism’s overall biochemistry. Therefore, it is important to understand separately the structure and properties of different biomolecules. Raman spectroscopy is very helpful in the study of the various biomolecules such as protein [219, 220, 241], nucleic acids [221, 223], lipids [225], carbohydrates [242], etc. Most of the characteristic bands are associated with the group CONH, called amide B (NH stretching, about 3100 cm−1 ), amide A (NH stretching, about 3500 cm−1 ), and Amide I–VII [219]. Most recently, the globular protein was studied by Raman spectroscopy, and it was predicted that hyper Raman may be the new tool to investigate the proteins as well as of biomolecules and more complicated biological structures with the 532-nm excitation [243]. Molecules of deoxyribonucleic acid (DNA) are essential to all living organisms-even to plants. It’s essential for life and its processes for an inheritance, protein-coding, and the genetic instruction guide. DNA contains instructions [244, 245] for the creation and reproduction of an organism or growing cell, and eventually death. Recently, Raman spectroscopy was used in the identification of DNA G-quadruplex (G4) formation and the distinction of different G4-folding topologies [246]. A lot of recent studies are still published recently using Raman spectroscopy to identify and study various biomolecules [247, 248].

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Cancer Detection

According to WHO statistics, nearly 9.6 million deaths were due to cancer in 2018. As with any disease, an early and quick cancer diagnosis is of utmost necessity. Traditional detection methods such as Computed Tomography (CT), Magnetic Resonance Imaging (MRI), Positron Emission Tomography (PET) scanning, etc. are difficult to use in intraoperative procedures as these techniques require comprehensive labeling. Accordingly, Raman spectroscopy was used to detect different cancer types [249– 261]. In 2017, Ming et al. diagnosed nasopharyngeal cancer using Raman spectroscopy although the deep anatomical position makes it very difficult to diagnose this form of cancer [257]. Also, Moradi et al. have shown the potential of this technique in identification between human ovarian cancer cells that were sensitive or resistant to cisplatin [259]. Whereas Jermyn et al. developed an intraoperative tool based on Raman spectroscopy to detect the invasive grade 2–4 gliomas and can detect as few as six cancer cells per 1 mm2 , which was much better than commonly used methods such as MRI [260]. As radiotherapy causes DNA damage and alterations of macromolecules of cancer cells, the Qui group has used laser tweezer Raman spectroscopy (LTRS) to study the effect of radiotherapy in the Nasopharyngeal carcinoma (NPC) treatment [262]. Raman spectroscopy can also detect other types of cancer as it can quickly distinguish differences in healthy and cancerous tissue molecular structures. Alexel et al. used Raman spectroscopy to identify and examine the impact of tobacco smoke on human blood components and revealed the profile of structural and chemical alterations that serve as a biomarker of physiological and pathological conditions in the tobacco-induced human blood components [263]. Leblond and his group performed a retrospective study of 65 patients using in vivo Raman spectroscopy to shed light on brain cancer molecular processes and classify oncogenic processes that characterize glioma [230]. Most recently, the accuracy of Raman spectroscopy in the detection and diagnosis of oral cancer was reviewed by the Li group [229].

4.5.3

Diabetes

According to IDF (International Diabetes Federation), about 463 million people are living with diabetes; this would grow to 700 million by 2045 [264]. The disease is characterized by elevated blood glucose levels due to malfunctioning insulin production. The utility of Raman spectroscopy in diabetes detection was used by the Birech group by identifying the biomarker bands in the spectra of diabatic rat’s blood. Researchers also revealed the herbal extract Rotheca myricoides Hochst with the aid of these biomarker bands had a greater anti-diabetic effect at a low dose (50 mg/kg of body weight) [236]. Noninvasive monitoring of blood glucose has been a longtime dream of treating diabetes. Some previous studies reported on glucose sensing, however, these reports did not reveal glucose Raman peaks. The first direct detection of glucose Raman peaks from in vivo skin was demonstrated by Kang et al. [265]. Keeping this point in focus, the Birech group developed a low-cost Raman sample

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substrate for the screening of metabolic diseases such as diabetes in whole blood by using Raman spectroscopy [266]. The excretion of urinary albumin remains the key biomarker for the detection of renal complications in type 2 diabetes. As the epidemic of diabetes grows, particularly in low-income countries, efficient and lowcost methods are needed to measure urinary albumin. Jose and his group performed a pilot study in this regard and evaluated the ability of Raman spectroscopy in urinary albumin assessment in patients with type 2 diabetes. This piece of the study revealed that Raman spectroscopy is capable of detecting low amounts of urinary albumin, indicating this method’s effectiveness for screening complications of type 2 diabetes renal [267].

4.5.4

Virus Capture and Identification

In structural biology, Raman spectroscopy has been named as “sleeping giant” because this technique offers a wealth of essential information for biomolecules, diabetes, and cancer, etc. [268]. Infectious and inflammatory diseases also require substantial attention because they encompass an enormous level of the worldwide burden on the global health care system. In such a manner, Raman spectroscopy has also been utilized for contemplating various infectious and inflammatory diseases including dengue [238, 269, 270], tuberculosis [271–276], sepsis [277–279], ulcerative colitis [280, 281], inflammatory bowel disease [282–285] etc. Nascent and reemerging viruses are responsible for a variety of recent epidemic outbreaks. A crucial step in the identification and prevention of outbreaks is the prompt and accurate characterization of emerging virus strains. Raman spectroscopy might be helpful in this regard for the rapid diagnosis and characterization of emerging strains of viruses. Among various virus infections, Dengue virus infection is caused by a mosquitoborne dengue virus (DENV) that belongs to the Flaviviridae family, claims several lives particularly in developing countries. In India, dengue is endemic in almost all states and its wrath can be understood by a Delhi example where this union territory reported its worst outbreak in 2015 with more than 15,000 cases [286]. Likewise, malaria influences more than 500 million individuals each year, and one child dies every 30 s [287]. Patel et al. conducted the first serum-based Raman spectroscopic analysis to stratify malaria and dengue, as the serum is found to be the first preference for dengue detection across different clinical samples [288, 289]. In the just mentioned study, Raman spectra were recorded for 130 subjects to generate a predictive model [288]. Raman spectra for malaria vs healthy controls (HC) and dengue vs HC recorded and represented in Fig. 9a, b, respectively, and major spectral peaks (cell-free DNA (1340 and 1420 cm−1 ), amide linkages (1280, 1302, and 1337 cm−1 ), β-carotene (1157 cm−1 ), Tyr (830 and 850 cm−1 ), CH2 deformation (1337, 1398, and 1445 cm−1 ), Phe (1004 and 1204 cm−1 ), Trp (1552 cm−1 ), creatine (846 and 908 cm−1 ), etc.) were identified as biomarkers for the identification of these diseases. This research work provides a detailed comparative description of the Raman spectra to distinguish between infectious disease (malaria and dengue) and the related clinical symptoms. Recently, the Terrones group presented a robust and high-throughput

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Fig. 9 Raman spectra of a, b comparison between Malaria vs HC, Dengue vs HC, Adapted with permission from Ref. [288]. Copyright 2019 American Chemical Society. c H5N2, H7N2, and reovirus collected from VIRRION, Adapted with permission from Ref. [237]. Copyright 2020 PNAS

sample preparation platform called VIRRION (virus capture with rapid Raman spectroscopy detection and identification) for swift enrichment of multiple viruses and label-free identification directly from clinical samples [237]. Using this technique, more than 100 Raman spectra were recorded and averaged for each strain to produce an accurate average fingerprint for each virus as shown in Fig. 9c. It is evident from Fig. 9c that each strain of the virus has a different fingerprint that could still be identified at concentrations as low as ~102 EID50 /mL. This sensitivity is equivalent to that of RT-qPCR detection which is an advantage of this VIRRION technique [237]. As the WHO also recognizes that early detection can halt the spread of viruses by enabling the rapid deployment of appropriate countermeasures, this assumption is also valid for highly contagious SARS-CoV-2 which is responsible for the current COVID-19 pandemic worldwide, as there is no treatment or vaccine [290]. Dutta and his group established a new statistical model for the detection of RNA viruses in saliva-based on Raman spectral characteristics for viral outbreaks, such as the ongoing COVID-19 pandemic [240]. It was found that 65 features among 1200 (within the range of 939–1054 cm−1 ) were sufficient to effectively distinguish the positive and negative viral samples as depicted in the form of a heat map, raising the accuracy of the model prediction to 91.6% [240]. A GUI-based analytical tool RNA virus detector (RVD) was developed using these 65-feature-based analyzes of the Raman spectra Dutta group. These latest studies further illustrate the ability of Raman spectroscopy in identifying the viruses. Therefore, there are other fields where this methodology can be applied effectively for further applications by making improvements.

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5 Conclusion Raman spectroscopy has the advantages of being non-invasive and able to provide real-time and in situ results, so it deserves to be studied and improved further to better serve field applications. However, some shortcomings are also associated with this technique as the Raman phenomenon is quite weak which results in poor sensitivity making it challenging to measure low concentrations of a substance. Even if the material is high fluorescent then it will be difficult to record spectra. Many advanced variants of Raman spectroscopy have therefore been discovered as a solution, such as surface-enhanced Raman scattering (SERS), tip-enhanced Raman scattering (TERS), coherent anti-Stokes Raman spectroscopy (CARS), etc. The overview, the technical advances, and their applications are demonstrated in upcoming chapters. Acknowledgements D. K. S. acknowledges financial support from the SERB-DST ECR project “ECR/2016/001289”. D. K. P. is grateful to DST, India for providing financial support under the INSPIRE Fellowship No. IF170625.

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Fundamentals and Applications of Surface Enhanced Raman Spectroscopy Bishnu Pada Majee and Ashish Kumar Mishra

Abstract As an advanced approach to Raman spectroscopy, Surface enhanced Raman spectroscopy (SERS) is known to be a powerful tool in detecting molecules at very low concentration level. The enhancement in Raman signals of probe molecules can be achieved by using the interaction between probe molecules and SERS active substrates. The fundamental concepts related to SERS study have been discussed in the present chapter. Electromagnetic and chemical interactions for improved Raman signals have been discussed. Recent advancement in SERS study with different metals and semiconducting active substrates have been provided as examples for the detection of organic molecules. Keywords SERS · Electromagnetic enhancement · Chemical enhancement · Enhancement factor

1 Introduction Among various sensing/detection methods, optical methods have the potential to detect the analyte molecules in a short time [1]. Raman spectroscopy is a fast and non-destructive optical technique that can provide the characteristic information of the probe molecule/analytes and hence it has been widely used in different applications in chemistry, physics, and medicine in the last few decades [2, 3]. Indian scientist Sir Chandrasekhara Venkata Raman discovered the phenomenon of Raman scattering in 1928 and received the Nobel prize in 1930 in physics for the same. In Raman spectroscopy, the scattered photon frequency is proportional to the difference in energy in the vibrational levels of the molecule. Raman spectroscopy technique is used as a compound fingerprint, like other spectroscopic methods such as Fourier transform infrared, UV–visible absorption and fluorescence spectroscopy techniques. However, Raman signal are quite weak due to the small scattering crosssection. In order to improve the Raman signal, surface-enhanced Raman spectroscopy B. P. Majee · A. K. Mishra (B) School of Materials Science and Technology, Indian Institute of Technology (Banaras Hindu University), Varanasi 221005, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 D. K. Singh et al. (eds.), Modern Techniques of Spectroscopy, Progress in Optical Science and Photonics 13, https://doi.org/10.1007/978-981-33-6084-6_7

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(SERS) was developed by utilizing interaction between probe molecule and substrate due to the plasmon resonance or chemical enhancement [4, 5]. The enhanced Raman signal of pyridine molecule adsorbed on a roughened silver surface was first observed by Fleishmann and his group in 1974 [5]. SERS has been widely used in the last two decades for the detection of industrial waste, chemical, food industries and medical science [6]. Materials, ranging from coinage metals (Au, Ag, Cu) to semiconductors, having high density of hotspots on their rough surface, can be used as active SERS substrate. In last few years, researchers have investigated different nanomaterials beyond the conventional metals (Au, Ag, Cu) and semiconducting metal oxides (TiO2 , ZnO, Cu2 O etc.). Carbon nanomaterials (Graphene, carbon nanotubes) and metal dichalcogenides (MoS2 , MoSe2 etc.) nanostructures have been identified as suitable SERS substrates in addition to conventional metals and semiconducting nanomaterials [1, 7–12]. Among newly studied materials, 2D materials are found suitable for the SERS applications due to the layer-dependent optical properties, high surface to volume ration and good stability [9]. Hence, researchers developed different 2D materials based active SERS substrates with different morphologies like flower, sheet etc. for the detection of organic pollutants. The chemically inert surface of 2D materials opposes the deformation and chemical reactions of the probe molecule with the surface and due to this, the reproducible enhancement is possible for quantitative analysis. In this chapter, we will discuss the basic principle of SERS, different ways of calculating enhancement factor, possible mechanism for signal enhancement and examples of different metals and semiconducting active SERS substrates for the detection of organic impurities.

2 Brief Discussion on Raman Spectroscopy The basic principle of Raman spectroscopy is based on the interaction between electromagnetic field (EMF) and materials/molecules, which results in inelastic scattering [13]. The incident EMF i.e. photon interacts with the analyte molecule, and a dipole moment is induced which is directly proportional to the polarizability of the molecule. The magnitude of induced dipole moment (μind ) depends on the strength of the incident electric field (Ein ) and the polarizability of the molecule (αm ) and it can be expressed as follows [14] μind = E in (ωinc ).αm

(1)

The efficiency of any scattering process is depending on the scattering crosssection and in case of Raman scattering, the efficiency depends as follows [15] Efficiency =

dσr d

(2)

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where σr and d are cross-section and element of solid angle, respectively. The differential Raman cross-section depends on the particular vibrational mode of a molecule, which is different for different methods. The Raman cross-section of a molecule for a given medium depends on the refractive index and the excitation wavelength. The Raman signal intrinsically weak due to the relatively low cross-section per molecule (~10−31 to 10−29 cm2 sr−1 ) as compared to the fluorescence spectroscopy (~10−16 cm2 sr−1 ) [15]. In Raman spectroscopy, one photon goes inelastic scattering out of 106 -109 incident photons resulting in the low signal strength and hence laser sources are used to enhance the signal strength. The classical theory explained the inelastic scattering of incident electric field Ein and the angular eigen frequency (ωvib ) of the vibrating molecule. This interaction results in three dipole components μind (ωinc ), μind (ωinc − ωvib ) and μind (ωinc + ωvib ) corresponding to Rayleigh, Stokes and Anti-Stokes, respectively, as shown in Fig. 1 (Left side) [14, 15]. In the Raman scattering process, the incoming photon does not absorb by the molecule rather it is scattered. The enhancement of the scattered signal depends on the resonance frequency. The SERS enhancement can be expressed as follows ISERS = Iinc (ωinc ) × I (ωs )

(3)

where, ωs = ωinc − ωvib . The above equation can be written in terms of the electric field as follows [14] ISERS = |E inc (ωinc )|2 |E(ωs )|2

Vibrational energy states

3 2 1 0

Raman process

(4)

FL process

Virtual energy states Stokes line

3 2 1 0 Incident photon

Rayleigh line

Anti-Stokes line

Vibrational energy states

)

Fig. 1 Schematic representation of the scattering process in Raman scattering (Rayleigh, stokes and anti-stokes line) and a fluorescence process (unlike Raman)

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where Einc (ωinc ) and E(ωs ) are the local electric field enhancement at frequency ωinc and electric field enhancement at the stoke frequency ωs . If the electric field value is very close to each other than the SERS intensity ISERS = |E(ωinc )|4

(5)

The above relation tells about the enhancement of the SERS and is equal to the fourth power of the electric field enhancement value. The electric field strength of dipolar radiation varies with distance r, i.e. E(r) ~ 1/r3 = r−3 . In the case of SERS intensity, the distance play a very crucial role and SERS intensity varies ISERS ~ 1/r12 = r−12 . The SERS is a surface selective effect and the bands due to in-plane and out-of-plane modes of an aromatic compound are differently enhanced because of different components of the tensor.

2.1 Comparison of Raman and Fluorescence Processes Raman scattering, an optical scattering process, is an instantaneous process where an incoming photon from the laser source at ωL excites a molecular vibration (ων ), then it emits a scattered photon at a frequency ωS = (ωL − ων ). In Raman process, incident photon does not get absorb in the molecule and the scattering process is generally excited in the transparency region of the molecule. In the non-resonant case, a Raman signal is occurred due to the interaction with the virtual state as shown in Fig. 1(Left side). In case of Fluorescence spectroscopy, absorption of a photon from ground state (S0 ) to the excited state (S1 ) occurs in step 1 due to the excitation source as shown of Fig. 1 (Right side). The first lag happens during that process and it undergoes a series of vibrational relaxation processes for few picoseconds to reach the vibrational ground state of S1 in step 2 as shown in Fig. 1(Right side). It remains for a few nanoseconds at the vibrational ground state of S1 and it undergoes the emission process from the ground state of vibrational state S1 to the state S0 in step 3 as shown in Fig. 1(Right side). The emission process is fully independent from the absorption process in fluorescence, while both photons (incident and scattered) are linked to each other in a coherent way in Raman process [16]. Raman and fluorescence processes are fundamentally different from each other; however, both the processes are two-photon processes.

3 Surface Enhanced Raman Spectroscopy The SERS signal amplification occurs due to the interaction among the incident light, analyte molecule and the active SERS substrate (metallic or semiconducting surface). The mechanism behind the SERS enhancement is an exciting area of science

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Fig. 2 Schematic diagram of SERS process Laser

SERS Signal

Analyte Molecule

Rough Surface

Nanostructured Substrate

to investigate. Most of the researchers have commonly used two theories, electromagnetic (EM) enhancement and chemical (CM) enhancement. The two mechanisms are applicable separately to metallic and semiconducting substrates. The excitation of localized surface plasmon resonance (LSPR) modes in metal substrates is known to be responsible for EM enhancement [17]. The LSPR occurs when the metal nanoparticle excites at the resonance frequency of the incident light. On the other hand, the CM enhancement happens due to the charge transfer (CT) between the probe molecule and semiconducting SERS substrate [15, 18]. A schematic representation of SERS process is given in Fig. 2, which suggest that the roughness of active SERS substrate, adsorption of analyte molecules, wavelength of incident laser and interaction between analyte-substrate play important role in SERS detection of analyte.

3.1 Theories for SERS Enhancement There have been constant scientific efforts to understand the reason behind the enhancement in Raman signal in SERS process. In this regard, mainly two different theories (electromagnetic and chemical mechanism) have been proposed to understand the signal enhancement for metallic and semiconducting nanostructures. They have been discussed in following sections.

3.1.1

The Electromagnetic Enhancement Theory of SERS

Enhanced SERS signal on metal substrates is explained on the basis of EM enhancement. The idea that surface plasmons play a key role to enhance the SERS signal was first discussed in 1980 by Gersten [19–21]. The collective oscillations of the conduction electrons in metal are called surface plasmons, which are at the core of SERS electromagnetic enhancement. Figure 3 shows the oscillations of conduction

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Fig. 3 Electromagnetic enhancement in SERS [15]

Electromagnetic Wave

Eloc

E0

+

+ + +

+

Metal Nanoparticle

−

− − −−

electrons in metal nanoparticles due to the oscillating incoming excitation (resonance excitation) of angular frequency (ωinc ) and amplitude (E0 ) causing a charge separation. The electron density (N) in metals is sufficiently high and the Coulomb interaction is main reason for coupled motion of the electrons. This collective motion of the electron is harmonic in nature and it is called surface plasmon frequency (ωSP ). This frequency is defined under random phase approximation as followsωSP =

4π N e2 ∈∞ m e

(6)

where ∈∞ and m e are the high frequency dielectric constant and the effective mass of electron, respectively [22]. The plasmon resonance lies in the visible or near UV region for metals like Ag and Au, however, the plasmon frequency in semiconductor materials lies in the infrared region due to the low density of electrons in the conduction band [23]. The optical properties are very crucial in SERS detection and these properties of bulk materials are characterized by the dielectric function√∈ (ω). This function is directly linked to the refractive index of materials i.e. n(ω) = ∈ (ω).The material is suitable for SERS applications if the real part of the dielectric function material is negative and has large value and its imaginary part of the dielectric function is small. The alkali and noble metals (Cu, Ag, Au) fulfill these properties and hence nanoparticles of these materials have been widely used for SERS applications [16, 24–26]. The silver is suitable for SERS applications as it has a very small imaginary part of dielectric constant in the visible and IR region. EM enhancement is occurred due to the excitation of LSPR mode because of resonance frequency of incident laser light at metallic nanostructures [17]. Local dipoles are created upon incident light which enhances the localized electric field around the metal nanostructures as shown in Fig. 3 and magnitude of the induced dipoles depends as discussed in Eq. 1 [15]. The sign of this dipole changes periodically with the external driving force i.e.

