Molecular Spectroscopy of Dynamically Compressed Materials (Shock Wave and High Pressure Phenomena) 9811924198, 9789811924194

Citation preview

Shock Wave and High Pressure Phenomena

David S. Moore

Molecular Spectroscopy of Dynamically Compressed Materials

Shock Wave and High Pressure Phenomena Founding Editor Robert A. Graham

Honorary Editors Lee Davison, Tijeras, NM, USA Yasuyuki Horie, Santafe, NM, USA Series Editors Frank K. Lu, University of Texas at Arlington, Arlington, TX, USA Naresh Thadhani, Georgia Institute of Technology, Atlanta, GA, USA Akihiro Sasoh, Department of Aerospace Engineering, Nagoya University, Nagoya, Aichi, Japan

Shock Wave and High Pressure Phenomena The Springer book series on Shock Wave and High Pressure Phenomena comprises monographs and multi-author volumes containing either original material or reviews of subjects within the field. All states of matter are covered. Methods and results of theoretical and experimental research and numerical simulations are included, as are applications of these results. The books are intended for graduate-level students, research scientists, mathematicians, and engineers. Subjects of interest include properties of materials at both the continuum and microscopic levels, physics of high rate deformation and flow, chemically reacting flows and detonations, wave propagation and impact phenomena. The following list of subject areas further delineates the purview of the series. In all cases entries in the list are to be interpreted as applying to nonlinear wave propagation and high pressure phenomena. Development of experimental methods is not identified specifically, being regarded as a normal part of research in all areas of interest. Material Properties Equation of state including chemical and phase composition, ionization, etc. Constitutive equations for inelastic deformation Fracture and fragmentation Dielectric and magnetic properties Optical properties and radiation transport Metallurgical effects Spectroscopy Physics of Deformation and Flow Dislocation physics, twinning, and other microscopic deformation mechanisms Shear banding Mesoscale effects in solids Turbulence in fluids Microfracture and cavitation Explosives Detonation of condensed explosives and gases Initiation and growth of reaction Detonation wave structures Explosive materials Wave Propagation and Impact Phenomena in SolidsShock and decompression wave propagation Shock wave structure Penetration mechanics Gasdynamics Chemically Reacting Flows Blast waves Multiphase flow Numerical Simulation and Mathematical Theory Mathematical methods Wave propagation codes Molecular dynamics Applications Material modification and synthesis Military ordnance Geophysics and planetary science Medicine Aerospace and Industrial applications Protective materials and structures Mining

More information about this series at https://link.springer.com/bookseries/1774

David S. Moore

Molecular Spectroscopy of Dynamically Compressed Materials

David S. Moore Moore Shock Spectra Santa Fe, NM, USA

Company’s website: https://mooreshockspectra.online ISSN 2197-9529 ISSN 2197-9537 (electronic) Shock Wave and High Pressure Phenomena ISBN 978-981-19-2419-4 ISBN 978-981-19-2420-0 (eBook) https://doi.org/10.1007/978-981-19-2420-0 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 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

Preface

The material in this book is the result of work done throughout my entire career, which has been primarily focused on observing, at the molecular level and in real time, the chemistry that occurs during and immediately after dynamic compression of molecular materials. The book is organized into nine chapters covering everything from very basic concepts to very detailed and in-depth discussions of seminal studies. Chapter 1 is a brief introduction to the rest of the book and outlines some of the terminology, units, and symbols used, which are sometimes different than those utilized in typical and historical shock physics papers. That choice was made in order to unify shock physics with accepted usages in physics and chemistry as codified by the Système International d’Unités and the International Unions of Pure and Applied Physics and Chemistry. Chapter 2 provides the reader the basic information on molecular spectroscopy needed to understand the terminology and concepts used in the remainder of the book. It starts with a discussion of spectroscopies involving electronic or ro-vibronic transitions in the ultraviolet, visible, and near-infrared spectral regions, and then presents ro-vibrational spectroscopic methods operating in the infrared regions. It then compares the information on molecular species available from these two kinds of spectroscopy, and its importance to experiments desiring to follow the progress of chemical reactions. The chapter continues with a detailed analysis of bandwidth broadening mechanisms including lifetime broadening (which also includes chemical bond lifetime), phase lifetime broadening (including collisional processes), inhomogeneous broadening, and Doppler broadening. It then presents vibrational hot bands and their contributions to ro-vibrational spectra under dynamic compression, and concludes with sections on optical systems and laser fundamentals, as these are vital tools for the spectroscopic methods discussed in this book. The Chap. 3 presents relevant details on the plethora of dynamic compression methods used to study the spectroscopy of shocked molecular materials. It begins with the experimental details of historical plate impact methods, using explosives or gas guns to accelerate flyer plates in embodiments aimed at collecting spectra. It then discusses the experimental details of the lower strain rate methods, from Kolsky bars, ramp compression, and dynamic anvil cells. It ends with a section on laser v

vi

Preface

shock compression methods concentrating on tabletop methods including ultrafast laser direct shock drive and laser driven flyers, but not including large-scale high compression systems like Omega or NIF. The easiest, and therefore, historically the first, method to obtain spectra of molecular samples under dynamic compression is by the use of UV-visible emission as it only involves a high-speed camera and a spectrometer. In contrast, UV-visible absorption experiments require a broadband UV-visible source and a way to direct the light through the shock-compressed sample, adding considerable complexity. The addition of a laser to induce fluorescence requires still more complexity but provides additional capabilities to observe molecular species. Chapter 4, therefore, carries the reader through the historical developments of electronic molecular spectroscopy methods used in dynamic compression research, as well as some seminal research results. It also compares dynamic spectroscopy of some materials to their spectroscopy at static high pressure/high temperatures. Unlike electronic transitions that involve the ro-vibrational levels within both the upper and lower electronic states encompassed by the transition, vibrational spectroscopies only involve the ro-vibrational levels within a given electronic (usually ground) state, so they are usually less congested and more information rich. Transitions between these ro-vibrational states occur via emission or absorption of photons in the infrared to radiofrequency regions or via inelastic scattering of photons as in the Raman effect. Chapter 5 leads the reader through the historical developments of infrared absorption vibrational molecular spectroscopy methods used in dynamic compression research, as well as some seminal research results. Chapter 6 carries the reader through the historical developments of Raman molecular spectroscopy methods used in dynamic compression research, as well as some seminal research results. Both chapters compare dynamic compression vibrational spectroscopy of some materials to their vibrational spectra at static high pressures and high temperatures. Because Raman scattering is a very weak process, some researchers have utilized coherent Raman processes in dynamic compression experiments to increase the signal strength, as well as allow very short time scale measurements. Chapter 7 presents the basics of several coherent Raman spectroscopy methods used in dynamic compression research, as well as some select, insightful, research results. It then compares dynamic spectroscopy of some materials to related results at static high pressure/high temperatures. Chapter 8 explores the use of X-ray absorption spectroscopy as an alternative method to observe molecules under dynamic compression. X-ray diffraction under dynamic compression to determine structure is well known, but is limited in its ability to observe molecular changes. Low energy X-ray absorption and emission exhibit peaks due to transitions at the carbon, nitrogen, or oxygen edges that are sensitive to the chemical bonding environment. The chapter outlines the methodologies, reviews some recent studies, and discusses future possibilities. The last Chap. 9 summarizes the main findings and conclusions in the book and attempts to point the way into the future.