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the incident electromagnetic wave and a Hertzian dipole on the nanoscale is generated. This Hertzian dipole can emit frequency at the same frequency as the incident wave. Another dipole moment is created in the analyte molecule due to the enhanced localized electric field around the metal nanostructure.

3.1.2

Chemical Enhancement Theory of SERS

In different SERS experiments, it is observed that the plasmon theory could not explain alone the overall enhancement. In a system, where both the mechanisms are simultaneously working, then the effects are multiplicative. Chemical enhancement is mainly depending on the local electronic structures of both the probe molecule and the SERS substrate [27, 28]. The chemical enhancement is the combination of non-resonant charge in the molecular polarization, charge transfer between analyte molecule and SERS substrate and enhancements from molecular excitation resonances. The non-resonant chemical enhancement is independent of the excitation wavelength as Raman scattering process is explained on the basis of virtual energy levels (shown in Fig. 1). However, the molecular excitation resonances arise when Raman scattering happens via electronic levels of the analyte instead of virtual states [29].

3.1.3

Relative Magnitude of Enhancement of Different Mechanisms

A unified explanation of SERS was explained by Herzberg–Teller coupling, which includes the surface plasmon resonance (SPR), charge-transfer resonance and molecular resonance [23, 30]. Figure 4 shows a hypothetical illustration of the spectral behavior and relative magnitude of the enhancement of different mechanisms. Nonresonant chemical enhancement is independent of excitation frequency and the effect is moderate, which produces Raman signal enhancements between 100 and 102 [29]. The excitation wavelength (resonance condition) plays an important role to enhance Fig. 4 Hypothetical example of the spectral dependence of SERS [29]

Total enhancement

Resonant Raman Electromagnetic Charge transfer Static chemical

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the signal. Even if the excitation wavelength cannot match exactly to the resonance condition, it can influence the SERS enhancement via an intensity borrowing mechanism [23]. Figure 4 depicts that the electromagnetic enhancement shows relatively higher magnitude of enhancement among the others enhancement mechanism.

3.2 Enhancement Factor (EF) Since the discovery of SERS, correct estimation for the “magnitude” of the enhancement remains a challenge for scientific community. The effect of SERS on detecting a probe molecule is characterized by Enhancement factor (EF), which is a pure number. The electromagnetic and chemical enhancement is contributed to the total EF in SERS study. The EF depends on the SERS substrate, excitation source and the analyte molecule and its value can be found anywhere in the range 103 –1014 [31]. In case of SERS measurement, a diversity of situations can arise such as single molecule, multiple molecules, distribution of analyte molecule on the surface and the averages over time etc., which make a single general definition of the EF impossible in a SERS process. Hence, there have been different ways of calculating EF. As an example, single-molecule enhancement factor (SMEF) is suitable for theoretical estimations of the EF, while substrate specific enhancement factor (SSEF) is the most useful and used definition [32]. Here, we will discuss different definitions for EF in SERS detection process.

3.2.1

The Single Molecule Enhancement Factor

It is the increase of the conventional Raman scattering at certain localized position for a given molecule at a specific point on the SERS substrate. It depends on the Raman tensor and orientation of the analyte molecule on the SERS substrate. It also depends upon the polarization and direction orientations of incident laser. Hence, to avoid conflicts with factor like the orientation of the molecule, the single-molecule enhancement factor (SMEF) can be expressed as follows I SM  EFSM =  SERS SM IRS

(7)

SM SM and IRS represent the intensities of SERS signal for single-molecule where ISERS and the average Raman signal per molecule in the absence of surface enhancement, respectively [31].

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3.2.2

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The SERS Substrate Enhancement Factor

Many of the SERS experiments show irrelevant and excess value of EF on substrates. Thus, it is important to define SERS substrate enhancement factors (SSEFs) separately. This enhancement factor has been widely used for calculating the average SERS EF and is expressed as follows [32, 33] ISERS NSurf EF = IRS NVol

(8)

where ISERS and IRS are the intensities of the SERS signal on the substrate and normal Raman signal on non-SERS substrate surface. The average number of molecules, NVol = CRS V is in the scattering volume (V) for the Raman measurement and the average number of adsorbed molecules is NSurf for SERS experiment. CRS is known as concentration of analyte molecules.

3.2.3

Analytical Enhancement Factor

The above definitions (SMEF and SSEF) of EF describe the intrinsic characteristics of the substrate and always do not directly relate to the experimental results. Hence, a question arises that how much SERS signal enhances under given experimental conditions as compared to the normal Raman. To address this question, analytical enhancement factor (AEF) is described as followsISERS CSERS AEF = IRS CRS

(9)

where CRS is the concentrations of the analyte and IRS is the normal Raman signal under non-SERS condition. CSERS is the possibly different concentrations (lowest detected concentrations) of the same analyte, which produces SERS signal of intensity ISERS under same conditions like excitation wavelength and laser power, objective lens and spectrometer etc. [31].

3.3 Charge Transfer Process Between Substrate and Analyte The reasons behind the enhancement in SERS signals of analyte using appropriate substrate need to be understood in order to get a complete idea of SERS process. There is no fixed mechanism which is unique for improved SERS signals in all substrates. As described in Sect. 3.1.3, different physical/chemical processes play

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key role in improving the signal. The SERS substrates have been mostly classified as metallic and semiconducting substrates. In this section, we will discuss the possible charge transfer mechanism in each of these substrates separately.

3.3.1

Metal Substrate and Analyte Molecule

The plasmon and molecular resonances occur in metals and analyte molecules, respectively. Once both metals and molecules come in to contact of each other, a charge transfer (CT)process occurs between them. Hence, in order to explain the SERS enhancement in metal-molecular system, three types of resonances are considered namely surface plasmon resonance, molecular resonance and the CT resonance at the fermi energy. In last few years, numerous experiments and theoretical approaches have been invoked to find the influence of these resonances. John R. Lombardi and Ronald L. Birke in 2008 explained the unified approach to SERS in metal-molecular system [23]. They observed that these three terms are not totally independent to each other. The wavelength dependence of these resonance showed that CT occurs between the metal conduction band and the molecular highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels [34]. They observed thatthe maximum enhancement occurs in that system when transition to or from the levels is located at Fermi energy level. The polarizability (α) of a molecule is the sum of the three terms (A, B and C terms). The CT process in metal-molecular system can be represented schematically, as shown in Fig. 5. The I and K are the molecular state and F is the charge-transfer state (Fermi energy state of metal).The A is a sum of terms with Frank–Condon integrals, which

K μFK

hFK F μIF I

B term

μIK

hIF C term

Fig. 5 Schematic of energy level diagram of metal–molecule system for B and C polarizability term [23]

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C term

B term

LUMO

hCK

LUMO

hCK EC μex

μmol

μCT

μex

μCT hIV

hIV

EV Semiconductor

μmol

HOMO Molecule

HOMO Semiconductor

Molecule

Fig. 6 The coupling diagram for B-term and C-term in semiconductor-molecule system [36]

vanishes at far from the resonance and it is considered to be responsible for resonance Raman. The A term allows only the totally symmetric Raman line. The others two terms B and C represent the Herzberg-Teller contribution and the CT transitions. In B and C terms, the transition borrows intensity from the nearby allowed molecular transitions via the Herzberg-teller coupling constant (h). The B or C terms must be involved in the intensity enhancement in non-totally symmetric modes. These terms may also affect the totally symmetric bands. When the excited wavelength falls in the region of a CT or molecular resonance, the high enhancement observed in Raman spectra. The SERS intensity of a system is proportional to the square of the polarizability and the equation of polarizability according to the previous work reported in metal-molecular system can be represented as follows [35] RIFK (ω) = 

(ε1 (ω) + 2ε0 ) + 2

μKI μFK h IF i|Q k | f    2    2 2 2 ωIK − ω2 + γIK ωFK − ω2 + γFK (10)

ε22 (ω)

where RIFK (ω) is a polarizability term and the Raman intensity (I) is the square of the polarizability term i.e. | RIFK (ω)|2 .[2] For the B term, the transition occurs from the ground state I to the excited state K via μ I K andthe CT transition from molecule to metal fermi state F via μ I F . The charge transfer state F and excited state K are connected via Herzberg-Telle vibronic coupling term hFK . For C term, the charge transfer transition from fermi state F to excited state K via μFK and the state F and I are connected through Herzberg-Teller vibronic coupling hIF . The denominator in the Eqs. 10 is the product of three different resonances.  These three terms are directly contributing to the SERS intensity. The first term (ε1 (ω) + 2ε0 )2 + ε22 (ω) is due to the plasmon resonance at ε1 (ω) = −2ε0 , where ε1 and ε2 are real and imaginary parts of the SERS substrate, respectively. The ε0 is the dielectric constant

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  2 2 of the surrounding medium. The second resonance term ( ωFK − ω2 + γFK ) shows the potential dependent and charge transfer resonance occurring at ω = ω FK . Third   2 2 − ω2 + γIK ) represents the molecular resonance occurring at resonance term ( ωIK ω = ωIK .[35] For further details on charge transfer study on metal–molecule system, one can go through Ref. [23].

3.3.2

Semiconducting Substrate and Analyte Molecule

In semiconductor-molecule system, mainly three resonances contribute to the SERS signal enhancement, which are exciton resonance, charge transfer and molecular resonances [36]. The intensity of a Raman transition depends on the polarizability tensor of the materials in the following form 2  ασρ I = 8π (ω ± ω I  I )4 I L /9C 4

(11)

where I L is the intensity of laser at angular frequency ω and the molecular transition frequency is ωI I between states I and I . The term ‘α’ is the polarizability of the molecule and the three directions in space (X, Y, Z) are represented by subscripts σ and ρ. The polarizability (α) of the molecule is the sum of the three terms and expressed asασρ = A + B + C

(12)

Lombardi and Birke discussed the enhancement mechanism in semiconductormolecule system. The charge-transfer transitions moment μ I C represents the charge transfer from HOMO level to the conduction band edge C of the semiconductor. Similarly, transition moment μV K represents the charge transfer transitions from valence band edge V to the molecule LUMO level. The CT can borrow the intensity either from the molecular transitions μKI or from the excitation transitions μVC . The SERS intensity is proportional to |R|2 and the A, B and C terms can be expressed as follows [36] A-Term: This term can be written as μIC μIC i|kk| f     2 (ε1 (ω) + 2ε0 )2 + ε22 (ω) ω2I C − ω2 + γIC

(13)

μVK μVK i|kk| f     2 2 − ω2 + γVK (ε1 (ω) + 2ε0 )2 + ε22 (ω) ωVK

(14)

RIC (ω) =  RVK (ω) = 

where the real and imaginary parts of the permittivity of the materials are ε1 and ε2 . The ε0 is the permittivity of free space and γ is the damping factor. The first term in both the equations in the denominator is the plasmon resonance term. The resonance terms in the above equations can occur via charge transfer either from HOMO level

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to conduction edge charge transfer (at ω = ω I C ) or the valance band edge to the LUMO level (at ω = ωVK .). B-Term and C-Term: The schematic representation of different transitions involved in B and C terms of Eq. 12 is given in Fig. 6. The B term can be expressed asμKI μIC h CK i|Q k | f    2    2 RICK (ω) =  2 2 ωKI − ω2 + γKI − ω2 + γIC (ε1 (ω) + 2ε0 )2 + ε22 (ω) ωIC (15) μVC μIC h IV i|Q k | f    2    2 RICV (ω) =  2 2 2 2 ωVC − ω2 + γVC − ω2 + γIC (ε1 (ω) + 2ε0 ) + ε2 (ω) ωIC (16) The B term explains the charge transfer from the molecule to the semiconductor. This term explains the charge-transfer resonance coupled with molecular resonance via Herzberg-Teller constant. The B term is obtained from charge transfer from molecular HOMO level to the CB edge at ω = ωIC . The large enhancement occurs when it happens at molecular transition at ω = ωIK or exciton transition at ω = ωVC . The intensity is borrowed from either molecular transition (Eq. 15) or exciton transition (Eq. 16). The h IV and h CK are the Herzberg—Tellervibronic coupling terms involved in charge transfer process. [36] The C term can be expressed asμVK μKI h IV i|Q k | f    2    2 RIVK (ω) =  2 2 2 2 ωKI − ω2 + γKI − ω2 + γVK (ε1 (ω) + 2ε0 ) + ε2 (ω) ωVK (17) μCV μVK h KC i|Q k | f    2    2 RKVC (ω) =  2 2 − ω2 + γVK ωCV − ω2 + γCV (ε1 (ω) + 2ε0 )2 + ε22 (ω) ωVK (18) The C term explains the charge transfer from the semiconductor to the molecule. It explains the charge transfer from VB edge to molecular LUMO level as shown in Fig. 6. In the case of semiconductor-molecular system, the enhancement occurs when molecular transitions (at ω = ωIK ) or an exciton transition (at ω = ωCV ) is coupled with other resonance via Herzberg-Teller coupling (h CK or h IV ). The intensity is borrowed from either molecular or exciton transitions as shown in Fig. 6.

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4 Examples of Different SERS Substrates Different metallic and semiconducting SERS substrates have been investigated in last few decades for the detection of organic impurities, biomolecules and/or drugs. Researchers have observed and analyzed the effect of shape, size, surface roughness etc. of nanomaterials substrates for SERS detection. Here, we provide some of the examples of metallic and semiconducting SERS substrates.

4.1 Metal SERS Substrates The metal nanostructures (Au, Ag, Cu etc.) are still known as the best candidates for high-efficiency SERS applications. There have been several studies especially on Au and Ag nanostructures for SERS detection of organic impurities and drugs. Tian et al. studied different shapes and size of Au nanostructures for SERS detection. They synthesized three different shapes of Au nanostructures- sphere, triangle and star shapes [37]. In this work, Au nanospheres were synthesized via the seed-mediated growth method and nanotriangles were prepared by chemical reduction method while nanostars were synthesized via surfactant-directed, seed-mediated growth method. The SEM images of prepared Au nanospheres, nanotriangles and nanostars in this work are shown in Fig. 7. Researchers used these nanostructures for the detection of Rhodamine 6G (R6G) molecules. The SERS spectra of R6G molecule in suspensions of Au nanostars, nanotriangles and aggregated nanospheres are shown in Fig. 7d. They observed that the enhancement increases as shape varies from nanospheres < nanotriangles < < nanostars. The different enhancement occurs due to the difference in number of intrinsic hotspots per particle. The number of hotspots per particle increases in the following order- nanospheres < nanotriangles < nanostars. In SERS enhancement, hotspots are the locations in very close to the nanostructures, in which the local electric (Eloc ) field enhanced largely due to the external electric field as compared to its surroundings electric field [38]. As a result, if any molecule present in a SERS-active hotspot then huge enhancement in the signal is observed. In another study on flower-like Au nanostructured array for SERS detection, Kim et al. prepared such structure with varying thickness from 5.1 to 49.6 nm using photolithography (top-down) and electro-deposition (bottom-up) methods [24]. Figure 8a shows the SEM image of the synthesized flower like Au nanostructure. The Authors detected two different analytes, Benzenethiol (BT) and Brilliant cresyl blue (BCB) and showed that rougher Au nanostructure exhibit higher SERS enhancement. Fig. 8b shows the SERS spectrum of BT indicating strong signal due to the hotspots at the surface of the SERS substrate. Figure 8c shows the SERS spectra of BCB molecule of different concentrations and the inset image shows the zoomed view of the lowest concentrations (1 nM). The main reason behind the SERS detection is electromagnetic enhancement due to the sharp tips and valleys on the Au nanostructures. The Ag nanostructures are also known as suitable SERS

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Fig. 7 The SEM image of the Au nanostructures. a nanospheres b nanotriangles and c nanostars and d The comparison of SERS spectra of 5 micro molar Rhodamine 6G. (Adapted with permission from [37]. Copyright 2020 Royal Society of Chemistry

substrate along with Au for the detection of organic and biomolecules. Wang et al. prepared silver/silicon nanoporous pillar arrays (Ag/Si-NPAs) by an immersionplating method for the detection of R6G molecules [25]. They controlled the size of the deposited Ag nanoparticles by tuning the immersing times (1, 3, 5 and 10 min). The FESEM image of the Ag/Si-NPA for immersion-plated for 3 min is shown in Fig. 9a, b. The SERS spectra of R6G (10–15 M) with different Ag/Si-NPA substrates are shown in Fig. 9c. Among the four spectra, the most intense peak was observed by the substrate immersion plated for 3 min and the weakest one was observed by the substrate immersion plated for 1 min. They observed the correlation between peak intensity and the deposited silver nanoparticles size. In addition to Au and Ag, Cu metal has also been used as SERS substrate. In one of the study, Maurizio et al. synthesized colloidal Cu spherical nanoparticles of mostly size around 3–9 nm by laser ablation method in aqueous solutions. The TEM image of the prepared spherical Cu nanoparticles is shown in Fig. 10a [26]. Figure 10b, c shows the SERS spectra of phen and bipy in aqueous Cu suspension and with deposited Cu particle layer. The SERS spectra under 785 nm excitations (deposited Cu nanoparticles) were found to be more intense and stronger as compared to 514.5 nm excitation (in aqueous colloidal suspension). The presence of localized

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Fig. 8 a The SEM image of flower-like Au nanostructure array SERS substrate. SERS spectra of b BT and c BCB molecules. Inset shows the close-up view of 10–9 M spectra. Adapted with permission from [24]. Copyright 2020 Institute of Physics

Fig. 9 a The FESEM images of Ag/Si-NPAs for immersion-plated for 3 min. and b at high resolution. c SERS spectra of R6G molecule at 10–15 M concentrations over Ag/Si-NPAs SERS substrate with different at different immersion-plating time: a 1 min b 3 min. c 5 min and d 10 min. Adapted with permission from [25]. Copyright 2020 Elsevier

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Fig. 10 a TEM image of Cu/phen colloid. Inset is the core–shell image of a Cu particle. b SERS spectra of Phen in Cu colloid (red) and from a dry Cu particle layer (blue) c SERS spectra of bipy in Cu colloid (red) and with a dry Cu particle layer (blue). Adapted with permission from [26]. Copyright 2020 American Chemical Society

surface Plasmon resonance (LSPR) band in the visible region is the main reason behind the origin of SERS spectra. Among conventional metals, Au is very costly, while Ag and Cu get easily oxidized which reduces their detection efficiency. In order to address this issue, researchers investigated Graphene as SERS substrate. Graphene is a single layer of sp2 hybridized carbon atoms having delocalized pi electrons. It neither acts as complete metal system nor like semiconducting system but it behaves like a semimetal. The use of graphene for SERS was performed in 2009 [11]. Ling et. al. used the graphene for the detection of dye molecule such as phthalocyanine (Pc), R6G, PPP, and crystal violet (CV) [39]. They prepared the Graphene substrate over SiO2 /Si via mechanical exfoliation using scotch tape. Figure 11a shows the Raman spectra of R6G in water (blue line) and the Raman spectra of R6G on singlelayer graphene substrate. In case of R6G in water, a strong fluorescence (FL) was observed and FL was reduced in case of SERS spectrum of R6G on graphene. The observation of the R6G peak on a graphene substrate attributed to graphene-induced FL quenching [11]. The Raman intensity of Pc molecule on monolayer graphene was found much higher than on the non-graphene area, as shown in Fig. 11c. This enhancement is due to the chemical mechanism and it is a short-range effect that required the coupling of molecule with substrate. Recently we also demonstrated the SERS applications of few layer reduced graphite oxide (rGO) prepared via hydrothermal method [40]. In this work, reduction of the prepared GO was performed using two different reducing agents, hydrazine hydrate and urea to make rGO-HH and rGO-Urea. The rGO-HH and rGO-Urea were used for R6G detection. To prepare the SERS substrate, rGO surface was modified with R6G solutions (different concentrations 10−3 M to 10−6 M) and the prepared mixture was drop casted over cleaned Si substrate. The SEM images of rGO-HH and rGO-urea are shown in Fig. 12a, b, respectively. The SERS spectra of R6G (1 mM to 1 μm) adsorbed on rGO-HH and rGO-Urea are shown in Fig. 12c, d. In this experiment, we detected upto micromolar concentrations of R6G on both the rGO samples. The main reason behind the detection of micromolar concentrations is charge transfer between the adsorbed R6G molecule and rGO.

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Fig. 11 a Schematic diagram of SERS detection over Graphene. b Under 514 nm laser excitation the Raman-FL spectra of R6G in water (10 μM) (blue line) and R6G on a 1L graphene (red line). c SERS spectra of Pc molecule on graphene (red) and on SiO2 /Si substrate (blue). Adapted with permission from [11, 39]. Copyright 2020 American Chemical Society

4.2 Semiconducting SERS Substrates In last few years, semiconducting metal oxides have been widely used as cost effective alternatives for metallic SERS substrates. Different researchers have demonstrated use of ZnO, TiO2 and CuO nanostructures for SERS detection of organic pollutants [12, 41–43]. Our research group has demonstrated the efficiency of hydrothermally synthesized semiconducting ZnO nanoparticles as SERS substrate. The SEM image of the synthesized ZnO nanoparticles is shown in Fig. 13a, which indicates the spherical shape of nanoparticles. We showed the detection of Methylene Blue (MB) and Methyl Orange (MO) of nano-molar concentrations using semiconducting pristine ZnO nanoparticle.[41] The SERS spectra of MB and MO for concentrations 10−5 to 10−9 M are shown in Fig. 13b, c, respectively. The graph clearly shows the significant peak intensity for 1626 cm−1 and 1390 cm−1 at their lowest concentrations (10−9 M) for MB and MO, respectively. The peak intensity of 1626 cm–1 and 1390 cm−1 peaks increase linearly with the concentrations of MB and MO, respectively as shown in Fig. 13d. Additionally, we demonstrated the self-cleaning property of used SERS substrate under UV light irradiation in aqueous medium. Further cleaned SERS substrates were reused for the SERS detection, which showed similar detection limit as of fresh ZnO nanoparticles-based SERS substrates suggesting the high degree of reusability of ZnO nanoparticles as SERS substrates.

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Fig. 12 SEM image of a rGO-HH and b rGO-Urea. SERS spectra of R6G at different concentrations on c rGO-HH and d rGO-Urea. Adapted with permission from [40]. Copyright 2020 Institute of Physics

In recent years, apart from Graphene, there have been many other 2D materials such as MoS2 , MoSe2 , SnS2 , WS2 etc. which are being investigated as semiconducting SERS substrates. These semiconducting 2D materials follow chemical mechanism for SERS detection [30, 44]. Recently, Hikari et.al. investigated the Raman enhancement of CuPc molecule on nine different 2D materials [30]. They observed that the Raman intensity depends on the thickness of the 2D materials and found that the intensity is inversely proportional to the thickness of the 2D materials. Figure 14a shows the sample preparation process of 2D materials via mechanical exfoliation method. They used a vacuum thermal evaporation method to deposit CuPc molecule over SiO2 /Si substrate. Figure 14b shows the Raman spectra of CuPc molecule on SiO2 /Si and on different 2D materials. They observed the enhancement effect on nine different 2D materials and they calculated it using flowing relationEF =

Ion 2D Ion sub

(19)

where Ion 2D and Ion sub are the intensities of the vibrational mode of CuPc molecule on 2D materials and SiO2 /Si substrate, respectively. They compared the EF of the

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Fig. 13 a SEM image of ZnO nanoparticles. SERS spectra of b MB and c MO on ZnO nanoparticles as SERS substrate. d SERS intensity versus dye concentrations for both dyes. Adapted with permission from [41]. Copyright 2020 Institute of Physics

Fig. 14 a The schematic diagram of sample preparation process. b SERS spectra of CuPc molecule over nine different 2D materials. c Enhancement Factor of different 2D materials of the 1528 cm−1 vibrational mode. Adapted with permission from [30]. Copyright 2020 American Chemical Society

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Fig. 15 a SEM image of few-layer MoS2 . b Raman spectra of R6G molecule over few-layer MoS2 substrate. Adapted with permission from [44]. Copyright 2020 American Chemical Society

1528 cm−1 mode on different 2D materials as shown in Fig. 14c. They found that WSe2 , SnS2 and WTe2 shows good enhancement for CuPc molecule which is comparable to enhancement using graphene as SERS substrate. They also observed the Raman spectra of CuPc molecule on different thicknesses of SnS2 flake, ranging from 14 to 45 nm and found that the Raman signal of the CuPc molecule decreases with increasing thickness. The enhancement is due to the chemical mechanism involved in SERS process comprising of three distinct processes: charge transfer resonance, molecular resonance, non-resonant ground state chemical enhancement. Our research group is also actively involved in SERS application of 2D materials. In one of the study, we demonstrated the SERS detection of R6G at nano-molar concentrations using chemical vapor deposition (CVD) grown pristine film of few layer horizontal MoS2 over Si substrate [44]. At the time of study, this was the highest SERS detection limit using pristine MoS2 nanostructure. Figure 15a shows the SEM image of the prepared few-layer MoS2 film suggesting the formation of uniformly distributed and interconnected islands of few-layer MoS2 . The Fig. 15b shows the SERS spectra of different concentrations of R6G molecules using few-layer MoS2 film, which indicate that the SERS signal intensity increases with the increasing concentrations of the analyte molecule. The surface roughness plays an important role in adsorption of dye molecule in a given length scale. The SERS enhancement in MoS2 -R6G system occurs due to the charge transfer and molecular resonance. Further, we also demonstrated self-cleaning property of MoS2 for under visible light illumination and reused the cleaned SERS substrate for R6G detection. The surface roughness, active sites, light absorption and dye adsorption properties of SERS substrate materials play an important role for the detection of organic pollutants. Hence, there have been multiple efforts on developing novel morphologies of different semiconducting materials to improve the SERS efficiency, which can enhance the light absorption, light trapping and dye adsorption [45, 46]. In this regard, we developed a film of vertically oriented few-layer (VFL) MoS2 nanosheets over Si as SERS substrate via CVD method for the detection of organic dyes [47]. Figure 16a shows the SEM image of the synthesized VFL-MoS2 film, which clearly

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Fig. 16 a SEM images of an interconnected network of vertically oriented few-layer MoS2 nanosheets. SERS spectra of b R6G and c MO dyes at different concentrations. d Raman intensity versus dye concentration of R6G and MO molecules for 611 and 1180 cm−1 peaks, respectively. Adapted with permission from [46]. Copyright 2020 American Chemical Society

indicates the presence of large number of thin edges. These edges are helpful for improved light adsorption by multiple reflection and larger accessible surface area for improved dye adsorption [46]. Figure 16b, c show the SERS signals for R6G and MO molecules adsorbed on the surface of MoS2 /Si at different concentrations (10−6 to10−10 M), respectively. The higher concentrations of dye molecules show higher peak intensities. In this study, we observed the variation of Raman intensities of 611 and 1180 cm−1 peaks versus dye concentration for R6G and MO molecules, respectively. We found that the slope of 611 cm−1 (of R6G) is higher than the 1180 cm−1 (of MO) as shown in Fig. 16d, suggesting higher enhancement factor (EF) for R6G compared to MO. We calculated the EF for both molecules and found value of EF of 8.6 × 104 and 5.8 × 104 for R6G and MO molecules, respectively. The high detection limit i.e. up to sub-nanomolar concentration (10−10 M) can be attributed to the Herzberg − Teller vibronic coupling in different resonances in analyte/VFL-MoS2 system. Both the molecules show absorption in the visible range (R6G-526 nm and MO-467 nm), which is very close to the resonance with the excitation laser source of wavelength 532 nm. Hence, three different mechanisms responsible for the enhancement are molecular resonance, CT resonance and the surface interaction between MoS2 and

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dye molecule. The vertical orientations of MoS2 provides a larger accessible area for effective dye absorption and also enhance the light trapping, which leads to an improved charge transfer process. Such nanostructures can be easily scaled up and used as SERS substrates for technological applications.

5 Summary In this chapter, we have discussed the basic principle of SERS process on metallic and semiconducting substrates. The mechanism for enhanced Raman signal in SERS process has been discussed for both the SERS substrates. The main origin of SERS on metallic substrate is the electromagnetic enhancement and for semiconducting substrate is chemical enhancement. Different calculation methods for enhancement factor (EF) have also been discussed in this chapter. At last, we have discussed few examples of SERS study on different metallic and semiconducting substrate for the detection of organic pollutants. Advancement in the area of 2D materials like Graphene, MoS2 etc. as SERS substrates have also been discussed.

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Tip-Enhanced Raman Spectroscopy Takayuki Umakoshi and Prabhat Verma

Abstract Visible light can interact efficiently with the vibronic and electronic systems of a sample and fetch rich information about the intrinsic features, such as the chemical, physical and biological properties of the sample. Optical techniques have therefore been convenient tools for a long time to analyze and image various materials. However, the spatial resolution in optical microscopy is restricted by the diffraction limit of light, making it impossible to study samples much smaller than the wavelength of the probing light. This restriction can be overcome if a conventional optical microscopy, such as Raman microscopy, is combined with near-field techniques. Tip-enhanced Raman spectroscopy (TERS) is such a technique. It utilizes a sharp metallic nano-tip to intensely enhance and strongly confine light within a tiny volume near the tip-apex and enables characterization of samples at the nanoscale. In this Chapter, we discuss the details of this technique and explain how light can be tightly confined into a nanometric volume for true nanoscale exploration of samples. TERS is still a young technique and has been going through a rapid development in the past two decades, which has not only made it more reliable and sturdier over the period, but has also brought this apparently complicated technique out from the laboratories of the veterans to the market for researchers who are experts in different fields. This has obviously happened with improved adaptability, flexibility and robustness with possibilities of a wide range of applications. We will discuss some interesting applications and related instrumentations for TERS. Keywords Tip-enhanced raman scattering · Plasmonics · Near-field optics · Near-field scanning optical microscopy · Raman spectroscopy

T. Umakoshi · P. Verma (B) Department of Applied Physics, Osaka University, 2-1 Yamadaoka, Suita 565-0871, Osaka, Japan e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 D. K. Singh et al. (eds.), Modern Techniques of Spectroscopy, Progress in Optical Science and Photonics 13, https://doi.org/10.1007/978-981-33-6084-6_8

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1 Introduction Tip-enhanced Raman spectroscopy (TERS) is an interesting combination of Raman spectroscopy and plasmonic near-field optical technique that takes Raman spectroscopy and microscopy to the next level beyond its conventional limits to establish it as an inevitable optical tool to investigate various materials at the nanoscale [1–3]. Raman spectroscopy has already been well-recognized as a powerful tool to investigate molecular vibrations by spectroscopically analyzing Raman scattering from molecules. Due to the inherent spectral and chemical specificities, it tells us which chemical bonds the molecules have and how the bonds vibrate [4]. As it is also an invasive and non-contact method, it has been widely used in a variety of research fields from material science to biological science as a versatile tool [5–7]. It has contributed in analyzing the physical properties of novel materials and in revealing molecular activities of biological systems. However, just like most of the classical optical techniques, Raman spectroscopy also suffers from the issue of limited spatial resolution due to the wave nature of light. This is because an incident light cannot be focused into a size smaller than approximately half of its wavelength due to the diffraction limit of light. For the visible light that is often used in Raman spectroscopy, the spatial resolution is limited to a few hundred nanometers. Improvement of the spatial resolution in Raman spectroscopy has been in high demand, especially since nanotechnology and nanoscience have become of great importance and interests in various scientific fields. TERS is one of the distinctive solutions to meet this demand. It is the only technique that measures scattered light from a nanometric volume of the sample and thus enables one to obtain Raman image of samples with a true nanoscale spatial resolution (typically 10–20 nm). Although there are several super-resolution optical microscopy techniques such as STED, PALM or STORM [8, 9], these techniques do not include a direct optical observation at nanoscale. TERS is the only technique where light is spatially confined to a true nanometric volume, which is done through a plasmonic approach known as the near-field scanning optical microscopy (NSOM). In NSOM, an aperture probe was initially used to generate near-field light at the tiny aperture, which is so-called aperture-type NSOM [10]. Later in 1994, Kawata et al. reported a new type of NSOM that uses an apertureless metallic tip instead of an aperture tip, which is known as scattering-type (or apertureless-type) NSOM [11]. In the scattering-type NSOM, a metallic tip that works as a plasmonic antenna is used to generate near-field light that is highly localized at the end of the tip apex [12]. Because the spatial extent of the near-field light is almost comparable to the size of nanometrically small tip apex, the tip apex practically behaves as a nano-light-source that invokes Raman scattering from a tiny volume of the sample directly beneath the tip apex. One can therefore detect Raman scattering signal from a nanoscale area of the sample with the near-field light. Therefore, by detecting Raman signal with the near-field light at the tip apex, nano-Raman spectroscopic analysis is possible. Actually, this is the basic idea of TERS. It was reported for the first time in 2000 by three different groups at almost the same time [13–15]. Since then, TERS has

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been a key technology in nanoscience, and many researchers have been involved in developing and improving TERS. In the past two decades after it was first reported in 2000, various new and exciting technological developments have been established to improve detection sensitivity, spatial resolution, imaging speed, and so forth. In particular, a spatial resolution of the single molecular level or even the atomic level has been recently achieved [16]. Measurement capability of TERS is still growing owing to the tremendous ongoing efforts from researchers. As a comparison, in terms of optical microscopy with the high spatial resolution, one may first come up with the super-resolution fluorescent microscopy, which was awarded the Noble prize in Chemistry in 2014 for its prominent contribution to the life science by imaging nanoscale details of biological samples [8, 9]. However, it is literally a technique to observe fluorescent label, which is tagged with a target molecule and therefore contains no information about the intrinsic properties of the target molecules. It is therefore not possible to observe an optical signal originating from the sample itself with this technique. In this sense, as TERS achieves the super-resolution imaging using a physically small light source at the tip apex, nanoscale Raman analysis is possible. Although both TERS and the super-resolution fluorescent microscopies are categorized in the super-resolution optical microscopy, TERS has such a clearly different property. Also, some other far-field techniques to improve the spatial resolution in Raman microscopy have been recently proposed and demonstrated [17, 18]. However, as mentioned above, only TERS can reach down to 10–20 nm or even single molecular level of the spatial resolution. In this chapter, we describe this powerful technique from the basic principle to recent important developments.

2 Basic Principle of TERS 2.1 Field Enhancement by Localized Surface Plasmon Resonance (LSPR) In Raman scattering process, when molecules are irradiated with light, they scatter light that has a wavelength shifted from that of the incident light [4]. As the wavelength shift occurs due to the interaction between the incident light and the vibration of molecular bonds, it contains a signature of the physical and chemical properties of the sample, and thus one can obtain information about molecular bonds by analyzing Raman scattering. Raman scattered signals are detected through a spectroscope in the form of Raman spectrum, an example of which is shown in Fig. 1a. Compared to fluorescence spectroscopy, a drawback in Raman spectroscopy is the low detection sensitivity due to the intrinsically weak Raman signal. This is why surface-enhanced Raman spectroscopy (SERS) technique has received much attention in wide research fields [19]. SERS can dramatically improve the sensitivity by strongly enhancing both the incident and the scattered light in Raman spectroscopy.

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Fig. 1 a Raman scattering process and a typical example of spectroscopically obtained Raman spectrum. b A comparison between normal Raman and SERS spectra of carbon nanotubes. c Enhancement process of Raman scattering through localized surface plasmon resonance of a single metallic nanoparticle

In general, SERS utilizes a metallic surface with nanoscale roughness as the substrate for holding the sample during Raman measurement. The roughness on the metallic substrate behaves as a closely packed collection of metallic nanoparticles that has free surface charge carriers, a quantum form of which is known as the plasmons. Depending upon the kind of metal and the quality of roughness, these plasmons can be resonantly excited, if irradiated with the light of a suitable wavelength that carries the same energy as the energy of natural oscillation of these plasmons. This phenomenon is known as the localized surface plasmon resonance (LSPR) [20, 21], which creates an oscillating electric field in the close proximity of the surface, resulting in the generation of localized near-field evanescent light in the immediate neighborhood of the surface. Under the resonance condition, strong plasmon oscillations are excited that generate highly localized and strongly enhanced light field in the vicinity of the metallic surface. This strongly enhanced light field drastically enhances Raman signals from molecules when the molecules sit close to the metallic structure. Raman spectra measured under such enhancement is called SERS, an example of which is shown in Fig. 1(b). Enhancement factor of Raman signal in SERS can be as high as 108 under certain conditions, which is high enough for the single molecule sensitivity [22, 23]. The enhancement in SERS is associated with the enhancement of the incident light as well as Raman scattered light. In addition to this plasmonic enhancement, the chemical enhancement resulting from a possible charge transfer may also contribute to the total enhancement in SERS [24]. Here, we will consider only the plasmonic enhancement. In short, it is known that polarizability α of a metallic particle is described as. α = 4πr 3

ε1 − ε2 ε1 + 2ε2

Here, we assume that radius r of the metallic particle is much smaller than the incident wavelength so that quasi-electrostatic approximation is satisfied. ε1 and ε2 are the dielectric constants of the metallic particle and the media surrounding the

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metallic particle, respectively. The dielectric constant is a function of the wavelength. At a certain incident wavelength that satisfies the condition of ε1 = −2ε2 , the polarizability α diverges to infinity, which indicates the resonant excitation of the localized surface plasmons in the metallic particle [25]. In reality, the polarizability α cannot be infinity as we have to consider damping factor of the metallic particles. However, one can still obtain enormous enhancement of Raman signals as mentioned above.

2.2 LSPR at the Tip for TERS While SERS provides huge enhancement in Raman scattering to an extent that even an extremely weak Raman scatterer such as an isolated single molecule can be detected, it does not solve the issue of poor spatial resolution of conventional Raman spectroscopy. This is because the light field is confined only in the direction normal to the metallic substrate, and is not confined in the lateral direction along the substrate. In order to achieve the lateral confinement of the light field, one must reduce the size of the substrate to the range of a few nanometers. This is realized in TERS, where the nanometric apex of the metallic tip used in TERS behaves as the tiny substrate of SERS. In this sense, TERS can be considered as a special case of SERS where a single metallic nanoparticle sitting right at the apex of the nano tip invokes SERS [1]. Figure 1c illustrates how localized surface plasmons can be resonantly excited in a single metallic nanoparticle by an incident light that results in the creation of enhanced near-field evanescent light in the close vicinity of the nanoparticle. This enhanced light field is confined in all three directions within a volume compared to or smaller than that of the nanoparticle. If a sample is placed in this enhanced and confined light, Raman scattering can be invoked only from a tiny part of the sample that is immersed in this confined light. In this way, Raman scattering can be measured from a nanometric volume of the sample. If one would like to measure Raman scattering from different areas of the sample, but at a spatial resolution of a few nanometers, then it would be necessary to move this metallic nanoparticle along the sample surface while measuring Raman scattering at every position of the nanoparticle. An easy way of doing this is by replacing the metallic nanoparticle with a metallic nanotip that can be conveniently scanned over the sample surface by means of a scanning probe microscopy (SPM) control, such as an atomic force microcopy (AFM) or a scanning tunneling microscopy (STM). The apex of a metallic nanotip can be approximated with a single metallic nanoparticle, as illustrated in Fig. 2, which would also generate a highly confined and strongly enhanced light field at the very end of the tip apex. When Raman scattering from a sample placed beneath the apex of a metallic nanotip is enhanced in this way, it is called the tip-enhanced Raman scattering. An important difference between SERS and TERS is the confinement of light in the lateral direction that provides high spatial resolution to TERS. Here one can position the tip apex at an arbitrary location and scan it over the sample surface under the precise control of a SPM. This makes it possible not only to obtain nanoRaman signal from any arbitrary location of the sample, but also to construct Raman

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Fig. 2 Schematic of TERS, TERS image and a TERS spectrum of carbon nanotubes

image of the sample at the spatial resolution of nanometer scale by raster scanning the tip over the sample surface. Such an optical nanoimage is called TERS image, an example of which for carbon nanotubes (CNTs) is shown in the inset of Fig. 2. This TERS image of CNTs is constructed from Raman intensity at the G-band, which is a representative Raman mode of CNTs appearing at around 1590 cm−1 [26, 27]. The spatial resolution in this TERS image is far beyond the diffraction limit of light, also shown in Fig. 2. Since full Raman spectra are measured at every position of the tip, each pixel of the image contains information of all Raman modes, and therefore one can construct TERS images at any desired Raman mode with a single measurement. Comparison of Raman images constructed from different Raman modes enables one to understand the detailed distribution of chemical bonds and related information at nanoscale because different Raman modes appear from vibrations of different chemical bonds. This is not possible in the case of SERS as the position of the nearfield light is fixed. Moreover, TERS image is usually accompanied with a topographic image obtained simultaneously through the SPM. Therefore, a correlative analysis between chemical information and morphology of sample is also possible. Although the fundamental principle can be similar to SERS, practically TERS has strong advantages owing to the spatial confinement and the position controllability of the near-field light. As already mentioned, the typical spatial resolution in TERS imaging is about 10–20 nm, and even a spatial resolution of single molecular level is possible under certain conditions.