Preface

vii

I sincerely hope that the material presented in this book will stimulate further advances in the field so that some of the remaining puzzles are solved. Santa Fe, NM, USA 2021

David S. Moore

Acknowledgments

I would like to acknowledge Dennis Vasilik, Michael Powell, and Margo Greenfield for their critical reading of the manuscript. I am especially indebted to Steve Schmidt for his guidance at the beginning of this scientific venture, as well as all of my mentors, collaborators, and colleagues around the world, and the large number of students and postdocs who contributed to the work at Los Alamos. Finally, I am inexpressibly grateful for the incredible lifelong support and encouragement from my wonderful wife Viera.

ix

Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Historical Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 1 1 2

2 Molecular Spectroscopy Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Electronic Versus Ro-Vibrational Molecular Information Available . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1 Electronic Spectroscopies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2 Ro-Vibrational Spectroscopies . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Bandwidth Broadening Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1 Population Relaxation (t 1 Time) . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2 Dephasing (t 2 Time) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3 Inhomogeneous Broadening . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.4 Doppler Broadening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Hot Bands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1 Hot Band Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Optical Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.1 Lenses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.2 Mirrors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.3 Dispersive Optics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.4 Spectrometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.5 Optical Conductance of Spectrometers . . . . . . . . . . . . . . . . . . 2.5 Laser Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.1 Laser Propagation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3 3 6 7 8 8 9 10 11 13 13 15 17 18 20 23 25 26 31

3 Dynamic Compression Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Explosively Driven Flyers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Gun-Driven Flyers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Kolsky/Split-Hopkinson Bar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Isentropic Compression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

33 33 35 37 39 39 xi

xii

Contents

3.6 Laser Shock Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.1 Direct Drive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.2 Laser-Driven Flyers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7 Shock to Material Energy Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

40 41 53 54 56

4 Electronic Molecular Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 UV–Visible Emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1 Pyrometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2 Molecular Emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 UV–Visible Absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1 Ultrafast Laser Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Laser-Induced Fluorescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

57 57 59 60 65 71 81 85 88

5 Infrared Molecular Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Infrared Absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.1 Time-Resolved Infrared Spectral Photography . . . . . . . . . . . . 5.1.2 Ultrafast Laser Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.3 Infrared Thin-Film Interference Effects . . . . . . . . . . . . . . . . . . 5.1.4 Ultrafast Infrared Absorption Methods . . . . . . . . . . . . . . . . . . 5.2 Other Complications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1 Path Integration Through a Shocked Sample . . . . . . . . . . . . . 5.3 Static High-Pressure Infrared Spectroscopy . . . . . . . . . . . . . . . . . . . . 5.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

89 89 93 97 100 101 110 110 112 113

6 Raman Molecular Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Raman Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.1 Raman Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.2 Experimental Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.3 Historical Developments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Stokes/Anti-Stokes Raman Temperature Measurement . . . . . . . . . . . 6.3 Surface Enhanced Raman . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

115 115 118 122 126 134 138 141

7 Coherent Raman Spectroscopies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1 Coherent Raman Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Stimulated Raman . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.1 Stimulated Brillouin Scattering . . . . . . . . . . . . . . . . . . . . . . . . 7.3 Stimulated Raman Gain and Loss Spectroscopies . . . . . . . . . . . . . . . 7.3.1 Stimulated Raman Gain and Loss Temperature Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4 Coherent Anti-Stokes Raman . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.1 Historical Coherent Anti-Stokes Raman (CARS) Shock Compression Experiments . . . . . . . . . . . . . . . . . . . . . . . 7.5 Raman-Induced Kerr Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

143 143 145 148 149 150 152 158 169

Contents

xiii

7.6 Interference Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 7.7 Sum Frequency Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 7.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 8 X-ray and Neutron Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 X-ray Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3 X-ray Diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.1 Small Angle Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4 X-ray Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4.1 EXAFS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4.2 XANES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4.3 X-ray Raman . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5 Neutron Resonance Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

183 183 184 184 185 186 189 191 194 196 198

9 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229

About the Author

David S. Moore is a Los Alamos National Laboratory Fellow, an American Physical Society Fellow, and an International Union of Pure and Applied Chemistry Fellow. He was a Los Alamos National Laboratory Director Funded Postdoctoral Fellow, an Alexander von Humboldt Fellow, and co-editor of the Wiley Handbook of Spectroscopy—2nd Edition. His work concentrates on revolutionary approaches to the study of shocked materials and the detection of explosives. He utilizes molecular spectroscopies, benchtop ultrafast laser methods, and unique diagnostics to obtain results pivotal to the molecular level understanding of material behavior at extreme conditions. e-mail: [email protected]

xv

Abbreviations

AOM AS-SRS ASTERISK BBDL C-4 CARS CCD CHNO CJ Comp-B COORS CPA CPM CSRS CW DAC DETA EXAFS FFT FID FROG FWHM HMX HNAB HNS HOMO HWHM IBM

Acousto-optic modulator Amplified spontaneous-stimulated Raman scattering Polarization control with polarization vectors oriented like an asterisk Broadband dye laser Composition C-4 (90% RDX, 5% plasticizer, 3% rubber, 2% mineral oil) Coherent anti-Stokes Raman spectroscopy Charged coupled device Carbon, hydrogen, nitrogen, oxygen materials Chapman–Jouguet Composition B (60% RDX, 40% TNT, 1% paraffin wax) Common old ordinary Raman spectroscopy Chirped pulse amplified Colliding pulse mode-locked Coherent Stokes Raman scattering Continuous wave Diamond anvil cell Diethylene triamine Extended X-ray absorption fine structure Fast Fourier transform Free induction decay Frequency-resolved optical gating Full width at half maximum High Melting Explosive—a.k.a. octogen, cyclotetramethylenetetranitramine, 1,3,5,7-tetranitro-1,3,5,7-tetrazocane Hexanitroazobenzene Hexanitrostilbene Highest occupied molecular orbital Half-width at half-maximum International Business Machines xvii

xviii

IFFT IRRAS IUPAC IUPAP LANL LANSCE Laser LCLS at SLAC LED LLNL LUMO MIR NBDL Nd:YAG NIR OMA OPA ORVIS PBX PE PETN PMMA PNNL PVN QD RDX