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3 Instrumentation for TERS 3.1 Experimental Setup Figure 3 shows one of the typical experimental setups for TERS. TERS realizes nano-Raman imaging by precise position control of near-field light by controlling the position of the tip apex. As it requires a SPM apparatus for the tip control, the instrumentation for TERS is usually much more complicated compared with that for SERS. In Fig. 3, an AFM system is integrated on an inverted optical microscope. Similar to an ordinary Raman spectroscopy, a single-mode continuous-wave (CW) laser that has extremely narrow band width is used as an incident laser. It passes through several optical components such as a beam expander, a wave plate, and a polarizer. It is then tightly focused through an objective lens to irradiate the tip apex located at the sample plane, which generates near-field light at the apex. Raman signals excited by the near-field light are collected through the same objective. In the detection path, an edge filter is set to block the Rayleigh scattering and allows only Raman signals to pass through. Raman signals are finally detected by a charge coupled device (CCD) detector through a spectroscope. Although it depends on the sensitivity required for a particular experiment, a Peltier-cooled or a liquid-nitrogencooled CCD is preferred for the sensitive detection of weak Raman signal. It should be noted that the near-field Raman signals are always accompanied with the far-field Raman signals, which are exited within the entire focal area by the incident laser.

Fig. 3 Schematic of a typical experimental setup of TERS. The inset shows a focus spot image obtained by the photo detector

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This creates an undesired far-field background in TERS measurements. A pure nearfield Raman signal is therefore obtained by subtracting Raman signal without tip (far-field signal only) from the Raman signal with tip (combination of near-field and far-field signals). One of the crucial processes in TERS measurements is to position the tip apex exactly at the center of the focus spot of the incident laser with nanoscale precision. Therefore, it is highly recommended to install piezo actuators to control relative position of tip and sample in both x- and y- directions, in addition to the z direction, which is for AFM feedback. By using x- and y- piezo actuators, one can rasterscan the tip within the focus spot while detecting the Rayleigh scattered signal by a photodetector so that one can obtain an image plotted by the tip-scattered intensity. It usually shows a concentric pattern formed by the incident laser, as shown in the inset of Fig. 3. This image represents the laser intensity distribution within the focus spot and helps one recognize the exact location where the intensity is the highest. By locating the tip apex exactly at the center of the concentric pattern using the piezo actuators, it is possible to properly excite near-field light at the tip apex with the maximum efficiency, and it is ready for the TERS measurements. Since the relative position of the tip and the focus spot is important for efficient excitation of the nearfield light, the positions of the tip and the incident focus spot are kept fixed, and the sample is moved in the x- and y- directions to search a region of interest or to facilitate the raster scanning for TERS imaging. Therefore, at least x- and y- piezo actuators are necessary for the sample stage. TERS imaging is performed by scanning the sample rather than scanning the tip. The sample stage scanner is synchronized with the CCD camera to obtain Raman spectrum at each pixel of the image. There are a few more important points about the experimental setup that should also be noted here. In the incident path in Fig. 3, a z-polarizer is inserted to convert linier polarization to radial polarization. As shown in Fig. 4a, the z-polarizer is composed of four or eight segmented half-wave plates, which rotates the linear polarization of incident light in each segment in such a way that the resultant polarization is converted into the radial polarization after the light passes through the z-polarizer [28]. When this radially polarized light bends into the focus spot as it gets focused through the objective lens, the lateral components of polarization cancel out while the z-components add up to result in the generation of a pure z-polarization at the focus spot, the direction of which is parallel to the tip axis. To excite the nearfield light underneath the tip, it is important that the plasmons oscillate parallel to the tip axis, i.e. in the z-direction. Therefore, a z-polarizer can efficiently induce near-field light at the very end of the tip. In contrast, a linear polarization in the absence of a z-polarizer generates the polarization parallel to the sample plane after the objective, because the z-components get cancelled in this case, as shown in Fig. 4b. It cannot thus efficiently excite near-field light at the tip apex. In addition to the z-polarizer, a spatial mask is also inserted in the incident path. It is basically an opaque disk with a diameter slightly smaller than the diameter of laser beam, which blocks the central part of the laser, and allows only the external part in the form of a ring to pass through. If the objective lens has sufficiently large numerical aperture (NA), the incident light from this external ring is focused at the sample plane at

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Fig. 4 a Z-polarization generated through a z-polarizer. b Polarization perpendicular to the tip axis, generated by focusing ordinary linearly polarized light

an angle larger than the critical angle, as illustrated in Fig. 5. Therefore, the total internal reflection occurs, which enables the so-called evanescent illumination to the tip apex, where no propagating light reaches the tip apex and it is illuminated only by the evanescent light. This reduces the possible far-field Raman scattering from the sample. Compared with the case without the mask, this evanescent illumination can therefore eliminate the unwanted far-field background scattering from the sample as Fig. 5 Evanescent illumination at the tip apex enabled by a spatial mask

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well as the possible scattering noise from the tip shaft since no incident light can transmit above the sample plane to hit the shaft or the upper part of the tip.

3.2 Some Advanced Instrumentations for TERS As positioning the tip into the focus spot is important, keeping the tip within the focus spot throughout the measurement time is also important. Since the near-field light at the tip apex can drastically reduce or vanish even with a position shift of a few tens of nanometers from the focus spot, it is crucial to suppress mechanical and thermal drift of the experimental systems. Every component such as the optical microscope, AFM, and cantilever tip can cause a mechanical or a thermal drift. One of the simple and effective ways to suppress the drift is to put the system in an enclosure box to block wind and sound from the external environment and to suppress the ambient temperature change or fluctuation. However, it is still not possible to completely eliminate the effect of the drift. It becomes more crucial, especially in the case of TERS imaging, as it requires to detect weak Raman scattering from multiple points. It usually takes close to 1 h or even more to obtain one TERS image of a reasonable size. In such a case, suppression of the drift by employing an enclosure is not enough, and a more active method such as compensation for the drift is required [29–31]. As the relative position between the tip and the laser focus can be shifted in any directions in the three dimensional space, drift compensation methods are required for all x-, y- and z- directions. In x- and y- directions, i.e. in-plane direction to the sample plane, the drift compensation can be easily achieved. As discussed with Fig. 3, one can obtain an optical image of the intensity pattern of the focus spot by scanning the tip on the focus spot, which can be used to position the tip apex in the right location within the focus spot. One can repeat the same process periodically at a certain time interval before a non-negligible drift occurs. For example, one can obtain the image of the focus spot every time after obtaining one line scan in a TERS image. If one finds that there is a shift between the tip position and the focus spot, the tip can be moved back to its original position before the next scan. In this way, if it takes, say, one minute to scan one line of the TERS image, it would be possible to compensate the drift every minute. For the compensation, it is of course better to make such an algorithm to automatically obtain the focus spot image and compensate the drift. It can be easily implemented by performing gaussian fitting to the focus spot image to find out the focus spot position. Although the tip-scanning configuration is discussed in Fig. 3, laser-scanning configuration with a galvano mirror system would be better because one needs to take the focus spot images multiple times during the TERS imaging. The galvano mirrors can in general scan so quick that the focus spot image can be obtained almost instantly [30]. In terms of the drift in z-direction, it is mostly caused by the drift of the objective lens. As the objective drifts in z-direction, the laser can be defocused at the sample plane, which would deteriorate the measurement. To actively compensate this drift, an objective lens positioner actuated by a piezo device is useful. It can precisely move

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the objective lens up and down with nanoscale precision. However, one first needs to sense the amount of drift to set off the feedback to the objective lens positioner. There could be several methods to do this. One of them is, for example, by utilizing a parallel-plate capacitor with variable capacitance. When the distance between the two metallic plates of a capacitor changes, the capacitance also changes with high sensitivity. If a capacitor is attached to the alignment system in such a way that the distance between the two metallic plates is mechanically changed with the change in separation between the sample and the objective lens, then one can precisely sense the change in this separation, which would correspond to the drift in the z-direction, by monitoring the change in the capacitance. Once the drift in z-direction is sensed by the capacitor in terms of an electric signal, it can send a corresponding feedback to the objective lens positioner [31]. Alternatively, an optical method could also be a good candidate to detect the drift in z-direction. A few major optics companies have already manufactured some optical microscopy systems that are integrated with the auto-focus system using optical drift detection. However, since they are not designed for nano-photonics applications, the movement of the objective lens is operated by a stepping-motor that moves in steps with jerk. It makes vibrations that affect SPM operation, and does not have enough accuracy. Therefore, a piezo-actuated system is highly recommended for TERS imaging. By implementing these compensations in three dimensions, one can keep tip within the focus spot for long time, which makes TERS more reliable and practical technique.

3.3 Laser Illumination Configurations Thus far we have discussed some practical suggestions for stable TERS measurements based on the TERS experimental system shown in Fig. 3. However, this is just one of several possible experimental configurations. One of the important bases to categorize the experimental setup is the configuration of laser illumination. In this sense, the TERS experimental setup can be characterized as the bottom-illumination, side-illumination, and top-illumination (Fig. 6). In the setup shown in Fig. 3, the bottom-illumination is used. The incident laser literally illuminates the tip from the bottom (Fig. 6a). In this configuration, the incident laser transmits through the substrate to illuminate the tip apex. Therefore, the substrate needs to be transparent. However, as one can bring the objective lens quite close to the substrate in this configuration, it is possible to use an objective lens with high NA that usually has short working distance (WD), which allows for an evanescent illumination and also provides high signal-collection efficiency [32–34]. In contrast, for the side- [35, 36] and top-illumination modes [37, 38], because one does not have to guide light from the bottom, it is possible to use opaque substrates and samples so that these configurations impose less restrictions on the sample choice. However, the disadvantage of these configurations is that the laser is illuminated from the same side of the substrate where the tip is located. This makes it technically

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Fig. 6 Laser illumination configurations for TERS. a Bottom-illumination, b side-illumination, and c top-illumination

difficult to bring the objective lens as close to the substrate as in the case of bottomillumination, making it impossible to use objectives with short WD or high NA. One must use an objective with longer WD or lower NA, which would have larger focus spot and lower detection efficiency. This would be an issue of concern, especially for TERS, a technique that observes very weak Raman signal. Historically, the top-illumination configuration emerged after the side-illumination. Therefore, the design of the top-illumination is improved to adopt an objective lens with somewhat higher NA compared with the side-illumination setup. However, to the best of our knowledge, maximum NA reported for top-illumination is approximately 0.7 with a WD of 10 mm. In order to avoid blocking the incident light with the body of the tip in top-illumination, one must use some special kind of cantilever tip, such as bird-beak-shaped cantilever tips, as illustrated in Fig. 6c. These cantilevers also help in reducing the WD.

3.4 AFM and STM for Tip Control The experimental configuration for TERS is also characterized by the type of SPM employed to control the tip position. Either a STM or an AFM is typically used for this purpose. In the case of STM, the substrate has to be conductive as it requires to facilitate a tunneling current [39, 40]. Also, if the substrate is conductive, but the sample is non-conductive and thick, then it is difficult to measure TERS as the tunneling current, which controls the tip position in z-direction, cannot be established. Therefore, small molecules or atomically thin samples are commonly used in STM-based TERS. It is often operated in vacuum or air, but it is not available in liquid environment. It would be therefore unsuitable if one wants to observe live bio-samples in their natural physiological conditions. In regards to the metallic tip, the most common tip probe used in STM is a tapered tungsten tip fabricated by electrochemical etching. However, since the tip material needs to be plasmonic in the range of visible wavelength for the purpose of TERS, gold is often used to fabricate a tip by the process of electrochemical etching [41, 42]. For this purpose, one end of

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a gold wire is immersed in an etchant solution, which is gradually etched through the electrochemical reaction induced by an applied voltage. The electrochemical reaction sharpens the gold wire, and finally an extremely sharp tip in conical shape is obtained as illustrated in Fig. 7a. Although gold is the most common material, silver and aluminum can also be used as a plasmonic material [43–45]. Usually the electrochemically etched tips are long and have smooth surface. They do not have any nano-sized plasmonic resonant structure that can be considered as an optical antenna. That is why they are often used in the gap-mode geometry to confine the light field. Since the STM-based TERS uses a conductive substrate, the gap-mode configuration is easily adopted. In the gap-mode regime, light is confined at the gap between the tip and the substrate. Compared with normal plasmon resonance mode with a single plasmonic structure, the gap-mode offers strong light confinements, however, with weak dependence on the incident wavelength because it does not have a plasmon resonance that one finds in the case of an antenna. The negative aspect of this is that one cannot selectively control the enhancement of one particular wavelength, but the positive aspect is that such system works readily with a large range of wavelengths. Moreover, the strong confinement contributes to strong signal enhancement as well as high spatial resolution. Recently, a single molecular resolution was reported in TERS [16], which was also achieved in the gap-mode configuration in STM-based TERS, as we will discuss later in this chapter. Although the gap-mode has several advantages, it can instantly disappear with increasing gap distance, similarly with the tunneling current. It thus requires a special care for thick samples. Another popular approach to control tips in TERS is AFM, as introduced in Fig. 3 [26, 32, 33]. Since AFM operates through weak forces such as van der Waals force between the tip and the sample, the sample does not have to be conductive. Therefore, either conductive or insulating, any types of samples can be measured with AFM-based TERS. Furthermore, AFM works not only in vacuum or in air but also in the liquid environment, which is highly compatible to biological samples [46– 48]. Recently, most of the commercially available AFM systems can work in liquid environment. Therefore, AFM-based TERS could be more versatile compared with STM-based TERS. A cantilever tip is usually utilized in AFM-based TERS as shown in Fig. 3, where the optical lever detection technique is used in most AFM to monitor applied forces between the tip and samples by detecting the deflection of the cantilever. The most common method of tip fabrication is to simply deposit plasmonic material on a cantilever tip via thermal evaporation [26, 33, 34]. A variety of AFM cantilever tips are commercially available. By evaporating silver or gold on the cantilever tip, granular plasmonic structures are formed on the tip body (Fig. 7b). Figure 7c shows a scanning electron microscopy (SEM) image of a cantilever tip after the thermal evaporation of silver. A metallic nanoparticle is often attached at the apex of the tip that facilitates LSPR so that near-field light can be efficiently excited at the apex. Since AFM-based TERS relies on the LSPR at the tip apex, it does not necessarily need the gap-mode configuration to confine the light, and hence the substrate does not have to be conductive. However, one can of course use a conductive substrate in AFM-based TERS if the gap-mode is preferred.

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The thermal evaporation of plasmonic materials relies on the process of random deposition of plasmonic nanoparticles. Therefore, one of the issues is the low yield of plasmonically active tips. No near-field light is excited unless a plasmonic particle sits at the tip apex. Therefore, precisely optimized deposition process is of great importance. In this regard, highly reproducible fabrication of plasmonic tips by the thermal evaporation has been reported [49, 50]. One straight forward way to have a plasmonic particle at the tip apex is to attach a gold nanoparticle directly on a tip apex using glue (Fig. 7d) [51]. Direct growth of silver nanoparticle on the tip apex through photochemical reaction was an alternate way that was also reported [52]. Figure 7e shows a SEM image of a tip where a silver nanoparticle was grown directly at the tip apex by the photochemical reaction. It is also possible to fabricate a plasmonic nanostructure at the tip apex by means of nano-lithographic techniques [53, 54]. However, as the fabrication process become rather complicated, the simple thermal evaporation technique has been still widely accepted in the field. As a part of the AFM family, shear force microscopy (SFM) has also been used in TERS. In this case, a tuning fork is utilized instead of the cantilever. An electrochemically etched tip, which is similar to the one used in STM-based TERS, is attached on the tuning fork, and the tip is controlled through the shear force between the tip and sample [55]. In recent years, some makers have started to manufacture TERS apparatus. For the researchers who are not familiar with TERS, this brings in some good opportunities for them to use TERS for their researches without the need of any expertise in related instrumentation. We also see that the number of publications in TERS research utilizing commercial instrumentation is gradually increasing. We believe that TERS would contribute more and more to various research fields in the near future.

4 Developments and Applications of TERS 4.1 Applications of TERS for Novel Nano-Materials We described basic instrumentations for TERS in the previous section. At last, we introduce some important evolution of TERS, especially focusing on the latest research developments to describe recent situation of TERS. After the first report of TERS in 2000 [13–15], one of the most common samples studied was CNTs, which is a robust one-dimensional advanced nano-material, wellsuited to demonstrate the strength of TERS. Some beautiful TERS images of CNTs have been reported from several groups [43, 49, 54, 56], an example of which is shown in Fig. 2. It made the research communities realize the strong imaging capabilities of TERS microscopy. Detailed structures as well as local optical responses of CNTs were visualized at nanoscale. For example, optical response change is clearly observed in Raman spectrum if some strain exists in CNTs. As Raman scattering originates from molecular bond vibrations, strains induced in molecular bonds drastically affect Raman scattering, which appears as broadening, position shift, and/or

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intensity decrease of Raman peaks. Figure 8a shows a TERS image of an isolated single-walled CNT, which was manipulated into the shape of the letters “CNT” [57]. A straight CNT was bent from several locations along its length in a controlled fashion so that it took the desired shape of “CNT”, which was created by pushing and dragging the CNT from different locations with the sharp end of an AFM tip. During this dragging process, the CNT was locally elongated and rolled as it bent into the desired shape. This created some tensile and torsional strain along the length of the CNT. Therefore, a local variation in the Raman shift of the G+ -band was expected along the CNT, which would represent the kind and the amount of local strain. In order to visualize these local strains along the length of the CNT, a TERS image shown in Fig. 8a was plotted by the Raman peak position of G+ -band, which is expected to shift depending on the kind and the amount of the strain. It is known that the peak position of G+ -band shifts to higher frequency if there is a torsional strain, whereas it shifts to lower frequency under a tensile strain. The peak-shift is color-coded in Fig. 8a, thus the color variation in TERS image shows how local strain varied within a single CNT along its length. As seen in the image, local strain within a CNT was clearly analyzed with the nanoscale spatial resolution. Figure 8a showed a beautiful example of how an AFM tip can be used to manipulate a particular physical property of the sample, such as the local strain within a CNT, by pushing the sample laterally by the tip apex, and then the same tip can be used in TERS to study this physical property at the nanoscale after coating it with a plasmonic metal. Another interesting application of tip to modify a physical property of a sample by a probe tip is when one applies a tiny pressure or force on the sample by pushing it with the tip apex in the vertical direction [58–60]. This can be done even with a metal-coated tip used for TERS measurement. In contrast to the previous case, when the sample is pushed in the vertical direction rather than in the lateral direction, the sample is sandwiched between the substrate and the tip apex. Therefore, instead of developing a local strain, it undergoes a local pressure

Fig. 7 Metallic tips for TERS. a Schematic of a electrochemically etched gold tip for STM-based TERS. b Schematic of a metallic tip fabricated by thermal evaporation. c SEM image of a metallic tip fabricated by the thermal evaporation for AFM-based TERS. d Schematic of a gold nanoparticle directly attached by glue at the tip apex. e SEM image of a silver nanopar-ticle directly grown on the tip apex through photochemical reaction. Reproduced from Ref. [51] with permission from the Japan Society of Applied Physics

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Fig. 8 TERS investigations of CNTs. a TERS image of a single CNT bent in the shape of letters “CNT”. Local strain distribution along the length of the CNT was visualized as frequency shift of Raman scattering. b Schematic of a tip with local pressure to a sample molecule showing small contact area. c Raman spectra of CNTs obtained under different local forces applied by the TERS tip. d Peak shift of Raman mode of a CNT under tip-applied pressure with respect to lateral position, showing a spatial resolution of 4 nm. e TERS image of two CNT bundles crossing each other in “X” shape. a is reproduced from Ref. [56] in accordance with the Creative Commons Attribution (CC BY) license, b–d are reproduced from Ref. [58] with permission from Springer Nature, and e is reproduced from Ref. [59] with permission from the American Physical Society

around the tip apex and the shape of the sample get elastically deformed in a very localized area under the tip. This locally deformed portion of the sample under the tip-applied force can show a shifted Raman peak in comparison to the Raman peak originating from the other part of the sample outside this localized area. The sample experiences a tip-applied force only at the contact point between the tip apex and the sample. Interestingly, since the tip apex usually has a round shape that can be considered as a hemisphere of a few tens of nanometers, the contact area between the apex and the sample could be much smaller than the size of the tip apex, as illustrated in Fig. 8b. Therefore, the deformation of the sample due to the tip-applied force is localized within a much smaller area of the sample in comparison to the sample area immersed into the localized near-field light at the apex. This indicates that if one can sense this local deformation of the sample in TERS, it would be possible to achieve much higher spatial resolution than in usual TERS. Indeed, this local deformation of the sample changes the molecular bond lengths locally, which reflects back in Raman scattering by inducing shifts in Raman modes, which can be observed in TERS spectrum. Figure 8c shows a series of TERS spectra measured from an isolated CNT around the spectral range of G+ -band, under different amounts of tip-applied forces, as indicated in the figure. Mode 1 that appears in all spectra is from the undeformed part of the CNT that is immersed within the confined nano-light, and Mode 2 that starts to appear at a tip-applied force of 1.5 nN is the shifted G+ -band originating from the very small part of CNT that is deformed due to the tip-applied pressure. This mode increases in intensity and shifts further as the tip-applied force increases.

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TERS can provide such a unique way, with which one can understand at nanoscale how chemical bonds change by an external force and/or how strong the chemical bonds are. The shift of Mode 2 from the original G+ -band at Mode 1 is large enough to experimentally measure. For example, it is about 10 cm−1 for a tip-applied force of 2.4 nN. Therefore, the shift can be easily observed in TERS measurements. Note that the contact area between the tip apex and CNT at this value of tip-applied pressure is ideally about 1 nm, even when the size of the tip apex was about 35 nm. This means Mode 2 can only be observed from a tiny part of a few nanometers of the CNT, while the enhanced Raman scattering in TERS experiment will have a spatial resolution comparable to the size of the tip apex. By taking this advantage, the authors scanned the CNT while measuring the shift in Mode 2 at a tip-applied force of 2.4 nN. As the tip passed over the CNT during a one-dimensional scan, Mode 2 was observed only within a small rage of about 4 nm, as shown in Fig. 8d. Therefore, with the inclusion of tip-applied force, an extremely high spatial resolution of 4 nm was demonstrated. This was the world record of spatial resolution in any kind of optical microscopy at the time of publication of this work in 2009. In another interesting work, an effect of local pressure applied to one CNT by another CNT was investigated by TERS (Fig. 8e) [61]. Here, two bundles of 2–3 semiconducting CNTs were placed in the shape of the letter “X”, where one CNT bundle lay on the other, so that a local pressure was induced at the crossing point of the two bundles. Through the detailed analysis of nano-Raman spectra, the authors revealed that a small part of the semiconducting CNTs turned into metallic CNTs only within an extremely localized area around the crossing point due to an alternation of the electric properties caused by a localized pressure-induced bond deformation inside CNTs. Although we do not discuss in details here, TERS is of course not limited only to CNTs or nano carbon materials, but a variety of samples from many different fields have been investigated by TERS. TERS is quite effective for advanced semiconducting materials as they are Raman active materials [62]. Recently, TERS investigation of 2D materials, such as graphene, MoS2 or WSe2 , has received great interests because of the attractive electric properties of these materials [33, 63–65]. The suitable structure of a 2D material with atomically thin thickness has also facilitated the use of TERS for nano-Raman analysis of 2D materials. Biomolecules such as DNA, nucleic acids, lipids, and polypeptides are also important targets for TERS. A number of papers have been published on bio-related TERS research, where nanoscale details of chemical bonds were well investigated and analyzed [60, 66, 67]. The critical next step will be TERS investigation of biological samples in their physiological environment. This usually means measuring a biosample, preferably live, when it is kept in a watery environment. An issue preventing from achieving this goal is contamination of tips during the measurement as TERS tips can be easily contaminated by substances floating in the liquid surrounding. The contamination may also emit background Raman signals that would deteriorate TERS measurements. Although some methods to protect the tip from contamination have

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been proposed [46, 68, 69], it has still been challenging to perform TERS measurements in liquid condition. Once TERS becomes a useful tool to observe living biological samples under their physiological conditions, it would show a strong impact to the related fields due to its label-free nature with a nanoscale spatial resolution to observe natural behaviors of bio-molecules. In an effort to add the benefits of nonlinearity in TERS and to observe non-living bio-molecules, an adenine crystal was investigated by combining the coherent antistokes Raman scattering (CARS) with TERS [70]. Also, recently stimulated Raman scattering (SRS) was demonstrated with TERS tip, where anomalous enhancement of SRS signals was reported [71]. The combination with coherent Raman techniques is another promising future of TERS.

4.2 Polarization Control in TERS One of the concerns in TERS has been to control the polarization of near-field light. It is easy to control and understand the polarization in far-field optics by simply using polarizers or related optical components. However, it becomes complicated in the near-field regime because the polarization of near-field light in TERS does not have a direct and simple relation with the polarization of the incident light as it is also strongly affected by the size, shape and the orientation of the plasmonic structure around the apex of the TERS tip. Even if the plasmonic structure at the tip apex is illuminated with a linearly polarized light, the resulting polarization of the near-field light may not be oriented to the same direction as that of the incident light. Even a slight difference in the shape, size or orientation of the plasmonic structure modifies the polarization of near-field light. In fact, this is one of the reasons why the near-field polarization has often been ignored from the considerations in TERS experiments, or at most has been considered based on some assumptions. However, since Raman intensity strongly depends on the polarization of excitation light, quantitative analysis is not possible without understanding the polarization of near-field light in TERS. Here, we introduce some techniques to study the polarization of near-field light that have been recently developed for TERS [72]. One can apply the defocused imaging technique for the evaluation of the polarization of near-field light generated at the tip apex. This technique is used to investigate the direction of the dipole moment of the plasmonic structure at the tip apex. It was originally developed to investigate the dipole moment of a single fluorescent molecule [73]. If the observation image is perfectly focused at the sample plane, a single fluorescent molecule would look like a single dot. However, by slightly defocusing the observation image, a unique defocused pattern appears, as shown in Fig. 9a. This defocused pattern shows a non-symmetric pattern with a dark spot, as indicated by the dotted enclosure. The location and the shape of the dark spot has a direct relation with the direction of the dipole moment. Therefore, one can understand the dipole moment by analyzing the defocused pattern.

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Fig. 9 a Optical image of a typical defocused pattern. b Optical images of defocused patterns of three different tips. c SEM images of the tips used for defocused imaging shown in b. Reproduced from Ref. [70] with permission from the American Chemical Society

This technique was used to investigate the dipole moment of plasmon oscillations at the tip apex. The polarization direction of near-field light corresponds to the direction of the dipole moment. However, because the TERS tips are typically fabricated by thermal evaporation of metal, they contain multiple plasmonic grains on the tip shaft as shown in Fig. 7c, each of which creates dipole oscillation of plasmons. Therefore, it is not possible to observe the defocus pattern of the dipole moment only from the plasmonic grain located at the very end of the tip. In order to solve this issue, an evanescent illumination was used through a spatial mask, as shown in Fig. 3. The penetration depth of the evanescent illumination is typically less than 100 nm from the substrate, and thus only the plasmonic grain attached at the tip apex is illuminated with it. Using this scheme, the authors could clearly observe a defocus pattern of the plasmonic dipole moment at the tip apex as shown in Fig. 9b. It was confirmed that every metallic tip created a signature polarization of near-field light even under the same condition of the incident illumination, which cannot be estimated only by observing the tip shape as shown by SEM images in Fig. 9c. TERS images of CNTs obtained with these tips with different near-field polarizations showed excellent agreement with estimations, which confirmed that this technique was adequate to evaluate near-field polarization in TERS. It was also demonstrated that the nearfield polarization can be controlled by precisely adjusting the tip position inside the focus spot as shown in Fig. 10 [74]. This development holds a potential to realize tipenhanced “polarization-dependent” Raman microscopy, which visualizes molecular orientations within a sample at the nanoscale spatial resolution. Understanding the optical properties of near-field light is highly essential to make TERS more precise and quantitative analytical technique.

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Fig. 10 a Focus spot image obtained by scanning a tip across the focus spot through the same way as shown in the inset of Fig. 3. b Defocus images obtained from the same tip when the tip was located at positions 1 to 3 shown in a. Different defocused patterns which indicate different near-field polarizations were obtained depending on the positions on the focus spot. Reproduced from Ref. [72] with permission from AIP Publishing

4.3 High-Speed Imaging As we mentioned in the introduction, one of the advantages of TERS is labelfree optical measurements, which is not possible with super-resolution fluorescence microscopy. This characteristic has encouraged researchers to apply TERS for biological studies in liquid environment to reveal natural behaviors of biological samples. We have already mentioned that one of the issues with TERS in liquid environment is the contamination of the tip. However, more critical problem would be the imaging rate of TERS. TERS is too slow to follow dynamic motions of living biological samples due to slow scanning rate of the tip and/or the sample stage. TERS typically requires at least several minutes for obtaining one TERS image, which would make the TERS image of a moving biological sample blur. Improving the imaging speed would open the doors for label-free observation of biological dynamics with a spatial resolution of a few tens of nanometers. Recently, a huge improvement was made in the imaging speed. By using highspeed AFM (HS-AFM) as a SPM in NSOM system, the authors achieved the frame rate of 3 s in a near-field fluorescence imaging (Fig. 11a, b) [75]. HS-AFM has been well recognized as a fast SPM, which has recorded movies showing structural dynamics of various proteins and has given a strong impact to life science [76, 77]. The integration of HS-AFM to NSOM holds great potentials to bring up NSOM to the next level by adding another aspect of “dynamics” with abundant optical information in NSOM. In their report, a DNA sample, which is a biological sample, was imaged in its physiological condition within 10 s (Fig. 11c). They further demonstrated successive near-field imaging of DNA fractions, in which dynamic photobleaching phenomenon was observed (Fig. 11d). Although it has been so far demonstrated only with fluorescence measurements in NSOM, we expect that the imaging rate of TERS can also be improved down to sub-second order in the near future to visualize nano-dynamics of biological samples.

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Fig. 11 a Schematic of the experimental setup of NSOM combined with HS-AFM. b High-speed near-field fluorescent image of a fluorescent bead. The imaging time was 3 s. c Near-field fluorescent image of DNA obtained within 10 s. d Successive near-field images of DNA fractions obtained at the rate of 8 s per frame. Reproduced from Ref. [73] with permission from Elsevier

4.4 Plasmon Nanofocusing for TERS LSPR has been a key science for the confinement of light at certain wavelength for NSOM and TERS. It is usually not possible to generate strong near-field light without LSPR. However, a completely different phenomenon, plasmon nanofocusing, has recently caught much attention as an alternative method for the generation of the near-field light. Plasmon nanofocusing is a phenomenon where plasmons propagate on a tapered metallic structure toward the apex and finally create strong near-field light at the apex by compressing their energy, as depicted in Fig. 12a [78]. One of the great advantages of plasmon nanofocusing is the suppression of background scattering by the incident light [79–81]. In an ordinary TERS, one has to directly illuminate the tip apex with the incident light, which creates strong background signal from the focus spot of the incident light that always accompanies near-filed signal, as illustrated in Fig. 12b. In contrast, in the case of plasmon nanofocusing, a plasmon coupler located at the tip shaft far from the apex is illuminated with the incident light, and near-field light is induced at the apex. Therefore, the incident light is spatially separated from the near-field light, and dose not generate background scattering. One can thus expect a drastic suppression of background signals from the incident light. A significant improvement of signal-to-noise ratio in TERS owing to this effect is therefore expected and has already been demonstrated [82]. While such a background suppression feature has attracted much attention in the field of nanophotonics, plasmon nanofocusing has another intriguing property. Because this phenomenon is based on the propagation of plasmons rather than the resonance, it can basically work at any arbitrary wavelength. This is not possible with LSPR because it is literally a resonant phenomenon that occurs at a certain wavelength or within a small wavelength range. One needs to precisely arrange the

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shape and size of plasmonic materials to tune the plasmon resonance wavelength depending on experimental requirements. In this sense, once a tapered plasmonic structure is fabricated, any wavelengths of light can be used with the same structure. Furthermore, multiple wavelengths can be applied at the same time to the same tapered structure, which enables the generation of a broadband near-field light [83]. The generation of broadband near-field light confined at the tip apex was experimentally demonstrated through the process of plasmon nanofocusing on a fabricated taped silver structure, where entire visible range of light was confined at the apex at the same time through broadband plasmon nanofocusing (Fig. 13) [84]. Further, tipenhanced CARS imaging was also demonstrated through plasmon nanofocusing, in which multiple wavelengths are involved for CARS process with a broadband femto-second pulsed laser [85].

Fig. 12 a Schematic of plasmon nanofocusing excited on a tapered metallic structure. b Schematic of excitation of LSPR and near-field light in the case of the normal TERS configuration. Direct illumination of the incident light to the apex creates background scattering.

Fig. 13 a SEM image of a tapered silver structure with a single slit as the plasmon coupler, fabricated on a cantilever tip. b Optical images of broadband plasmon nanofocusing, observed through several band-pass filters at different wavelengths. Reproduced from Ref. [81] in accordance with the Creative Commons Attribution (CC BY) license

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Being based on a completely different mechanism to generate near-field light, plasmon nanofocusing provides distinct properties of the background suppression and the broadband, which holds a great promise for the next generation of TERS techniques.

4.5 Single Molecular Resolution Last but not least, a dramatic progress that has recently been seen in TERS is the spatial resolution. Spatial resolution in TERS had typically been 10–20 nm, and only in some particular cases, such as using tip-pressure, the limitation could be pushed down to a few nanometers of spatial resolution as described earlier. It has been a common understanding in TERS for a long time that the typical spatial resolution in TERS is limited by the size of the tip apex, which is typically about several nanometers. However, single molecular spatial resolution has suddenly been achieved in TERS, which was first reported in 2013 [16]. In this work, STM-based TERS was utilized under the conditions of ultrahigh vacuum and low temperature. Even though a solid physical reason to achieve such ultrahigh spatial resolution was not provided at that time, it has had an impact high enough to stimulate the TERS community or even other research fields outside the nanophotonics field. Since then, several research papers have been published on the ultrahigh-spatial-resolution TERS [86, 87]. Moreover, sub-molecular spatial resolution was recently reported in 2019 [88], in which surprisingly the authors reported a spatial resolution of around 1Å. The mechanism of such high spatial resolution is still under debate, but one of the ideas that have started to be somehow accepted in the community is that an extremely tight confinement of light field may be achieved between a few atoms protruded at the tip apex and the substrate in the gap-mode regime, so-called the “pico-cavity”. In this sense, only if a single or a few atoms protrude from the tip apex, atomic level confinement of light field can be possible when the tip is brought to an angstrom distance from the substrate. Although it has a limitation in the gap-distance and it may work only for studying atomically thin molecules, there is no exaggeration in saying that TERS has now become a powerful tool to study states of a single molecule at a sub-molecule spatial resolution.

5 Conclusion In this chapter, we described from basic instrumentations to recent developments and trends in TERS. It has been around 20 years since the invention of TERS, and during this journey TERS has been recognized and well-accepted as a unique and powerful tool of Raman spectroscopy and imaging with the nanoscale spatial resolution. At the same time, we have witnessed tremendous efforts made by many researchers that has kept TERS evolving in different aspects, which further encourages researchers

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to develop new technologies to advance TERS. Such enthusiasm has already made TERS even commercially available for researchers in various other fields to include and make use of TERS in their research easily. For the future of TERS, the single molecular resolution would still be a hot trend in the application of TERS for coming several years. As the spatial resolution has already been extensively improved in TERS, an improvement in the temporal resolution would be the next big challenge, the crucial first step of which has already been made by integrating HS-AFM as discussed earlier. We believe that in the near future, TERS will enable video-recording of molecular activities through a video rate high-speed nano-Raman imaging. From technical viewpoints, TERS is still a labbased technique as it is highly complicated for people from other fields to implement it easily. Also, measurement reproducibility is still an issue and should be further improved in the future. If TERS reaches a level where it becomes simple and easy to use and show a certain level of stability and reproducibility for various types of samples, it would become a truly universal technology, which would expand the TERS community extensively. We do believe that it would come true in not-so-distant future.

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Coherent Anti-Stokes Raman Scattering: Basics, Theoretical Background, and Applications Alireza Mazaheri Tehrani, Faezeh Mohaghegh, and Arnulf Materny

Abstract Coherent anti-Stokes Raman scattering (CARS) as a highly efficient nonlinear vibrational spectroscopy technique has found wide applications in physics, chemistry, and biology. Measurement of temperature and species concentration in combustion exhaust, fundamental studies of dynamics of energy transfer between different vibrational modes of sample molecules and high precision rapid imaging of biological species in living cells are just some of the applications. This chapter aims to briefly introduce the technique, starting with basics and experimental conditions, followed by some examples of recent applications. Keywords Coherent anti-Stokes Raman scattering · Nonlinear optics · Vibrational spectroscopy · Nonlinear four-wave mixing · Surface-enhanced CARS

1 Introduction Vibrational spectroscopy has for decades successfully provided a powerful, nondestructive and versatile tool for studying molecular structures. The vibrational energies are unique features of the molecular samples and can be considered to be “molecular fingerprints”. Their detection allows for a rapid characterization yielding detailed structural information about the molecular species [1, 2]. Vibrational energies typically can be found in a wavenumber range from a few cm−1 to a few thousands of cm−1 ; they are far smaller than the electronic energies. The vibrational lines are relatively narrow and the spectrum therefore offers more detailed information about structure and also interactions of the molecules with their surrounding compared to absorption or emission spectra, which occur due to electronic transitions with typically broad spectral features. The lines assigned to vibrational modes reflect with high spectral resolution intra- and intermolecular properties, which is used in many fields of science, like physics, chemistry, biology, or material science.