RIKES SAM SASR SBS SERS SFG SHG SIP-CARS SLM SNL SRGS SRLS TATB TEM TMD TNB

Abbreviations

Inverse fast Fourier transform Infrared reflection–absorption spectroscopy International Union of Pure and Applied Chemistry International Union of Pure and Applied Physics Los Alamos National Laboratory Los Alamos Neutron Scattering Center Light amplification by stimulated emission of radiation Linac Coherent Light Source at the Stanford Linear Accelerator Center Light-emitting diode Lawrence Livermore National Laboratory Lowest unoccupied molecular orbital Mid-infrared Narrow band dye laser Neodymium yttrium aluminum garnet laser Near-infrared Optical multichannel analyzer Optical parametric amplifier Optically recorded velocity interferometry system Polymer bonded explosive Potential energy Pentaerythritol tetranitrate Polymethyl methacrylate plastic Pacific Northwest National Laboratory Polyvinyl nitrate Quantum dots Royal Demolition Explosive—a.k.a. hexogen, cyclonite, cyclotrimethylenetrinitramine, 1,3,5-trinitroperhydro-1,3,5triazine Raman induced Kerr effect spectroscopy Self-assembled monolayer Stokes/anti-Stokes Raman Stimulated Brillouin scattering Surface-enhanced Raman spectroscopy Sum frequency generation Second harmonic generation Spectral interferometric implementation of CARS Spatial light modulator Sandia National Laboratory Stimulated Raman gain Stimulated Raman loss; sometimes called Inverse Raman Triamino trinitrobenzene Transverse electromagnetic mode Theoretical maximum density Trinitrobenzene

Abbreviations

TNT TOF TRISP UDE UIUC UV WSU XANES XAS XRD XTX

xix

Trinitrotoluene Time of flight Time-resolved infrared spectral photography Ultrafast dynamic ellipsometry University of Illinois-Urbana Champaign Ultraviolet Washington State University X-ray absorption near-edge spectroscopy X-ray absorption spectroscopy X-ray diffraction Extrudable explosive

Chapter 1

Introduction

1.1 Motivation It is my observation that historical advances in the understanding of the molecular changes in condensed materials induced by dynamic compression events have run parallel to the advances in the experimental probes used to measure those changes. Traditionally, insight into condensed material dynamic compression behavior has been obtained from bulk property measurements, such as density, mechanical deformation, and hydrodynamic effects. These measurements are usually sufficient to understand the continuum level material response when the state reached is steady, but are difficult to interpret when reactions or other time-dependent phenomena occur. To gain a microscopic or molecular level understanding of these time-dependent phenomena, a large number of spectroscopic methods have been developed and applied. These methods have run the gamut from emission spectroscopy to techniques as complicated as time-resolved infrared spectral photography or coherent Raman spectroscopy, all with the aim to provide species-specific information about dynamic compression induced chemistry (or indeed any chemistry that occurs at dynamic extreme conditions). One particularly thorny example of these endeavors is the quest to observe the chemical processes (reaction paths and kinetics) occurring in detonation initiation, resolved in space and time. It seems that Mother Nature doesn’t particularly want us to succeed, as you will see in succeeding chapters of this book, but we are persistent and capable and the future looks bright.

1.2 Historical Events It was noticed historically (and even today, as can be seen in movies and television shows) that explosive events are usually very bright optically, at least to the naked eye. A number of early explosive researchers thought to gather that light and put © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 D. S. Moore, Molecular Spectroscopy of Dynamically Compressed Materials, Shock Wave and High Pressure Phenomena, https://doi.org/10.1007/978-981-19-2420-0_1

1

2

1 Introduction

it through a spectrometer or record it with high-speed cameras, or even combine the two! One of these researchers was Anatoly Dremin, who in the early 1960s very simply focused the explosive light emission onto a high-speed camera. Even in the simple liquid explosive nitromethane, he observed that the emission had spatial intensity variations across the emitting face of what was considered to be a uniform steady detonation. These features were larger in spatial extent when the nitromethane was diluted with acetone and were quite large indeed just before the mixture was no longer detonable [1]. It took a number of years before emission spectra were successfully recorded, which was done by inserting a spectrograph in front of the high-speed camera. It sounds simpler than it is. One of the first successful integrations of these two devices, in an experiment designed to record emission spectra of detonating hexanitrostilbene, was achieved by Robert Setchell in the 1980s [2, 3]. Despite very careful characterization of the timing and detector response, the emissions were quite difficult to interpret, although some resemblance to emission features from diatomic radicals and water was suggested. The puzzling part is: why would the emission features, if they originated in the high-pressure/high-temperature state of the reacting condensed explosive, be assignable to gas-phase emission spectra? It is known that spectra, particularly ro-vibronic spectra, but also UV/visible emission and absorption spectra, broaden and shift with pressure and temperature [4–8]. This puzzle is discussed further in this book and is one of the main reasons for writing it.

1.3 Terminology For consistency and agreement with accepted standards, this book utilizes the International System of Units (SI) of the BIPM (Bureau International des Poids et Mesures), as well as terminology, symbols, and units as compiled by the International Unions of Pure and Applied Chemistry (IUPAC) and Physics (IUPAP). These are most easily accessible via the IUPAC Green (Quantities Units and Symbols in Physical Chemistry) or Gold (Compendium of Chemical Terminology) Books [9, 10]. A large number of books and papers on dynamic compression utilize slightly different symbols than those used here. The IUPAC/IUPAP symbol conventions are utilized herein.

Chapter 2

Molecular Spectroscopy Basics

This chapter presents the basic information on molecular spectroscopy needed to understand the terminology and concepts used in the remainder of the book. It starts with a discussion of spectroscopies involving electronic or ro-vibronic transitions in the ultraviolet, visible, and near-infrared spectral regions and then presents rovibrational spectroscopic methods operating in the infrared regions. It then compares the information on molecular species available from these two kinds of spectroscopy and its importance to experiments desiring to follow the progress of chemical reactions. The chapter continues with a detailed analysis of bandwidth broadening mechanisms including lifetime broadening (which also includes chemical bond lifetime), phase lifetime broadening (including collisional processes), inhomogeneous broadening, and Doppler broadening. It then presents vibrational hot bands and their contributions to ro-vibrational spectra under dynamic compression and concludes with sections on optical systems and laser fundamentals, as these are vital tools for the spectroscopic methods discussed in this book.