A. Mazaheri Tehrani · F. Mohaghegh · A. Materny (B) Department of Physics and Earth Sciences, Jacobs University Bremen, Bremen, Germany e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 D. K. Singh et al. (eds.), Modern Techniques of Spectroscopy, Progress in Optical Science and Photonics 13, https://doi.org/10.1007/978-981-33-6084-6_9

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There are in principle two main vibrational spectroscopy methods. IR absorption spectroscopy makes use of vibrational resonances in the infrared energy range. Raman spectroscopy is based on inelastic scattering of higher-energy photons (generally in the range from near IR to UV) from the molecules of the sample. In this process, the photons loose (“Stokes scattering”) or gain (“anti-Stokes scattering”) energy by an excitation or de-excitation of molecular vibrations or rotations (in the following, we will mainly refer to vibrational spectra), respectively. Both methods provide a relatively fast, label-free, and accurate measurement of a huge variety of samples, like gases, liquids, powders, thin films, nanostructures, etc. The application ranges from a quick identification of the species to an in-depth investigation of the sample structural state and changes. Despite having great advantages, both of these methods suffer from certain disadvantages, which limit the range of their applications. For example, low spatial resolution of few microns due to longer infrared wavelength makes IR absorption spectroscopy less attractive for microscopic investigations [3, 4] and absorption of infrared radiation by water molecules in the sample is a hurdle for using this technique in aqueous environment or for living species [3]. Raman spectroscopy, in contrast, using a shorter excitation wavelength, features a higher spatial resolution and can be easily applied in aqueous solutions due to transparency of water in the visible wavelength range. However, systems emitting fluorescence in many cases let Raman signals vanish in the intense emission background. In addition, Raman signals are relatively weak due to the very low scattering cross section – roughly a single inelastic scattering event per million photons. This limits the access to low concentrations of sample molecules and in many cases results in relatively long measurement times due to the need for longer integration [5]. Without special techniques, which allow for a suppression of the fluorescence and/or an enhancement of the scattered signal, a number of samples cannot be investigated. While researchers have heavily utilized these methods for several decades (since the discovery of the “Raman Effect” in 1928) to advance the scientific research, the discovery of the laser in 1960 [6] has drastically improved the successful application of this method, by providing a well-defined, coherent, and intense optical excitation source. At the same time, this has opened a completely new front in optics, the socalled “nonlinear optics”, which only plays a role when very high field intensities are applied as they for example are found in tightly focused laser beams or for very short laser pulses. A few years later, in 1965, two scientists, Maker and Terhune, working in a research laboratory of the Ford Motor company in Michigan [7], did research in pursuit of a so-called “Raman Laser”—a laser system based on light amplification of a stimulated Raman signal in a Raman active medium. They found that when exciting with a laser beam together with a second beam red-shifted (“Stokes shifted”) by the energy of a vibrational mode, a coherent laser-like signal at the anti-Stokes energy, i.e. the laser energy increased by the mode energy, could be observed. This laid the foundation for a technique, which in 1974 was named “coherent anti-Stokes Raman spectroscopy” (CARS) by R. F. Begley et al. [8]. It could be demonstrated that CARS has significant advantages compared to normal spontaneous Raman spectroscopy, which include five order of magnitude higher conversion efficiency, fluorescence-free

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spectrum on the higher energy side of the excitation, and a high spectral resolution, which in principle eliminates the need for a monochromator when lasers with narrow bandwidth are used [8]. The advance of the laser technology in the following years made high-power tunable laser systems available, which resulted in an increased research activity on CARS spectroscopy. The CARS technique rapidly became one of the most popular nonlinear spectroscopic methods to study fluorescing samples, gases in discharges, plasmas, combustion and atmospheric chemistry [9, 10]. The elimination of the fluorescence background in particular opens a wide range of applications to study biological samples, which previously was not possible using spontaneous Raman spectroscopy due to the strong emission background typical for many bio-chemical compounds. Also for CARS, limitations have to be overcome like e.g. the existence of a third-order nonlinear “non-resonant” background, which will be discussed later in this chapter. However, the aforementioned advantages have made CARS a great non-destructive tool featuring a chemical contrast without the use of labels (like e.g. dyes) to investigate and rapidly image biological samples and living cells in their natural environment. Applications of CARS in microscopy have been intensively studied and reviewed elsewhere [11–13] and are not the focus of this chapter. Our contribution will not be able to cover the full range of CARS theory, instrumentation, and applications, which easily would fill a book on its own. However, we would like to give the reader an idea of the usefulness of this nonlinear optical spectroscopy technique. In this chapter, we will first briefly present the theoretical background of the nonlinear CARS process, which is also of relevance for understanding the required experimental setup. Then, we will focus on only two recently introduced CARS techniques, femtosecond-time-resolved CARS (tr-CARS) and surface-enhanced CARS (SE-CARS).

2 Theoretical Background In this section, a brief description of the underlying theory behind the CARS process is presented, which will provide the background required for understanding the method and is important for designing and performing an experiment successfully and for interpreting the results correctly. For a more in-depth theoretical description, we refer the interested reader to more detailed publications [14–17]. To put it simple, CARS is only one of the possibilities that nonlinear optics would offer, in which three incoming photons nonlinearly interact with a medium and a new photon is generated. These are the so-called “four-wave-mixing” (FWM) processes. Since with increasing order of nonlinearity, the resulting signals become smaller, FWM does not play a role unless the sample is irradiated with a very high intensity of light or generally electro-magnetic radiation. In this case, the well-known principle of “superposition” does not describe this process any more. This also explains why such nonlinear optical phenomena—while theoretically predicted already in 1931 [18]— have not been observed until about thirty years later [19, 20] after the Ruby laser

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was developed in 1960 [6]. The principle of many nonlinear optical processes, were later developed and presented by a Dutch-American physicist and Nobel laureate, Nicolaas Bloembergen in his book entitled “Nonlinear Optics” [21]. In order to better understand the aspect of response of the matter to the external electric field one should consider that the external field is simply polarizing the material and the amount of this externally induced polarization depends on the material structure and the strength of the external driving field. It is actually the nature of this dependence that defines the (non-)linearity of the response of the material. The dependence of the macroscopically induced dipole moment, the polarization P (per unit volume) on the driving field E is described by P = ε0 χ E, where ε0 is the electric permittivity of free space. The susceptibility χ is a tensor and is representing the material properties and response, and, therefore, is also determined by symmetry considerations. Keeping in mind that polarization and field are vectors and the susceptibility is a tensor, we are in the following simplifying the expressions using a scalar notation. Additionally, frequencies have to be assigned to the electric fields and the generated polarization and also the susceptibility is a function of frequency. To keep this introduction as simple as possible, we will only refer to this where specifically required. The polarization P, can be expanded in a Taylor series in terms of different powers of the electric field: P = ε0 χ (1) E + ε0 χ (2) E 2 + ε0 χ (3) E 3 + · · · = P (1) + P (2) + P (3) + · · ·

(1)

This gives rise to a linear term, P (1) , and nonlinear terms P (2) , P (3) , . . . , of the polarization, which are characterized by the nonlinear susceptibilities, χ (i) of i-th order. These susceptibility terms decrease in magnitude following their order (χ (1) > χ (2) > χ (3) > . . . ). Typical values for the susceptibility of third order, χ (3) , can be found in Table 1. It is also worth mentioning here that due to symmetry considerations—which are out of the scope of our chapter—the second order dielectric susceptibility χ (2) vanishes for isotropic material with inversion symmetry, e.g. gases and liquids. Hence, for example, the second harmonic generation, SHG, which in principle is a three-wave-mixing process, does not occur in such materials and the third-order term, with χ (3) = 0 would describe the first nonlinearly active process that can be observed. The process of third order, in which we are interested here, is CARS, which is a four-wave-mixing process, described by tensor elements of χ(3) . Using a nomenclature, which is especially useful for time-resolved CARS, in the nonlinear Raman process, a pump pulse at frequency ω p , a Stokes pulse at frequency ωs , and a probe pulse at frequency ω p , generate a nonlinear polarization, which is the source term of the “anti-Stokes signal” at frequency ωas . The following relation reflects the energy conservation, which is only involving photon energies, which means that the molecule will be energetically unchanged after the nonlinear scattering process in contrast to the spontaneous Raman scattering:

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ωas = ω p + ω p − ωs

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(2)

The induced nonlinear polarization of third order then can be written as: P (3) (ωas ) = χ (3) E p (ω p )E p (ω p )E s (ω S )

(3)

Here, the frequencies ω p and ω p can be different, however in most experiments pump and probe pulses are used with the same frequency ω p = ω p and originate from the same laser source. However, in time-resolved CARS, pump and probe laser pulses in general can be delayed against each other. In order to resonantly excite a vibrational mode of the molecule under investigation oscillating with frequency ω R , the frequency difference between pump and Stokes laser is set such that ω p −ω S = ω R is fulfilled. In order to obtain a CARS spectrum, typically the Stokes laser frequency is tuned (e.g. using a dye laser) over the range of Raman resonances while a detector records the changes of the anti-Stokes signal intensity. Filters or a monochromator can help to filter out the exciting laser frequencies, however, as will be discussed below, a spatial separation in principle allows for a background-free detection of the CARS signal even without spectral filtering. Another, more elegant method for obtaining a nonlinear Raman spectrum within a shorter time is to use a broad-band laser emission as Stokes laser. Then, a monochromator is required, which disperses the anti-Stokes signal and a CCD detector records a spectrum over the range defined by the spectral width of the Stokes laser excitation. The spectral resolution is now defined by the spectral widths of the pump and probe lasers and the spectral resolution of the spectrometer used for dispersion and detection of the anti-Stokes signal. Figure 1 shows on the left-hand side the energy diagram of a typical CARS experiment, here assuming the interaction with a diatomic gas molecule. The righthand side of the figure depicts another process, which also can contribute due to a twophoton absorption step not involving any Raman resonance. This purely electronic process can create a non-resonant (“resonant” referring to vibrational or rotational energies) background, which in most cases is unwanted. It can result in line shifts and in non-symmetric line shapes, which will not be discussed in more detail here. A quantum mechanical description yields the Raman-resonant and the nonresonant terms of the nonlinear susceptibility χ (3) . Without derivation, we give the result in Eq. (4) according to Lotem et al. [23]: χ (3) =

AR At   + ωt − 2ω p it ω R − ω p − ωs − i R

(4)

in which A R and At represent the cross sections of the Raman scattering and two photon absorption transition processes, respectively. A two-photon electronic resonance is assumed at energy ωt . The half widths at half maxima of the Raman-resonant CARS line and the non-resonant background signal are  R and t , respectively.

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Fig. 1 Energy diagram of CARS (left) and creation of a “non-resonant” background signal due to a purely electronic process involving a two-photon absorption (right)

The first term of Eq. (4) is maximaized when the resonance denominator becomes minimum, which is achieved by tuning the difference between pump and Stokes frequencies to match the vibrational frequency ω R . These resonances yield the desired CARS spectrum. The second term does not depend on a vibrational (or rotational) mode energy and will yield a non-negligible contribution if a two-photon absorption transition (ωt ) will be close to 2ω p . Since the anti-Stokes signal intensity is proportional to the square of the polarization, a mixture of the Raman-resonant and non-resonant terms result in CARS spectra e.g. showing asymmetric line shapes as already mentioned above. While in the wave description of the fields given by E(r, ω) = exp[i(ωt − kr)], the frequency will contribute to the “energy conservation” given by Eq. (2), also the wavevector k has to be considered, which is related to the momentum. Again, without derivation, we obtain for the anti-Stokes signal intensity the following expression [13]: Ias

 2  (3) 2 2 sin k2 L ∝ I p I p  I s χ  L k L

(5)

2

where I p I p Is is the product of the laser intensities and L represents the interaction length defined by the overlap of the laser foci and the sample properties (e.g. film thickness). In this expression, the term in round brackets describes the so-called phase-matching condition, which depends on the “phase-mismatch” k, which results from a vector diagram of the wavevectors assigned to lasers and anti-Stokes signal:

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k = k p + k p − ks − kas

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(6)

From Eq. (5), it is clear that for an increase of the efficiency of the process several parameters have to be considered, namely, laser intensities, third-order nonlinear susceptibility (depending on material and Raman-resonance conditions) and also the phase-matching condition. Figure 2 shows the strong dependence of the CARS efficiency on the phase mismatch k. Only when k = 0, an intense CARS signal can be expected. The “momentum conservation” condition given by Eq. (6) also defines the required directions of lasers and the direction of the resulting signal, which is spreading within a very narrow angle range like a laser, and which concentrates the signal spatially resulting in signals that in some cases can be seen with naked eye. The required arrangement of lasers resulting in a phase-matched signal can be achieved using different geometries within a plane or also within a three-dimensional scheme, like the so-called “Folded BoxCARS” geometry suggested by Shirley et al. [24]. For this, one has to keep in mind that the wavevectors are also determined by the dispersion relation where the index of refraction is a function of frequency ω: Fig. 2 Dependence of CARS efficiency represented by the exponential term in  2   Eq. (5), sin k2 L / k2 L , on phase mismatch k plotted as function of kL/2

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Fig. 3 Diagram showing the so-called “Folded BoxCARS” arrangement fulfilling the phase-matching condition given in Eq. (6)

k(ω) =

ωn(ω) c0

(7)

Only in gas phase, where n(ω) ≈ 1 can be assumed for the different frequencies involved in the CARS process, a collinear arrangement of lasers and signal will approximately fulfill the phase matching condition in a “macroscopic” setup. Another exception is when the interaction length with the sample is extremely small like for tightly focused beams in a microscope arrangement. Then, the phase-matching condition is sufficiently relaxed so that also for a collinear beam arrangement still viable signal intensities can be achieved. Otherwise, well-defined angles between laser beams and anti-Stokes signal direction are required. This is shown in Fig. 3 for the case of a Folded BoxCARS arrangement. This geometry has advantages. Firstly, the signal is well separated from the laser beams, which reduces unwanted background from the lasers. A combination of an aperture and spectral filtering for the signal allows for the detection even of signals at very low wavenumbers like e.g. rotational transitions or acoustic phonons. Secondly, for femtosecond time-resolved experiments, which will be discussed in more detail in this chapter, the clear separation of the pulses with pump, probe, and Stokes frequencies, results in a relatively simple setup.

3 Applications 3.1 Femtosecond Time-Resolved CARS (Tr-CARS) In frequency-domain, CARS spectroscopy is attractive due to its capability of yielding intense Raman spectra, which are background- and fluorescence-free and which can be obtained with extremely high spectral resolution when narrow-banded laser sources are used. However, also in time-domain CARS has proven to be a very useful tool. Using ultrashort laser pulses typically on a femtosecond timescale, allows for a coherent excitation of vibrational modes. Since femtosecond pulses are spectrally broad, high spectral resolution cannot be achieved. However, a coherent superposition of vibrational modes will be generated by the interacting laser pulses,

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which then develops in time and yields information about coherence lifetime and vibrational relaxation processes. These superpositions can involve different overtones of vibrational modes and/or different vibrational modes of the sample resulting in vibrational wave packets and beating phenomena. Some examples are given in the following showing how possible applications of femtosecond time-resolved CARS can look like. In order to follow the ultrafast dynamics of vibrational excitations, the principle of a typical pump-probe scheme used in time-resolved spectroscopy is also applied for tr-CARS. In the first step, a combination of “pump” and “Stokes” pulses interact with the sample in a precise spatial and temporal overlap. This interaction defines “timezero”. The wavenumber difference between pump and Stokes lasers define, which vibrational states will be coherently excited. The bandwidths of the exciting pulses will determine which vibrational (or rotational) states are accessed simultaneously. The resulting superposition of wave functions results in beating, which develops over time and is determined by both the lifetimes of the coherence (phase relations) and the population of the exited states. Interactions with the molecular environment can e.g. result in loss of coherence or vibrational relaxation, etc. In a second step, the so-called “probe” pulse interacts with the sample with a welldefined time delay after time-zero. An anti-Stokes signal will be generated, which depends on the temporal development of the initially prepared nonlinear polarization. The transient signal is modulated by the beating of the coherently excited states and at the same time shows a decay over time, which reflects the lifetime of the excitation. Figure 4 demonstrates the principle of femtosecond time-resolved CARS in a schematic way, showing a real CARS transient obtained from a polydiacetylene (PDA) single crystal in our laboratories. Without going into details, the following should be mentioned. The pump and Stokes lasers are spectrally broad as indicated in the scheme, which shows the pulses along a wavenumber axis. The probe pulse is assumed to be identical with the pump pulse except for the timing. Pump and Stokes lasers are tuned such that several vibrational modes of the PDA are coherently excited (e.g. C = C stretching vibration as most intense feature in the transient at time-zero, here assigned to the arbitrary number “5”). The resulting anti-Stokes signal is also spectrally broad and has (with relatively pure resolution) spectral features, which can be assigned to vibrational modes. The transient CARS signal shown as 3-dimensional plot yields this purely resolved spectrum probed by the time-delayed probe pulse for different delay times (cuts parallel to the relative-wavenumber axis). Looking along the time axis (cuts parallel to the delay-time axis), one recognizes beatings. These are due to the coherent superposition of different modes. However, additionally, one observes that obviously certain vibrational modes only start to contribute after a certain delay time (e.g. the mode with the arbitrarily assigned number 4). While the beating directly is related to the coherence, the delay of a mode excitation points to an intramolecular vibrational energy transfer. As will be shown below, a Fourier transform of the transient signal yields the mode frequencies involved in the coherent superposition. We would like to point out that there are many degrees of freedom, which allow for a great variety of experimental schemes [25]. We have already mentioned the

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Fig. 4 Understanding the concept of the use of CARS in a pump-probe scheme. The laser pulses are spectrally broad and are displayed on a wavenumber axis. The spectrum of the probe laser is assumed to be identical to that of the pump laser. Due to the coherent Raman-resonant excitation of vibrational modes (here “1” to “5”) by the pump-Stokes pulse pair, the probe laser results in an anti-Stokes signal, which resembles a purely resolved Raman spectrum, which varies with time delay as shown in the 3-dimensional CARS transient shown. Beatings and energy transfer can be observed (for a further discussion, see main text)

wavenumber difference between pump and Stokes lasers, the spectral widths of the pulses, and the timing between pump-Stokes pulse pair and probe pulse. However, also between pump and Stokes laser a time delay can be introduced, e.g. allowing the molecular system to “develop” before the vibrational states are excited. Additionally, the wavelength of the laser pulses can be varied in order to make use of electronic resonances. Polarized laser light can help to access information about symmetry dynamics in the systems. An introduction of a chirp to the laser pulses can help to perform controlled mode excitations. Here, we will restrict the discussion to the simple “pump-probe scheme” as described above, but it is important to know that even more can be done. A typical experimental setup as commonly used for tr-CARS is depicted in Fig. 5. In this setup, the required pump, Stokes, and probe (assuming ω p = ω p ) pulses are

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Fig. 5 Experimental setup of time-resolved femtosecond CARS. Adapted with permission from [23]. Copyright © 2014 American Chemical Society

produced by two optical parametric amplifiers (OPAs), which are pumped by 150 fs (775 nm, 1 kHz repetition rate, 1 mJ energy/pulse) pulses from a Ti:sapphire laser. The pulses then are compressed to 80 fs using prism compressor pairs as shown in Fig. 5. Using an optical beam splitter, the output of one OPA is split to form a set of pump and probe pulses. The output of the other OPA is directly used to generate Stokes pulses. The computer-controlled delay stages, fitted with “retro-reflector” mirrors, are used to provide temporal delays between the pulses in a Michelson-type setup. As was already mentioned above, a well-suited geometric arrangement of the laser pulses fulfilling the phase-matching conditions is the folded BoxCARS arrangement. The spatially separated pulses are focused into the sample where they spatially overlap under the desired angles resulting in a forward-directed CARS signal, which is spatially separated from the exciting and probing laser pulses. A lens is used to collimate the signal beam and a mask blocks the lasers and leaves through the anti-Stokes signal. For initial alignment, temporal and spatial overlap of all three pulses are verified using the cross-correlation signal produced in a BBO crystal at the position where the sample will be placed. This also allows for an estimation of the temporal width of the pulses interacting with the sample. The anti-Stokes signal light originating from the sample is dispersed by a spectrometer equipped with a CCD camera, which detects the broadband CARS spectrum. The spectrum is then recorded as a function of time delay between the pump-Stokes pulse pair and the probe laser pulse yielding a 3-dimensional signal as shown in Fig. 4. Namboodiri et al. [23] have used a tr-CARS experiment to monitor the evolution and dephasing time of reflecting the vibrational dynamics of several variations of 1,3-dialkylimidazolium ionic liquids (ILs) with bis(trifluoromethylsulfonyl)imide [NTf2 ] as anion. In their experiment, vibrational states around 1400 cm–1 have been coherently excited, and the dynamics of vibrational modes were investigated as a

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function of time. These authors have successfully shown the transfer of vibrational energy to the modes beyond the excitation spectrum due to interionic interactions. Figure 6 shows an example of a signal recorded in this work, highlighting the ultrafast dynamics of the system. Here, two modes centered at 1215 and 1500 cm −1 are the main features seen in the nonlinear Raman spectrum for the IL [BMMIM] [NTf2 ]. A complex temporal behavior including decay and beating structure can be observed. For an analysis, corresponding Fourier transform calculations are shown in the figure, which yield even more detailed information about the vibrational modes involved in the observed dynamics. For example, the major peaks in the FT spectrum obtained from the transient signal at 1215 cm−1 are 98 and 203 cm−1 . They correspond to the beating between vibrational modes at 1223/1120 (103) and 1323/1120 (203) cm−1 . The 1323 cm−1 mode can be assigned to the asymmetric stretch of the anionic SO2 and cationic ring Fig. 6 Signals recorded in [BMMIM] [NTf2 ]. The upper contour plot displays the transients as a function of detection wavenumber. The dominating transients at 1215 and 1500 cm–1 and their corresponding Fourier transform spectra are plotted below. The red lines in the transient diagrams represent fitted single-exponential functions. Adapted with permission from [23]. Copyright © 2014 American Chemical Society

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vibrations, the 1223 and 1120 cm−1 modes belong to the asymmetric stretch and the symmetric stretch vibrations of SO2 , respectively. A dephasing time T 2 of 0.72 ps for the Raman signal at 1215 cm−1 can be determined. Among other interesting findings, the tr-CARS experiments revealed that also beatings with modes could be detected, which have not been directly excited by the pump-Stokes laser pair. This pointed to an efficient interionic vibrational energy transfer, which again helped to better understand coupling mechanisms in the molecular IL system. There are many more examples where tr-CARS has been applied to obtain detailed information about vibrational or rotational dynamics of molecules. In this short chapter, we will not be able to discuss further applications. However, as last comment, we would like to add that besides getting information about dynamics, the delay between the laser pulses can also be used to suppress the non-resonant background signal mentioned above (see e.g. Fig. 1). Since this two-photon absorption process is purely electronic and therefore has an extremely short lifetime, it vanishes as soon as a short time-delay is introduced between the laser pulses. Using pulses in the picosecond time range still allows for a good spectral resolution, which now yields nonlinear Raman spectra free of the undesirable background.