2.1 Electronic Versus Ro-Vibrational Molecular Information Available In contrast with classical systems, in the quantum mechanical realm, a system that is bound or otherwise confined spatially can only possess discrete energy values called energy levels. When we consider electrons involved in the bonds between nuclei in a molecule, these energy levels can be electronic in nature, but also vibrational or rotational. The lowest energy state is called the ground state. For a diatomic molecule (or one bond of a polyatomic molecule), the energy levels can be depicted as shown in Fig. 2.1, plotted as potential energy versus internuclear separation. Also shown in that figure are the vibrational levels (horizontal solid lines) and the dissociation

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 D. S. Moore, Molecular Spectroscopy of Dynamically Compressed Materials, Shock Wave and High Pressure Phenomena, https://doi.org/10.1007/978-981-19-2420-0_2

3

4

2 Molecular Spectroscopy Basics

Fig. 2.1 Plots of potential energy versus internuclear separation for the ground electronic and one excited electronic state for an arbitrary molecule. Also shown are the vibrational levels (solid horizontal lines) and dissociation energies. Rotational energy levels are not shown, but are sublevels within each vibrational state. Example electronic ro-vibrational transitions are shown (upward pointing arrow is absorption; downward pointing arrow is emission)

energies (horizontal dashed lines), as well as example transitions (upward pointing arrow is absorption and downward pointing arrow is emission). Transitions between the energy levels cover the electromagnetic spectrum from radio frequencies to X-rays for rotational transitions to inner shell electronic transitions, respectively. The absorption and emission arrows in Fig. 2.1 depict typical molecular electronic transitions that occur in the UV-visible frequency range. For a molecule composed of n atoms, there are 3n degrees of motional freedom, including three translations (in the x, y, and z directions), three rotations about the principal axes of the inertial ellipsoid of the molecule (note: for linear molecules, there are only two rotations because the rotation around the molecular axis doesn’t change the atomic coordinates), and 3n-6 (note: 3n-5 for linear molecules) vibrations (motions that change the distances between the atoms, e.g., the bond lengths and the angles between them). A simple model of the bond in a molecule is that of two masses joined by a weightless spring. Hooke’s law states that the force F necessary to move a mass m a distance x from an equilibrium position is proportional to the spring force constant f , e.g., F = − f x, so that f is a measure of the strength of the bond between the atoms. The minus sign shows that the force is directed in the opposite direction from the elongation. From Newton’s second law, the force is also proportional to the mass times its acceleration, which is the second derivative of the elongation with respect to time. F =m

d2x =−fx dt 2

(2.1)

2.1 Electronic Versus Ro-Vibrational Molecular Information Available

5

Solving for x gives the equation for a harmonic oscillation x = x0 cos(2π νt + φ)

(2.2)

with ν the oscillation frequency and φ the phase angle. For a diatomic molecule with atomic masses m1 and m2 , m in the above equation is the reduced mass 1/m = 1/m 1 +1/m 2 . Substituting for m and solving for ν give the equation for the vibrational frequency of a diatomic molecule: 1 ν= 2π

   1 1 f + m1 m2

(2.3)

It was found convenient for molecular spectroscopists to convert frequencies from Hertz to wavenumber units (waves per unit length): ∼

ν= ν/c = 1/λ usually expressed as waves per centimeter, cm−1 (also called wavenumbers). The potential energy (PE) of such a diatomic molecule during a vibration can be found by integrating Hooke’s law, which gives the equation of a parabola P E = f r 2 /2

(2.4)

where r is the internuclear distance from the potential energy minimum (at r = r 0 ) along the bond. In real life, molecules are not completely harmonic, but behave anharmonically because the force required to compress a bond is larger than the force required to stretch that bond. Figure 2.1 shows this anharmonic behavior, where the curve at smaller internuclear separations is much steeper than at larger. Many different approximate functions have been used to represent these anharmonic potentials, such as the Morse function: 2   P E = D 1 − exp(−r f e /2D

(2.5)

where D is the dissociation energy and f e is the harmonic force constant. For polyatomic molecules, it is more convenient to describe the vibrational motions in terms of normal modes rather than individual bond motions. Each normal mode involves a certain type of motion and its associated symmetry. The term “normal” applies because each mode occurs without movement in any of the other normal modes, so that they are mathematically orthogonal. By converting the molecular motion equations from Cartesian coordinates to the normal mode coordinates, the mathematics is made much simpler and allows Group Theory to be used to determine

6

2 Molecular Spectroscopy Basics

symmetries as well as Raman or infrared activity. These attributes will be made much clearer in Chaps. 5, 6, and 7. In the gas phase, molecules are free to rotate. However, in the condensed phase, intermolecular interactions (e.g., from crystal lattice neighbors or collisions) inhibit free rotations so that the rotational quantum numbers are no longer valid and the rotational spectrum collapses. Pressure and temperature affect the populations of the quantum energy levels and their widths via various broadening mechanisms. All of these subjects and their relevance to dynamically compressed materials are discussed in this and subsequent chapters.

2.1.1 Electronic Spectroscopies Transitions between electronic levels in molecules occur via emission or absorption of photons in the near-infrared to UV spectral regions, as shown by the up and down arrows in Fig. 2.1. The transition probability is governed by the Einstein coefficients A (for spontaneous emission) and B (for absorption and stimulated emission). The frequency ν of the emitted photon can be calculated from the Bohr condition E 2 − E 1 = hν (where E 2 is the energy of the upper state and E 1 the lower, and h is Planck’s constant). Spontaneous emission occurs via the “spontaneous” decay from the upper to lower electronic levels. Absorption and stimulated emission depend on the presence of a photon at or near the energy of the transition. Sometimes stimulated emission is referred to as negative absorption. For molecules, the electronic levels also contain vibrational and rotational energies, the ro-vibrational states. Some vibrational states are depicted in Fig. 2.1. The presence of these additional energy levels within each of the electronic manifolds involved leads to complex emission and absorption spectra, especially for larger molecules. In the gas phase, the molecules are free to rotate. The rotational energies are typically smaller than vibrational energies, so that rotational features appear as fine structures on the vibrational transitions. Pure ro-vibrational spectra can be obtained in the infrared (or Raman), as described below. Purely rotational transitions occur at much lower frequencies and their spectra are often recorded using radiofrequency methods, but these are not discussed further in this book as they are not used (to the knowledge of this author) in dynamic compression studies of molecular materials. In condensed phases, rotational motions are hindered (liquids) or inhibited entirely (solids) by neighboring molecules, leading to a collapse of the rotational fine structure. In addition, there are other broadening mechanisms that lead to further complexity (vide infra). The result is typically a broad and sometimes featureless electronic absorption or emission spectrum. Example spectra for nitromethane (no fine structure) [11] and for nitrogen dioxide (ro-vibrational structure) [12] are shown in Fig. 2.2.

2.1 Electronic Versus Ro-Vibrational Molecular Information Available

7

Fig. 2.2 Top: Nitromethane UV absorption spectrum; Bottom: Nitrogen dioxide UV/visible absorption spectrum. Figures were generated from data in Refs. [11, 12]

2.1.2 Ro-Vibrational Spectroscopies Unlike electronic transitions involving the ro-vibrational levels within the upper and lower electronic states involved in the transition, vibrational spectroscopies only involve the ro-vibrational levels within a given electronic (usually ground) state. Transitions between the ro-vibrational states occur via emission or absorption of photons in the infrared to radiofrequency regions. The spectra can be obtained via dipole processes as described above for the electronic absorptions, as in infrared absorption spectroscopy (see Chap. 5). Alternatively, Raman methods can be used (see Chap. 6). Both kinds of spectra for nitromethane are shown in Fig. 2.3. Infrared and Raman spectroscopy are complementary tools for obtaining vibrational spectra. Depending on the nature of the vibration, which is determined by the symmetry of the molecule, vibrations may be active or forbidden in the infrared or Raman spectrum. Infrared active are vibrations that modulate the molecular dipole moment. Raman active are vibrations that modulate the molecular polarizability (a longer discussion of polarizability can be found in Chaps. 6 and 7). Vibrations that are forbidden in both spectra are called silent. Vibrations of molecules with a center

8

2 Molecular Spectroscopy Basics

Fig. 2.3 Infrared absorption (black) and Raman (red) spectra of nitromethane. The infrared spectrum was generated using the data in Ref. [13]. Raman spectrum taken by the author

of symmetry that are infrared active cannot be Raman active and vice versa, which is the rule of mutual exclusion.

2.2 Bandwidth Broadening Mechanisms Molecular spectral bandwidths are broadened via several physical processes. These are usually divided into homogeneous and inhomogeneous types. Homogeneous broadening is due to population and phase relaxation processes as discussed in Sects. 2.2.1 and 2.2.2. Several kinds of inhomogeneous broadening processes are caused by intermolecular interactions as detailed in Sect. 2.2.3.

2.2.1 Population Relaxation (t1 Time) Population relaxation is the process by which a non-equilibrium electronic and/or ro-vibrational population (e.g., caused by a shock wave) returns to the Maxwell– Boltzmann population distribution given by the temperature of the sample. This broadening mechanism is related to pressure- or collision-induced changes in population lifetimes as well as the lifetimes of molecules (when created or destroyed by chemical reactions). The relationship between lifetime and bandwidth is shown in Fig. 2.4. Under dynamic compression at high rates, sample temperatures can be nonuniform depending on the design of the experiment, so that different population distributions can exist until thermal equilibrium is reached, complicating data analysis. Later chapters discuss some specific examples especially for ultrafast shocks.

2.2 Bandwidth Broadening Mechanisms

9

Fig. 2.4 Homogeneous bandwidth of a single vibrational transition centered at 750 cm−1 at five different lifetimes from 1 ps to 62 fs calculated using the Heisenberg uncertainty equation as described in the text

2.2.2 Dephasing (t2 Time) Pure dephasing describes the adiabatic modulation of the electronic and/or rovibrational energy levels of a transition caused by thermal fluctuations of its environment. In other words, dephasing is caused by the dynamical interactions of a molecular state with its environment and is measured by a dephasing time, t 2 . In the Markovian limit (and no inhomogeneous broadening—see below), the spectral linewidth Γ is given by the Fourier transform of the sum of the population and dephasing rates: Γ =

1 1 + π t1 2π t2

(2.6)

The bandwidth from either the t 1 (population) or t 2 (dephasing) time can be estimated using the time–bandwidth expression from the Heisenberg uncertainty principle: Et ≥ /2. Figure 2.4 shows the calculated bandwidth of a single vibrational transition centered at 750 cm−1 at five different lifetimes from 1 ps to 62 fs. A more common method is to use the damping coefficient Γ in a lineshape function like a Lorentzian  y = A (x − x0 )2 + Γ 2

(2.7)

10

2 Molecular Spectroscopy Basics

Fig. 2.5 Comparison of Gaussian and Lorentzian lineshape functions, both centered at 750 cm−1 and with damping coefficient Γ = 15 cm−1

or a Gaussian   y = Aex p −[(x − x0 ) Γ ]2

(2.8)

where x is the wavenumber or wavelength, x 0 is the center of the band, and A is a parameter related to the height of the band. The difference between these two lineshape functions, a comparison that will be important later in this book, is illustrated in Fig. 2.5. Note particularly how the Gaussian approaches zero much more quickly than the Lorentzian in the wings of the band. Usually, dephasing times are much shorter than population relaxation times, so that the dominant contribution to Γ is from dephasing. However, t 1 can be extremely short if it is dominated by the lifetime of the molecule and reactions are happening very quickly (as in shock-induced chemistry).

2.2.3 Inhomogeneous Broadening Inhomogeneous broadening is similar to proximity broadening. It is due to small changes in a molecule’s energy levels due to the presence of other molecules at fluctuating or different distances as in fluids or amorphous solids. This particular broadening mechanism is quite important in shock compression experiments where chemical reactions result in multiple species (e.g., a soup of reactants, intermediates, and products) at high pressure. Homogeneous versus inhomogeneous broadened lines are compared in Fig. 2.6. Shock-compressed condensed phase materials when they undergo reaction usually produce a mixture of products and at intermediate times a mixture of reactants,

2.2 Bandwidth Broadening Mechanisms

11

Fig. 2.6 Top: Homogeneously broadened lines of width νH of different strengths; Bottom: Inhomogeneously broadened line of width νINH composed of multiple homogeneously broadened lines

products, and intermediates. This soup of materials results in another type of broadening that most closely resembles inhomogeneous broadening. Figure 2.7 illustrates one type of inhomogeneity—mixture heterogeneity—by comparing a pure material (sugar) to that same material in a mixture (sugar water). Even in this very simple case, comparing Raman spectra of pure sugar versus solvated sugar, where the mixture is just sugar and water, shows dramatic broadening and shifting of the vibrational spectroscopic features.