3.2 Surface-Enhanced CARS (SE-CARS) CARS spectroscopy in frequency and time domain has interesting applications. The use in frequency domain as alternative for linear (spontaneous) Raman spectroscopy has not been further discussed in the chapter since it appears to be obvious. From the examples given in Sect. 3.1, the attractiveness for time-resolved vibrational spectroscopy has been demonstrated. However, there are also more “specialized” developments. In our contribution, we would like to only mention one of them, “Surfaceenhanced CARS” (SE-CARS). This is an example, which demonstrates that on the one hand an interesting CARS technique exists, which is attractive for certain applications, but on the other hand the mechanisms involved are not completely understood yet. Surface-enhanced Raman scattering (SERS) [26, 27] is a well-known and still rapidly growing technique, in which the relatively weak spontaneous Raman scattering is drastically enhanced by several orders of magnitude when the molecules are placed in close proximity of a metallic (typically nano-structured) surface. There are two main mechanisms, which result in the signal enhancement. When the light couples to the surface electrons of the metal, plasmons (quanta of the electron oscillation) are excited resulting in “surface plasmon polaritons” (SPPs), which in principle concentrate the energy of the field in a very close proximity of the surface. This results in an “electro-magnetic enhancement” effect where both exciting laser field and signal field are affected. Additionally, the coupling of the metal electrons and the molecular electronic system can result in resonance effects, which contribute to the SERS process as “chemical enhancement” mechanism. The enhancement can be so

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big that single molecule sensitivity [28–30] becomes feasible. While in principle this can be used for enhancing weak CARS signals in frequency-domain spectroscopy, also a combination with time-resolved ultrafast vibrational CARS spectroscopy [31– 34] is possible. In another word one could “watch a molecule breathing” [35] or study molecular dynamics of nanostructures. Namboodiri et al. [36], have performed SE-CARS, using silver sols, prepared based on the well-known methods by Lee-Meisel [37] and Hiramatsu et al. [38]. Firstly, they have verified the SE performance of the silver sol in a standard SERS experiment. Figure 7 shows the Raman spectra of 10–2 M pyridine recorded with and without the presence of silver colloid. A drastic enhancement can be observed. Then, they have used the samples varying the concentration of the silver sol in order to test SE-CARS with femtosecond laser pulses. For this, they have used an experimental set up similar to that depicted in Fig. 5. Figure 8 shows the CARS spectra obtained for different concentrations of the silver colloids. Interestingly, these authors have observed a relatively small tenfold enhancement, which occurs only in a very narrow excitation profile around a pump wavelength of 550 nm. That was attributed to the non-uniformity of the silver sol in use. Obviously, only a small portion of silver nanoparticles had the right shape and size to be excited and simultaneously transfer the excitation energy to the vibrational modes of the sample. A much bigger enhancement would have been expected since the electro-magnetic enhancement effect should now contribute to the enhancement not only of one laser field, but three laser pulses should be affected. It might be that the specific phase-matching and coherence requirements of CARS prevent the expected drastic enhancement. Nevertheless, this experiment demonstrated that a localized enhancement of tr-CARS spectra is possible when molecules are in close proximity to metal nanostructures. Fig. 7 Raman spectra of 10−2 M pyridine recorded without (dashed) and with (solid) the addition of silver colloid. Adapted with permission from [36]. Copyright © 2010 Elsevier B.V

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Fig. 8 Surface-enhanced CARS spectra obtained from pyridine. The CARS spectrum from pure pyridine is shown as solid curve. The other spectra are for different concentrations of silver colloids (dashed curves) as indicated in the graph. Adapted with permission from [36]. Copyright © 2010 Elsevier B.V

Another study was performed few years later by Hua et al. [39], in which they used randomly aggregated gold nanoparticles on a glass surface, on which a pyridazine solution was added. These authors also found relatively small enhancement factors. They have nicely adjusted their gold nanoparticle absorption spectrum (plasmon resonance) to include all three pulses, pump, Stokes, and probe (in their experiment ω p = ω p was chosen) as shown in Fig. 9. Nevertheless, these authors also found that the overall enhancement they were able to achieve was several orders of magnitude smaller than expected based on the huge enhancement factors observed in SERS experiments. The authors have concluded that although there should be locally very huge enhancement of the CARS signal, the non-constructive interference arising from field contributions from many arbitrary hotspots, eventually diminishes the overall enhancement of the signal. This supports the assumption that phase-matching and coherence conditions are partially not fulfilled by the enhanced portions of the electro-magnetic fields. Fig. 9 Absorbtion spectrum of the aggregated gold nanoparticles (black, NPs) overlapping with the laser pulse spectra (pump, Stokes, and probe) used in the SE-CARS experiments. Adapted with permission from [39]. Copyright © 2014 American Physical Society

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Fig. 10 a–c Experimental SE-CARS spectra of pyridazine on random gold NP aggregates from different focal spots on the sample. Adapted with permission from [39]. Copyright © 2014 American Physical Society

Additionally, in the experiments no time delay between the exciting laser pulses was introduced, which resulted in non-resonant background. As described earlier, the non-resonant background nonlinearly mixes with the Raman-resonant signal producing non-symmetric line shapes, which even can look like negative signal peaks. Figure 10 shows spectra obtained from different spots on the surface. Due to the arbitrary arrangement of gold aggregates, different enhancement of the Ramanresonant part occurs, which results in a variation of CARS spectra. While silver or gold clusters and aggregates are expected to have very inhomogeneous field contributions, which obviously are not enhancing CARS signals with the highest possible efficiency, a well-structured SE substrate should be better suited for SE-CARS. In their work, Steuwe et al. [40] used a well-defined reproducible nanostructured surface. Commercially available Klarite™ (Mesophotonics Ltd, U.K.; gold-coated inverted pyramid nanostructures) was covered by a benzenethiol monolayer. Figure 11 shows the substrate (a) and the frequency-resolved SE-CARS image (b) for a Raman-resonance tuned to the ring stretching mode of benzenethiol at 1070 cm−1 . The surface structure is very regular, which allows for a coupling of the laser light to the surface plasmons such that locally efficient enhancement occurs. This can be seen from the spatially dependent CARS intensity. An enhancement factor of 105 compared to standard CARS could be obtained. The authors have demonstrated that SE-CARS in this case is more than 103 times more sensitive than SERS measurements [40]. An interesting review of recent advances in SE-CARS can be found in Ref. [31].

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Fig. 11 a SE-CARS image of benzenethiol monolayer on the commercially available plasmonic surface Klarite™. The color of each pixel tracks the peak intensity at 1070 cm−1 extracted from each SE-CARS spectrum taken with integration time of 10 ms every 300 nm using a 100 × objective. b Scanning electron microscopy image of the same area showing the pyramidal pits of Klarite™. Adapted with permission from [40]. Copyright © 2011 American Chemical Society

In Fig. 12, the model of the SE-CARS process of the benzenethiol molecules on the Klarite™ surface involving the interaction with pump, Stokes, and probe laser light is shown. The nano-structured surface allows for an excitation of surface plasmons in a well-defined way. The so-formed SPPs localize the fields and result in the enhancement effect. Vibrational modes of the molecules attached to the surface

Fig. 12 Model for surface-enhanced CARS. Incoming pump and Stokes radiation couples into respective plasmons that interact coherently with a molecule on the surface. The outgoing CARS scattered plasmon is coupled into an emitted anti-Stokes photon. Adapted with permission from [40]. Copyright © 2011 American Chemical Society

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are excited and the anti-Stokes signal is generated, which finally couples out of the surface. Recently, we have demonstrated that a combination of femtosecond time-resolved tr-CARS spectroscopy and SE-CARS can be used to access ultrafast vibrational dynamics with nanometer spatial resolution beyond the diffraction-limit of an optical microscope [41]. While a standard optical microscope was used to focus the light of the pump, Stokes, and probe laser pulses onto the nano-structured sample surface, a scanning near-field optical microscope was used to detect the anti-Stokes signal. Using a gold-coated optical fiber tip with an aperture size of e.g. 100 nm, resulted in the detection of a locally enhanced (“tip-enhanced”) CARS signal.

4 Conclusions In this chapter, we have given a short introduction to coherent anti-Stokes Raman scattering (CARS) spectroscopy. We have introduced basics, which explain the nature of the nonlinear optical process and demonstrated the fundamental properties explained by energy and momentum conservation. From this, the requirements for the realization of an experimental setup have been deduced like phase-matching geometries, etc. Advantages of the nonlinear Raman technique over the linear spontaneous Raman scattering have been pointed out. Among others, there are the avoidance of fluorescence background, the considerable signal intensity due to the generation of a laser-like coherent signal, the achievable high spatial resolution of the technique, and the relatively simple use of CARS for time-resolved vibrational spectroscopy. We have not discussed the more obvious application of CARS for frequencydomain vibrational spectroscopy as a counterpart to linear spontaneous Raman spectroscopy. However, we have selected two specific fields of applications, which on the one hand demonstrate the usefulness of the four-wave mixing approach and on the other hand also show that there are still open questions, which require further research. As first application, we have introduced time-resolved CARS (tr-CARS). Using femtosecond laser pulses, the observation of coherent vibrational excitations is possible giving access to ultrafast vibrational dynamics. Vibrational energy transfer and the coupling between different vibrational modes can thus be investigated on an elementary time scale. The usefulness of the tr-CARS technique has already been demonstrated for different molecular systems, however, there are still many possible applications, which have not been tackled up to now. The combination of the surface-enhancement (SE) effect occurring when electromagnetic fields interact with molecules in contact or in close vicinity of nanostructured metal surfaces can also enhance CARS. However, the SE-CARS effect is less efficient compared to SE-Raman scattering (SERS) when using standard substrates like colloidal silver or gold. The required phase-matching and coherence conditions can to some extent be locally fulfilled when very regular SE-CARS substrates are used. Besides the enhancement of CARS signals also extremely high

Coherent Anti-Stokes Raman Scattering: Basics, Theoretical … Table 1 χ3 values for some materials

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Material

Wavelength / nm

χ3 / 10–22 m2 /V2

Reference

Quartz

1313

4.2

[22]

Ethanol

1313

7

[22]

Cyclo-hexane

1313

8.7

[22]

Water

1053

3.9

[22]

Polydiacetylene

1053

600

[22]

spatial resolution beyond the diffraction limit of optical microscopes can be achieved when tip-enhancement techniques are applied. A combination of SE-CARS and trCARS allows for the detection of ultrafast vibrational dynamics with ultrahigh spatial resolution. Many potential applications can be envisaged, but at the same time still a better understanding of the underlying enhancement mechanisms is required.

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Optical Cavity and Laser-Based High-Resolution Spectroscopic Techniques and Applications

Modern Experimental Techniques in Ultrafast Atomic and Molecular Physics P. Madhusudhan, Rituparna Das, Pranav Bharadwaj, Pooja Chandravanshi, Swetapuspa Soumyashree, Vinitha Nimma, and Rajesh K. Kushawaha

Abstract Electron dynamics in atoms and molecules occur on attosecond (10−18 s) timescale while nuclear dynamics take picosecond (10−12 s) to femtosecond (10−15 s) duration. Science at these timescales is known as ultrafast science. The advent of femtosecond laser and modern experimental techniques made it possible to chase the nuclear and electron dynamics in atoms and molecules, resulting in an avenue for controlling molecular reactions. A new nonlinear phenomenon involving the interaction of intense laser light with atoms known as high harmonic generation has been discovered, which has given birth to attosecond science. Attosecond light pulses have allowed probing and controlling the electron dynamics on their natural time scales in atomic and molecular systems. It is even possible to make molecular movies using modern electron or ion imaging techniques in conjunction with femtosecond and attosecond light pulses. Several experimental techniques have been developed and used in ultrafast sciences in the last 2–3 decades which has facilitated the study of ultrafast molecular reactions. In this chapter, a description of the key experimental techniques of ultrafast science is presented.

1 Introduction Following molecular reactions on their natural timescale has been of prime interest to many chemists and physicists, as it will eventually allow them to control the reactions. The advent of femtosecond laser and the further development of various ions or electrons imaging techniques and the pump-probe experimental scheme has made it possible to chase the electron and nuclear dynamics in atomic and molecular systems on their natural time scales and also control the chemical dynamics [1–5]. Experimentally, the quantum control of molecular dynamics [6] is not a straightforward task and is still a topic of ongoing research. At the center of this research is lightP. Madhusudhan · R. Das · P. Bharadwaj · P. Chandravanshi · S. Soumyashree · V. Nimma · R. K. Kushawaha (B) Physical Research Laboratory, Ahmedabad, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 D. K. Singh et al. (eds.), Modern Techniques of Spectroscopy, Progress in Optical Science and Photonics 13, https://doi.org/10.1007/978-981-33-6084-6_10

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matter interaction, in which intense femtosecond-attosecond light pulses interact with atoms and molecules. During the interaction numerous phenomena like multi-photon ionization (MPI) or above-threshold ionization (ATI) [7–11], tunnel ionization (TI) [12–14], high harmonic generation (HHG) [15–17], laser-induced electron diffractions (LIED) [18, 19], frustrated double ionization of atoms and molecules in strong laser fields [20, 21], and many more take place. The field of attosecond science is evolving rapidly. Nowadays, many research groups worldwide possess ultrafast lasers and high harmonic generation setup, which they utilize in the generation of attosecond pulses trains [22, 23] or even isolated attosecond pulses [24]. The femtosecond and attosecond light pulses have given the opportunity to understand the fundamental mechanism of any photo-induced molecular reaction. In experimental ultrafast atomic and molecular physics, the knowledge of ultrafast laser, femtosecond light pulse characterization techniques, electron/ion coincidence technique, time of flight mass spectrometer, recoil ion momentum spectrometer/COLTRIMS/reaction microscope and Velocity Map Imaging (VMI) spectrometer is essential. In this chapter, these experimental techniques are discussed which will be indispensable for graduate students.

2 Femtosecond Laser The 2018 Nobel Prize in Physics was awarded to Gérard Mourou and Donna Strickland for introducing the chirped pulse amplification technique [25] which has created the shortest and most intense laser pulses known to mankind. A femtosecond laser is essentially based on chirped pulse amplification which consists of mode-locked oscillator, pulse stretcher, amplifier, and pulse compressor. Chirped pulse amplification process starts by passing short laser pulses from a mode-locked oscillator through a grating or prism-based pulse stretching unit, followed by the amplifier where amplification of these pulses is performed. Finally, the stretched and amplified pulses pass through a pulse compression unit made of gratings or prism, and intense femtosecond light pulses are created. A schematic diagram of the typical components of a femtosecond laser is shown in Fig. 1. In the following sections, the basic principle of a femtosecond laser is discussed.

Fig. 1 Block diagram of femtosecond chirped pulse amplifier

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2.1 Femtosecond Oscillator The femtosecond oscillator produces seed pulses for the chirped pulse amplifier (CPA). These pulses are femtosecond light pulses generated in an optical cavity/resonator based on the mode-locking principle. The oscillator comprises four major components viz a gain medium (Ti:sapphire crystal), a set of mirrors forming the optical cavity/resonator, a pump laser, and dispersion compensation optical components. A schematic diagram of the femtosecond oscillator is shown in Fig. 2. In an optical cavity, light bounces between the mirrors and forms standing waves or modes. Generally, these modes oscillate independently in the cavity without any fixed-phase relationship among them and thus, they have random phases. In order to generate short (nanosecond) to ultrashort (pico- to femto-second) pulses, having a constant phase relation between the adjacent modes is of utmost importance. The technique of stabilizing the phases of the modes or creating a fixed phase relationship between the adjacent modes is termed as mode-locking. In a mode-locked oscillator, the modes constructively interfere with one another periodically, thus producing an intense burst or a pulse of light. The mode-locking methods can be classified as active mode-locking or passive mode-locking. In active mode-locking, a modulator (electro-optic) is placed in the cavity and modulated by an external signal. In passive mode-locking, a saturable absorber is placed in the cavity which ensures that the pulse gets modulated without any intervention. The absorption of light by a saturable absorber is intensitydependent. As radiation passes through the saturable absorber material, its atoms get excited to a higher energy level at a rate faster than de-excitation to the ground state. This depletes the ground state of the absorber thus, saturating the absorption of light. Therefore, a saturable absorber selectively absorbs or attenuates low-intensity constant wave light (pulse wings) and transmits light of relatively high intensity. In each round trip of the oscillating pulses, this process repeats, and high intensity light gets

Fig. 2 Schematic diagram of typical femtosecond oscillator

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amplified which leads to a train of pulses, resulting in the mode-locking of the oscillator. A successful passive mode-locking technique is the Kerr-lens mode-locking (KLM) which exploits the Kerr lensing effect in the gain medium. The combination of the Kerr lensing and a slit can be considered as a virtual saturable absorber. In KLM, the intense modes experience higher gain which favors mode-locking. For more details on active and passive mode-locking techniques, the readers are advised to go through these references [26–28].

2.2 Femtosecond Laser Amplifier A mode-locked femtosecond oscillator emits pulses having energy ∼nanojoules and repetition rate ∼80 MHz. For many atomic and molecular physics experiments and applications of femtosecond pulses to other branches of science, high power laser pulses are essential. Direct amplification of the oscillator pulses may damage the Ti:sapphire crystal. To tackle this problem, the idea of pulse stretching was introduced which was innovative and successful. However, pulse stretching followed by amplification by a single pass of the pulse through the amplifier unit may not give several watts of output power. In order to achieve high power femtosecond laser output, two design schemes of amplifier have been developed viz a multipass amplifier and a regenerative amplifier. In the multipass amplifier, the stretched seed beam passes through the gain medium (Ti:sapphire crystal) multiple times (typically 10 or more times) and gets amplified at each pass. In this design, each pass is well separated in space from the other pass. The schematic diagram of a multipass femtosecond amplifier is shown in Fig. 3, in which only four passes are shown for simplicity and explaining the working principle of the multipass amplifier. The main advantage of the multipass amplifier is that there are no dispersive materials in the cavity. The regenerative amplifier is similar to the femtosecond oscillator. In this amplifier, the seed beam passes through the gain medium multiple times without spatial separation and gets amplified. Pockels cells are used to control the passage of the

Fig. 3 Schematic of multipass femtosecond amplifier

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Fig. 4 Schematic diagram of femtosecond laser, PRL Ahmedabad India

seed beam. In the femtosecond laser lab of PRL, there is a regenerative femtosecond amplifier which produces 25 fs, 1 KHz, 800 nm, and 10 mJ laser pulses. The working principle of this amplifier is briefly demonstrated in Fig. 4. Before entering the pulse stretching unit, the oscillator’s seed pulses pass through a spatial spectrum filter to remove the central part of the spectrum (800 ± 50 nm). This process is essential since the emission spectrum intensity of the Ti: Sapphire crystal peaks at 800 nm, thereby distorting the gaussian seed pulse into a non-gaussian pulse. Hence 800 nm region is removed using a filter, and the remaining light is stretched using a diffraction grating. This stretched pulse is incident on the amplifier cavity at Brewster’s angle along with the light from the pump source to amplify the signal significantly. The valley in the spectrum is flattened. In the pulse compression region, the amplified pulse undergoes a phenomenon similar to the stretcher region but reversed. The stretched pulse is compressed using a similar grating. To maximize stretching and compressing as well as increase the yield of the amplified laser pulse, multiple round-trips are performed instead of a single pass.