2.2.4 Doppler Broadening Line broadening occurs due to the Doppler effect because, at temperatures above absolute zero, the molecules are moving relative to the spectroscopic detector. The different velocities of the molecules in an ensemble result in different Doppler shifts, the sum of which produces line broadening. The broadening depends on the frequency ν0 of the molecular transition, the mass of the molecule m, and its temperature T according to (k is the Boltzmann constant):

12

2 Molecular Spectroscopy Basics

Fig. 2.7 Illustration of one type of inhomogeneity (mixture heterogeneity). Raman spectra of pure sugar and aqueous sugar solution showing dramatic broadening and shifting of the vibrational spectroscopic features in the solution spectrum

 ν = 2ν0

2ln2kT mc2

 21 (2.9)

In comparison with the other broadening mechanisms discussed above for dynamically compressed materials, Doppler broadening is relatively insignificant, as shown in Fig. 2.8 for m = 28 (like molecular nitrogen) at ν 0 of 750 cm−1 as a function of Fig. 2.8 Calculated Doppler width of a mass 28 vibrational spectral band centered at 750 cm−1 as a function of temperature

2.3 Hot Bands

13

temperature. Note the ordinate scale is in 10–3 cm−1 , so that this broadening mechanism is a factor of a thousand or so smaller than the other effects in these materials under the typical conditions of a dynamic compression experiment.

2.3 Hot Bands Vibrational hot bands, which occur due to the thermal population of excited vibrational levels, effectively result in spectral broadening. Raman and infrared absorption spectroscopies have been often used to interrogate the vibrational spectra of shockcompressed molecular materials [5, 14–25]. The goal of these types of experiments is to provide information on the intramolecular potential energy surfaces at extreme conditions and to understand how shock waves induce chemical reactions, especially in energetic materials. In experimental coherent anti-Stokes Raman (CARS, see Chap. 7) studies of shock-compressed diatomic molecular materials such as N2 , CO, and O2 , vibrational hot bands were clearly observed. Because they were well resolved as well as adequately fit using gas phase anharmonicities and simple pressure and temperaturedependent frequency shifts, they could be used to estimate vibrational temperature [26–33]. The vibrational spectra of polyatomic molecules are more complicated than diatomics because anharmonicity couples the normal modes via cross terms in the wave equations [34]. Even in the gas phase, these anharmonic coupling coefficients have been determined for only a few polyatomic molecules [35, 36]. For studies of polyatomic molecules under shock compression, the thermal population of low-lying vibrational levels will result in measurable intensities of the hot bands in the vibrational spectra, complicating their analysis. Most vibrational spectroscopic studies of shock-compressed polyatomic molecules, such as water, carbon tetrachloride, benzene, and nitromethane, have ignored the effects of hot bands [22, 24, 37–42]. The spectral resolution in these studies was found to be inadequate to allow an estimate of vibrational frequency shifts or vibrational temperature. This inability to resolve the hot bands in polyatomic molecules has resulted in their neglect in the shock compression literature to date.

2.3.1 Hot Band Theory For polyatomic molecules, the vibrational energy G(vi ) expressed in terms of anharmonic coupling coefficients x ij (neglecting vibrational angular momentum interactions) can be expressed as follows [43]:

14

2 Molecular Spectroscopy Basics

G(v1 , v2 . . . ) =

i

 gi  gi + ωi vi + xi j (vi + )(v j + g j /2) + . . . 2 2 i j≥i (2.10)

where vi is the vibrational quantum number, ωi the fundamental frequency, and gi the degeneracy of normal mode i. The frequency of a general transition originating in the state (v1 , v2 . . . ) and ending at the state v1  , v2  . . . can be estimated as follows (using Eq. 2.11): 

   g ωi v1 + i G v1  , v2  . . . − G(v1 , v2 . . . ) = 2 i  

 g  xi j v1 + i v j + g j /2 + 2 i j≥i  

gi − ωi vi + 2 i

  gi  v j + g j /2 − xi j vi + 2 i j≥i

(2.11)

The hot band (vi + vj – vj ) [notation from Ref. [43]] of this transition originating from the state [vj ] will be at frequency:



vi v j = vi + (gi + 1)xii vi + xi j v j

(2.12)

j=i

Consider a fundamental vi and a hot band (vi + vj – vj ). The separation between these two features is x ij . If x ij is large enough to allow the bands to be resolved, then both can be measured adequately enough for further interpretation. When the bandwidths are comparable to or larger than typical x ij , the bands overlap, even with very high instrumental resolution [44]. This inability to resolve the hot bands in polyatomic molecules has resulted in their neglect in the shock compression literature to date. Examples of both situations are illustrated in Fig. 2.9, where the hot bands of shock-compressed liquid nitrogen are clearly resolved using coherent anti-Stokes Raman spectroscopy—see Chap. 7—and were used to estimate vibrational temperature. Those of shock-compressed liquid nitrous oxide are not resolved, which makes the fitting of the spectrum to extract vibrational frequency shift with pressure as well as estimating vibrational temperature difficult [45].

2.4 Optical Systems

15

Fig. 2.9 Top: Coherent anti-Stokes Raman (CARS) spectrum of shock-compressed liquid nitrogen showing several spectrally resolved vibrational hot bands (drawn using data from Ref. [27]). The shock had reflected off the LiF window in this experiment, so that some small peaks from the doubly shocked (P2) N2 are barely visible. The calculated spectrum using the CARS equations is shown as the dashed line; Bottom: CARS spectrum of shock-compressed liquid nitrous oxide (N2 O) with unresolved vibrational hot bands (the ambient N2 O vibrational mode spectral feature is evident at 1284 cm−1 as the shock had not quite reached the confining window)

2.4 Optical Systems Spectroscopic methods rely on the use of optics to transport the electromagnetic radiation from the source to the detection system. These optical systems can be composed of imaging elements such as lenses and mirrors or conducting elements such as optical fibers, or both. The radiant flux Φ transported by any optical system is given by the radiance L of the light source times the optical conductance (similar to etendue or throughput) of the optical system G times a transmission efficiency τ: Φ = LGτ

(2.13)

16

2 Molecular Spectroscopy Basics

Fig. 2.10 Schematic diagram of the concept of optical conductance showing the symbols used in Eq. (2.14)