2.3 Carrier-Envelope Phase (CEP) and CEP Stabilization Carrier-envelope phase (CEP) or carrier-envelope offset (CEO) of an ultrashort pulse is defined as the phase difference between the carrier wave and the pulse envelope (Fig. 5). It is an important feature of few-cycle ultrashort pulses. Practically, there is a pulse to pulse CEP change associated with the carrier wave due to the dispersive medium in the cavity, which changes the carrier-envelope offset in each round trip. As a result, the carrier-envelope phase of femtosecond oscillator pulses varies as a function of time.

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Fig. 5 Varying CEP of ultrashort pulses. CEP of 0 and π are mainly perspective dependent

The offset is measured in radian and varies from [0, 2π ]. The rate of change of offset in the pulse train is considered as ‘offset frequency’. Absolute carrier-envelope offset is generally of no physical importance since CEP keeps varying from pulse to pulse; hence relative CEO is used in CEP stabilization or CEP tagging for CEP sensitive experiments. The spatial asymmetry of the electric field of the ultrashort pulses (800 nm central frequency) with a gaussian profile contributes significantly to these sub-fs pulses, while the effect is averaged out for temporally longer pulses. Measurement of CEP can be done primarily by two methods: • f-2f spectrometer (non linear methods): This is a ‘self-referencing’interferometric technique. A single pulse is split, and one of the pulses is stretched by passing it through a material with high non-linearity and is spanned as an octave. The other pulse then interferes with the stretched pulse at a non-linear crystal. Beats observed at the interference are a quantitative measure of the carrier-envelope phase. For more details on the f-2f spectrometer and CEP stabilization, the readers are advised to go through these references [29, 30]. • Carrier-Envelope Phase Meter (CEPM): This technique is based on the abovethreshold ionization (ATI) and the stereographic detection by dual time-of-flight spectrometers. The parametric asymmetry plots (PAPs) can be derived from the ATI spectra, which can be used to characterize the few-cycle pulses. More details on CEPM (dual time-of-flight spectrometers) [31], CEP tagging experiment [32], and their extensive comparison are found here [33–35].

3 Femtosecond Pulse Characterization Techniques Ultrashort pulse characterization involves the measurement of the intensity and phase, of an ultrashort pulse, as a function of time or frequency. Measurement of ultrashort pulses is extremely important for determining the temporal resolution of experiments, generating shaped pulses [36] which have various applications [37], the effect of pulse shape (like chirp) on experimental results [38, 39], etc. Characterization of ultrashort pulses has been very challenging since their creation, as the femtosecond laser pulses are the shortest events ever created by man. Initially, autocorrelators were used for pulse characterization, but they do not give complete information about the pulse. All detectors (like power meters and photomultipliers) have a slow response time (∼ ns) and hence, they only measure the time-averaged

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  ∞  E(t)2 dt . They do not give any information about the −∞

temporal or spectral phase. An ultrashort pulse is mathematically represented as E(t) =



I (t)ei(ω0 t−φ(t)) ,

(1)

where I is the intensity, ω0 is the central frequency and φ(t) is the phase. The above equation can be written in the frequency domain as E(ω) =



S(ω − ω0 )ei(φ(ω−ω0 )) ,

(2)

where S(ω) is the spectrum of the pulse, and φ(ω) is the spectral phase. The spectral phase is related to the instantaneous frequency ω(t) as ω(t) = ω0 −

dφ . dt

(3)

Let us first take a look at the primeval techniques of ultrashort pulse characterization, before moving on to the latest characterization techniques.

Fig. 6 Schematic diagram of field autocorrelator (M = mirror, BS = beam splitter)

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• Field autocorrelation: This technique is based on the Michelson interferometer, where the laser pulse is first split into two—a reference and a gate pulse (replica of the reference but delayed by a small time interval τ ). The gate pulse is then scanned over the reference pulse for different time delays as shown in Fig. 6. The signal measured by the detector (power meter, photomultiplier, etc.) is therefore, given by  IField =

∞

−∞

  E(t) + E(t − τ )2 dt ,

(4)

∞ where the cross-term IAC (τ ) = −∞ E(t)E ∗ (t − τ ) dt is the autocorrelation function. Taking the Fourier transform of the autocorrelation function IAC (τ ) gives the spectrum of E(t). • Intensity autocorrelation: This pulse characterization technique is slightly different from the previous one. Here, the reference and gate pulses are focused and overlapped in a nonlinear second harmonic generation (SHG) crystal. The   ∞ autocorrelation signal −∞ I (t)I (t − τ )dt travelling parallel to the optic axis is retained while the non-collinear gate and reference beams are eliminated. The autocorrelation signal (intensity) is measured with a slow detector like a power meter or a photomultiplier. This technique helps us in measuring the time duration of the ultrashort pulse. • Interferometric autocorrelation: Also termed as phase-sensitive autocorrelation or fringe-resolved autocorrelation (FRAC), this method was developed by JeanClaude Diels [40]. The experimental setup includes a Michelson interferometer, with the reference and gate pulses overlapped in a collinear fashion inside a nonlinear SHG crystal. Thus, there is an interference between the second harmonic signal generated by the two interacting beams and those generated by the two beams individually, resulting in interference fringes as a function of delay. By plotting the intensity vs time delay between the reference and gate pulses, we can obtain the interferogram or the FRAC trace which is given by Diels et al. [40]:  IFRAC (τ ) =

−∞ ∞

 = + 

∞

   E(t) + E(t − τ ) 2 2 dt

 I (t)2 + I (t − τ )2 dt

−∞  ∞

∞

−∞





 I (t) + I (t − τ ) Re E(t)E ∗ (t − τ ) dt + −∞  ∞ 

2 ∗ 2 Re E(t) E (t − τ ) dt + I (t)I (t − τ )dt.

(5)

−∞

One advantage of this technique is that it gives some phase information and is sensitive to the pulse shape, unlike the previous two methods [40]. However, as the pulse becomes more complex, the phase information is washed out by the fringes.

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The major disadvantage of all the above techniques is that the temporal or spectral phase of the ultrashort pulse cannot be measured, without which complete information of a pulse is not available. For the complete ultrashort pulse characterization, the following techniques were eventually developed.

3.1 Frequency Resolved Optical Gating (FROG) FROG was the first technique to measure the spectral phase of ultrashort pulses completely and was developed by Kane and Trebino in 1993 [41]. Using this technique, we can determine the intensity I (t) and phase φ(t) of a pulse in real-time. The FROG setup, as shown in Fig. 7, is based on the intensity autocorrelation technique, but here the slow detector is replaced by a spectrometer. Thus, at each delay of the gate pulse with respect to the reference, we can record the spectrum of the autocorrelation signal (hence, the name frequency-resolved). The plot of the wavelength vs. delay is called the FROG trace or spectrogram from which we can determine the intensity as a function of time or frequency. The phase information can be obtained using a two-dimensional phase-retrieval problem. The FROG trace (intensity vs. time or frequency plot) is given by

Fig. 7 Schematic diagram of intensity autocorrelator/FROG. The intensity autocorrelator utilizes a slow detector while the FROG setup replaces the detector by a spectrometer (M = mirror, BS = beam splitter)

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  2   2      IFROG (ω, τ ) ∝ Esig (ω, τ ) = FT Esig (t, τ ) = 

∞

−∞

Esig (t, τ )e

−iωt

2  dt  , (6)

where Esig (t, τ ) = E(t)Egate (t − τ ),

(7)

and E(t) and Egate (t − τ ) refer to the reference and time-delayed gating pulses, respectively. The FROG technique has been used with different beam geometries and phase distortions like second-harmonic generation (SHG FROG), third-harmonic generation (THG FROG), polarization gating (PG FROG), and self-diffraction (SD FROG), the details of which can be found in [42, 43].

3.2 Spectral Phase Interferometry for Direct Electric-Field Reconstruction (SPIDER) In 1998 Iaconis and Walmsley reported a new interferometric technique of ultrashort pulse characterization, which they named Spectral Phase Interferometry for Direct Electric-field Reconstruction (SPIDER) [44]. The SPIDER is based on the measurement of spectral intensity and spectral phase using spectral shearing interferometry. It utilizes non-iterative algorithms for the direct reconstruction of the electric field and is, therefore, capable of real-time ultrashort pulse characterization. We can characterize pulses of duration 10−11 s or shorter using this technique. The schematic diagram of the SPIDER is shown in Fig. 8. The spectral intensity is measured with the help of a spectrometer. For determining the spectral phase, first, the pulse is split into two with a beam splitter (BS)—one pulse is stretched (chirped) from ∼fs to ∼ps by passing it through a dispersive optical element (like a glass slab or a pair of gratings), while the other unchirped pulse is passed through a mismatched Michelson interferometer producing a pulse pair having a time delay of τ between them. The two unchirped replica pulses and the chirped pulse are made to interfere in a nonlinear crystal (BBO) where they are upconverted to two blue pulses by sum frequency generation (SFG). As each of the fundamental unchirped pulses overlap with different parts of the chirped quasi-monochromatic pulse, the upconverted blue pulses have different central frequencies (or are spectrally sheared). The upconverted blue pulse pair is then sent to a spectrometer, which generates the interferogram. An inversion algorithm is used to retrieve the spectral phase by comparing the recorded interferogram to a calibrated interferogram that serves as a reference. Mathematically we can represent two duplicate pulses having a time delay τ between them and spectrally sheared by Ω as: E1 (t) = E(t) eiω0 t

(8)

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Fig. 8 Schematic diagram of SPIDER (M1-M11 = mirrors, BS1-BS3 = beam splitters, DS1, DS2 = delay stages)

E2 (t) = E(t + τ ) ei(ω0 +Ω) t ,

(9)

where Ω is called the spectral shear. On interference, the resultant spectral interferogram can be written as 2  (+) (−) ˆ (ω)e−iωτ + Sˆ ac (ω)eiωτ , S(ω) = Eˆ I (ω) = Sˆ dc (ω) + Sˆ ac

(10)

where     ˆ − ω0 )2 + E(ω ˆ − ω0 − Ω)2 , Sˆ dc (ω) = I (ω − ω0 ) + I (ω − ω0 − Ω) = E(ω (+) ˆ − ω0 − Ω), and (ω) = Eˆ ∗ (ω − ω0 )E(ω Sˆ ac (−) ˆ − ω0 )Eˆ ∗ (ω − ω0 − Ω). (ω) = E(ω Sˆ ac

The retrieval of the spectral phase is explained in the flow chart in Fig. 9. Figure 10 shows the pulse characterization of the femtosecond laser pulses of PRL, Ahmedabad, using SPIDER. Here, the time domain display illustrates the pulse reconstruction from the SPIDER interferogram. The black curve indicates the intensity pulse shape while the blue pulse shape indicates the Fourier limit of the measurement. The vertical blue cursors indicate the time window for reconstruction, and the pink curve gives the temporal phase. The measured pulse duration of PRL’s femtosecond laser is 27 fs.

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Fig. 9 Flow chart depicting spectral phase retrieval in SPIDER

Fig. 10 Pulse characterization of PRL’s femtosecond laser using the SPIDER

3.3 GRating-Eliminated No-Nonsense Observation of Ultrafast Incident Laser Light E-Fields (GRENOUILLE) Autocorrelators and FROG have a number of disadvantages. Firstly, the reference and gate pulses need to be carefully aligned in the second-harmonic crystal, and the alignment should be maintained when the gate pulse is delayed. Secondly, the phase-matching bandwidth condition requires the use of a thin SHG crystal, which results in a weak second-harmonic signal. GRENOUILLE, developed in 2001 [45], is a better alternative to autocorrelators and FROG as it gets rid of the complexity, high cost, and maintenance issues. Figure 11 shows the comparison between the FROG and GRENOUILLE setups [46]. The GRENOUILLE setup is shown in Fig. 12. The first cylindrical lens focuses the beam in the thick SHG crystal along the horizontal axis with a range of incidence angles. The Fresnel biprism splits and superimposes the beam at the focus inside the crystal, with the delay between the superimposing beams varying in the horizontal position. Thus, the combination of the cylindrical lens, Fresnel biprism, and the SHG crystal ensures the self-gating process while also maintaining the beams’ alignment in space and time. A thick SHG crystal is used as its purpose is twofold. Firstly, it enhances the second-harmonic signal strength. Secondly, it acts as a spectrometer

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Fig. 11 Schematic diagram comparing FROG and GRENOUILLE setups. This figure is reprinted from [46] with permission from The Optical Society (OSA)

by emitting a second-harmonic signal of varying wavelengths for beams incident at different vertical angles of incidence (therefore, satisfying different phase-matching conditions). A pair of cylindrical lenses is then used to image the spectrum in a camera. The wavelength is mapped along the vertical direction while the delay between the pulses is mapped along the horizontal direction, which gives the trace. Figures 13 and 14 show the femtosecond pulse characterization performed for the Ti: sapphire laser pulses of PRL, Ahmedabad.

4 Femtosecond Pump-Probe Techniques Time-resolved studies of photo-induced processes in atoms and molecules (gasphase samples) require the pump-probe technique and the electron/ion detection and momentum mappings techniques such as reaction microscopy or velocity map imaging spectrometry. In this section, we discuss the femtosecond pump-probe technique only. In the femtosecond pump-probe technique, the laser pulse is split into two - a

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Fig. 12 GRENOUILLE from above and side views. This figure is reprinted from [46] with permission from The Optical Society (OSA)

Fig. 13 [Left] The measured FROG Trace, [Right] retrieved FROG Trace, of the the PRL’s Femtosecond laser pulse using GRENOUILLE

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Fig. 14 [Left] Temporal intensity and phase, [Right] spectral intensity and phase, of the PRL’s Femtosecond laser pulse using GRENOUILLE

pump pulse and a probe pulse. The probe pulse is delayed by some interval τ (by increasing the path length it traverses) and then recombined with the pump pulse (in a Mach-Zehnder interferometer setup), followed by their interaction with the molecular (or atomic) beam in the Velocity Map Imaging Spectrometer. The pump and probe pulses usually have a pulse width of 25 fs or even shorter. In order to ensure that the two pulses overlap in time, they are first combined in a nonlinear BBO (barium borate) crystal. The pump and probe pulses’ intensities are kept low initially such that the second harmonic signal generated by each of the pulses individually is nearly insignificant. Only when the two pulses overlap entirely in time that the resultant second harmonic signal is significantly intense. In the Velocity Map Imaging Spectrometer, the time and spatial overlap of the pump, probe, and the pulsed molecular beam is monitored by the TOF spectrum. When these pulses overlap in time, the TOF signal is much higher as compared to when there is partial or no overlap. The relative intensities of the pump and probe pulses depend on the experiment to be performed. When τ = 0, the pump and probe pulses overlap with each other and they interact simultaneously with the molecular (or atomic) beam. When τ = 0, there is a timedelay between the two pulses. In that case, the pump pulse first excites or ionizes the molecules (or atoms), and then the probe pulse probes the evolution of the excited molecules or ions on the potential energy surfaces (PESs) at different time delays. Various parameters like the yield of ions, kinetic energy release (total kinetic energy of all ions, electrons, and neutral particles created during interaction), etc. studied as a function of the pump-probe delay gives us information about how photoinduced process in atoms/molecules proceed with time. Figure 15 demonstrates the pump-probe setup in combination with the velocity map imaging (VMI) spectrometer. Figure 16 is an example of how the pump-probe technique can be used to perform time-resolved studies of molecular reactions. In this case, the H+ 3 formation

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Fig. 15 a Schematic diagram of the pump-probe setup, b schematic diagram of time-resolved experiments performed in the Velocity Map Imaging (VMI) spectrometer using the pump-probe technique, and c fragmentation dynamics of a molecule AB with pump-probe setup

+ Fig. 16 Normalized H+ 3 and D3 yields from ethylene glycol, acetone, and CD3 OD as a function of pump-probe delay. This figure is reprinted from [47] with permission from Springer Nature

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from polyatomic molecules such as ethylene glycol, acetone, and CD3 OD has been + investigated using pump-probe techniques. The yields of H+ 3 and D3 have been measured as a function of pump-probe delay, and the results are plotted in this figure. + The time-dependent variation of the H+ 3 and D3 yields have been observed. This information can be used to estimate the lifetime of the precursor ion. Details about the mechanism of H+ 3 formation from polyatomic molecules in an intense laser field using the pump-probe technique is available in Ref. [47].

5 Experimental Techniques for Detection and Imaging the Momentum of Electrons and Ions The interaction of intense femtosecond pulses with the molecules can be used to study the ultrafast electron and nuclear dynamics in such a system. The electrons, ions, and neutral particles are generated during this interaction. It would be possible to reveal the ongoing processes [48] induced by femtosecond pulses by detecting and measuring the energy/momentum of the electrons/ions/neutral fragments. Different electron/ion spectrometers have been developed over the past few decades and used in ultrafast AMO physics to understand the ultrafast dynamics in atomic and molecular systems. Some of these spectrometers are the Time-Of-Flight Mass Spectrometer (TOF MS), COLd Target Recoil Ion Momentum Spectrometer (COLTRIMS), and the Velocity Map Imaging (VMI) spectrometer. The working principle of these spectrometers is discussed in the following subsections.

5.1 Time-of-Flight Mass Spectrometer (TOF MS) Time-of-flight Mass spectrometry is the technique of detecting ions on the basis of their mass-to-charge ratio. The time-of-flight (TOF) mass spectrometer works on the principle of measuring the time-of-flight of ions traveling over a distance of known length. The schematic diagram of a typical TOF mass spectrometer is shown in Fig. 17. The linear TOF mass spectrometer consists of a repeller plate, an extractor plate, a drift tube, and a detector. The region between the repeller and the extractor plates is called the interaction region. The separation between these plates and the length of the drift tube are determined by the Mclaren criteria based on the space focusing condition and will be discussed later in this section [49]. In 1955, Wiley and McLaren reported an improved time-of-flight mass spectrometer based on dual electrostatic fields (extraction (Es ) and acceleration regions (Ed )) for guiding the ions to the detector, and a field-free region known as the drift tube. This design of the TOF spectrometer is known as the classic Wiley-McLaren TOF spectrometer. In the absence of the second field region (Ed is zero), this spectrometer

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Fig. 17 Schematic diagram of the TOF mass spectrometer

forms a single-field TOF MS. The single or double field mass spectrometers are used on the basis of the resolution required in the experiment. The ions formed in the interaction region are repelled by the repeller and then accelerated by the electric field(s) in the extraction region. In the extraction region itself, the ions of different mass-to-charge ratios are separated from one another and form different peaks in the TOF spectrum. In the drift region, the ions travel with constant velocity. A detector (microchannel plate (MCP)) is situated at the end of the drift tube which detects the ions impinging on it. In order to determine the dependence of the TOF on the mass-to-charge ratio of the ions, let us consider the case of a single-field TOF MS. The distance s travelled by an ion of mass m and charge q in the extraction region Es is given by 1 s = uts + ats2 = 2



   pz 1 qEs 2 ts + t , m 2 m s

(11)

where pz is the initial momentum of the ion along z-axis (TOF-axis is considered to be the z-axis). Solving the quadratic equation for ts , we obtain ts =

pz ± − m



pz m

2

    qEs qEs . + 2.s. m m

(12)

Velocity ‘v’of the ion at the end of the extraction region is given by v 2 = u2 + 2as.

 v=

pz m

2

  qEs .s. + 2. m

Time spent by the ion in the drift tube tD = ion is given by

D . v

(13)

Thus, the total time of flight of the

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  2 − pmz

  s + 2.s. qE m D   +     . 2 qEs pz s m .s + 2. qE m m

pz m

±

T = ts + tD =

(14)

For ions having zero initial momentum (pz = 0), the TOF reduces to √ T=

D 2qsEs /m + . qsEs /m 2(qEs /m)s

(15)

√ Since s, D, and Es are known quantities, this equation is of the form T = a + b. m/q, where a and b are constants. This equation is called the calibration equation and √ it is m/q used for calibrating the TOF MS. This shows that the TOF is proportional to √ m/q value will have smaller TOF than ions with of the ions. Ions with smaller √ higher m/q value. Since the ionization region is a small spherical region of volume