Figure 2.10 illustrates the concept of optical conductance and shows two components of an optical system in a medium with an index of refraction n. The first component has surface area F 1 and the second F 2 . The differential surface elements of these are dF 1 and dF 2 . The length of a line between the two surfaces is a12 , and it contacts the surfaces at angles with respect to each surface normal of α 1 and α 2 . The optical conductance is given by the following integral:  G=n





2 F1

F2

 cosα1 cosα2 d F1 d F2 2 a12

(2.14)

An example of this concept of optical conductance is a lens with surface area F radiating into (or irradiated from) a solid angle Ω in a medium with an index of refraction n. That lens surface area has an optical conductance given by G = n 2 FΩ

(2.15)

Quite often, the radiation paths from one optical element to another approximate a truncated cone. It is then convenient to know the solid angle Ω of a cone with half-angle Θ which is given by  Ω = 4π sin2

Θ 2

 ≈ π sin2 Θ

(2.16)

so that its optical conductance G can be approximated for small half-angles by G ≈ Fπ N A2

(2.17)

where NA is the numerical aperture NA = n sinΘ. When two optical elements with areas F 1 and F 2 are far apart (e.g., F 1 and F 2 100 λ) [311, 312]. Tyndall scattering has nearly no wavelength dependence and a fairly constant scattering cross section. Mie scattering has a wavelength dependence between λ−4 and λ depending on particle size. Rayleigh scattering has a λ−4 dependence on wavelength. Those latter two dependences mean that UV to blue absorptions could be due to particle scattering as well as electronic absorption at later times after shock passage. Holmes et al. observed increased blue absorption in shock-compressed benzene near its Hugoniot cusp (onset of reaction) and stated it was due to the formation of carbon particles at late times because of the wavelength

4.3 UV–Visible Absorption

81

dependence of the absorption [278, 310]. In Chap. 8, we will discuss X-ray small angle scattering methods to measure particles. Another option will be discussed in the ultrafast laser methods Sect. 4.3.1. It assumes that the scattering is diffuse so that it can be measured at right angles to the incident and reflected light beams of a typical absorption experiment. Regardless, it is important in UV/visible absorption spectroscopy experiments to recognize that particle light scattering can play a role.

4.3.1 Ultrafast Laser Methods The above UV/visible opacity and scattering experiments compellingly suggested that thin samples of reactive materials were needed in order to measure shock-induced absorption spectra because of the strong opacity that grew with reaction onset [39, 41, 283]. The step wave loading method achieves the required thin samples, but results in temperatures lower than single shock experiments [7]. To overcome the shock-induced absorption in thicker samples, a group at Los Alamos National Laboratory utilized laser-driven shocks and ultrafast laser methods to measure UV–visible absorption spectra of singly shocked very thin samples [298, 313–318]. Their experimental setup is depicted schematically in Fig. 4.24. The UV–visible continuum used in this apparatus was produced by self-focusing and self-phase modulation band broadening induced by focusing part of the compressed ultrafast laser pulse into a CaF2 window, as described in Refs. [315–318]. The Los Alamos apparatus allows recording a reference spectrum simultaneously with the sample spectrum using an imaging spectrometer so that the two spectra are dispersed onto two different spatial regions on the CCD. That process allows

Fig. 4.24 Schematic diagram of the ultrafast laser shock and UV–visible absorption apparatus. The imaging spectrometer allows sample and reference spectra to be recorded simultaneously in two different spatial locations on the CCD

82

4 Electronic Molecular Spectroscopy

accounting for pulse-to-pulse spectral fluctuations. The shocked spectrum is normalized by the preshocked spectrum of the sample to determine the spectral changes induced by the shock. This normalization can be done in either transmission or absorption. For example, the absorbance difference between the preshocked and the shocked material can be obtained using the following equation: ⎛ A = − log⎝ 

Tsample TReference Tsample TReference



⎞



⎠

Shocked

(4.9)

Preshock

where T is the transmission spectrum of the sample/reference in the shocked or preshocked state. Because the LANL group’s experiments were done in reflection mode off of a shocked metal surface, there was a concern about changes in the time-dependent reflectivity of that metal surface following release into the sample. This effect can be approximately measured using a nonreactive sample. Measuring the reflectivity of the metal surface without a window is problematic because the metal releases into the air rather than a more shock impedance-matched material. This effect was illustrated dramatically in Figs. 13 and 14 of Reference [317] and also for the case of nitromethane and the inert DETA in Fig. 4.25. There was also a concern that part of the transmission loss could be due to light scattering instead of absorption. To examine this possibility, an experiment was performed that used a photodiode off-axis to the laser to collect diffusely scattered photons. The photodiode signal level from a shock-compressed TNT sample was compared to that from a near-perfect scatterer (polyfluoroethylene—PTFE). The scattered signal level for shocked TNT was found to be smaller than that which could be attributed to absorption [319]. Similar experiments should probably be performed for all of these kinds of experiments to differentiate between absorption and scattering processes, if possible. The Los Alamos group used this methodology to record UV–visible absorption spectra of a wide variety of shock-compressed liquids and transparent solids over a number of years including CS2 [316]; polyvinyl nitrate, single crystal RDX, single crystal PETN, and sapphire [298]; cyclohexane, cyclohexene, 1,3-cyclohexadiene, benzene, water, acetonitrile, acrylonitrile, tert-butylacetylene, and phenylacetylene [313]; and nitromethane and amine-sensitized nitromethane [315]. They found that shock states that departed from the Hugoniot of the starting material (in either direction—above or below) generally also exhibited UV–visible absorption features. The appearance of UV–visible absorption indicated the onset of shock-induced chemistry, but elucidation of the chemical processes involved necessitated the use of vibrational spectroscopic methods (see Chaps. 5–7). Some of these UV–visible absorption spectra as a function of shock pressure are shown in Fig. 4.26 along with their shock velocity/particle velocity data. Of course, the products/intermediates from shock-induced chemistry must have different

4.3 UV–Visible Absorption

83

Fig. 4.25 Comparison of shock-induced absorption using ultrafast laser methods for an inert liquid—a diethylene triamine (DETA)—versus a reactive liquid—b nitromethane with 5% DETA— at the same shock pressure (19 GPa). The loss of light in (a) is due primarily to the loss of reflectivity of the aluminum surface in this experiment. Adapted with permission from Ref [315]. Copyright 2014 American Chemical Society

densities from the starting material for the shock states to depart from the extrapolated reactant Hugoniot. For condensation reactions, the products are usually more dense (as for acrylonitrile, phenylacetylene, and CS2 ) versus reactions that produce less dense products (as for nitromethane) as illustrated in Fig. 4.26. As the LANL group’s UV/visible absorption experiments only covered the first 350 ps of time after the arrival of the shock, the shock pressures required to observe either departure from the Hugoniot or the UV–visible absorption features compared to longer time scale plate impact experiments could be explained using the diagrams in Fig. 4.27. The surface shown in the left-hand panel is obtained assuming Arrhenius kinetics governed by the following equation:

84

4 Electronic Molecular Spectroscopy

Fig. 4.26 UV–visible absorption and shock velocity/particle velocity data for four different materials showing a correlation between departure from the unreacted Hugoniot and onset of observable absorption. Used with permission from Ref. [320]. Copyright 2017 AIP Publishing LLC

 ∂λ = k = (1 − λ)A exp − E a T dt

(4.10)

Figure 4.27 illustrates the fact that the shock pressure (and therefore temperature) at which reaction appears in the data depends on the observation time. Observing the reaction for longer times allows the reaction to proceed so that products appear. If the laser shock experiments on thin samples could be held for longer times comparable to the plate impact experiments, the products would be observable at similar lower shock pressures. Regardless of how the UV–visible absorption spectra are obtained, they all are quite broad and essentially featureless. Although the appearance of strong UV– visible absorptions is a strong indication that chemistry is happening, they do not absolutely confirm what species are evolving after the shock passage so that the chemical pathways can be unraveled. This shortcoming was the incentive for investigations of other spectroscopic methods, such as vibrational spectroscopy, with the hope that the greater specificity to the molecular structure would further this goal.

4.4 Laser-Induced Fluorescence

85

Fig. 4.27 Effect of observation time on Hugoniot cusp location in particle velocity/shock velocity space using the example of phenylacetylene and Arrhenius kinetics. Each curve in the bottom panel can be viewed as a slice from the 3-D surface in the top panel. Adapted with permission from Ref. [320]. Copyright 2017 AIP Publishing LLC

4.4 Laser-Induced Fluorescence Due to the strong gray body emission observed in many of the emission spectroscopy studies, many researchers chose to add a laser to their experiments and use it to induce fluorescence from molecular species. Campillo and coworkers explored a number of these types of experiments using laser-driven shocks, starting with the measurement of the shock pressure-induced fluorescence frequency shifts and the pressure-dependent fluorescence lifetime of anthracene [115, 321]. They then used the fluorescence lifetime of crystal violet to deduce the shock pressure-dependent viscosity of glycerol [116] and measured the

86

4 Electronic Molecular Spectroscopy

Fig. 4.28 Shock-induced temperature rise in water versus shock pressure measured using the fluorescence intensity enhancement of fluorescein dye. Figure drawn using data from Ref. [114]

temperature of shock-compressed water via the thermal fluorescence band broadening of fluorescein [322], shown in Fig. 4.28. They were also able to measure the uniaxial strain in ultrafast laser shock-compressed ethanol by measuring the energy transfer rate between donor and acceptor dye molecules using streak camera fluorometry [323]. A typical apparatus used for these kinds of experiments is depicted schematically in Fig. 4.29. Time scale and experiment duration are achieved via timing between the drive laser and excitation laser, and the streak camera streak rate. The Washington State University group of Y. M. Gupta expanded the fluorescence work using gun flyers and larger samples to minerals such as ruby [324]. They also

Fig. 4.29 Components of a laser-driven shock fluorescence detection apparatus

4.4 Laser-Induced Fluorescence

87

used the fluorescence from dyes like Rhodamine 6G to examine the microscopic response of shocked liquids [325, 326] and found fluorescence as well as absorption from an intermediate species in shock-induced decomposition of amine-sensitized nitromethane [327]. The only significant changes to the apparatus shown in Fig. 4.29 for use on a gundriven flyer system are replacing the drive laser with the gun/projectile and the metal film/sample with a typical gun target. In both cases (laser and gun), there should be a transparent window on the backside of the sample (not shown). More recently using laser-driven flyers, the Dlott group used fluorescence methods to investigate the behavior of fluorescent molecules at extreme conditions [328] and also to probe pressure distributions in shocked microstructured materials [329]. For these measurements, they added an ultrafast laser to excite a fluorescent dye (Rhodamine 6G). The time of arrival of this excitation laser was adjusted to selected times following flyer plate arrival at the sample, and a streak camera was used to record the time-dependent fluorescence and extract lifetimes. They concluded that fluorescence lifetimes are better than intensity measurements for these purposes as they are insensitive to the motion and optical properties of the shocked sample. They found a linear relationship between fluorescence lifetime and pressure. They also measured time-dependent fluorescence depolarization ratios as a method to determine shear deformation rates. Fluorescence provides a capability that to date has not been sufficiently exploited, at least to the mind of this author. It provides a means for spatially resolved probes of pressure in shocked microstructured materials and in situ ultrafast measurement of pressure in transparent or translucent materials. Further investigation into the relationship between pressure and temperature in fluorescent probes might allow in situ and even spatially resolved temperature measurements. In particular, fluorescent nanoparticles provide a means to survive the shock compression conditions as in situ probes. An early exploration by Zhuravlev et al. utilized transient absorption of semiconductor nanocrystals (a.k.a. quantum dots) to follow their behavior under static high pressure [330, 331]. The work was extended to the pressure dependence of photoluminescence by several groups [332–339], but most relevantly to shock compression studies by the Dlott group [329, 340–342]. An example of the capability is shown in Fig. 4.30 [343], where CdSe quantum dots embedded in a polyvinyl alcohol thin film redshift under uniaxial compression due to their deformation under the uniaxial strain. The emission from these same quantum dots shifted to the blue in shocked fluids. In either case, the redshift was found to be proportional to strain and the blueshift to hydrostatic pressure [342].

88

4 Electronic Molecular Spectroscopy

Fig. 4.30 Photoemission from ~40 nm diameter CdSe quantum dots (QD) embedded in a polyvinyl alcohol thin film during a 6.5 GPa shock experiment. The time traces were recorded using a stream camera during continuous laser excitation. PDV velocity traces of the front and back surfaces (blue) of the film allow calculation of the longitudinal strain (green) with time during the experiment. The QD emission peak redshift tracks the strain very accurately, whereas the QD emission intensity lags behind due to photophysics. Used with permission from Ref. [343]. Copyright 2019 Springer Nature

4.5 Summary The capabilities of the plethora of UV–visible spectroscopies explored to date as well as others yet to be explored provide excellent opportunities to obtain important information about the fates of molecules under both dynamic and static loading. Newer fluorescence methods utilizing probes of pressure and temperature are an exciting area for further development, as they may open new avenues to obtain molecule-specific information.

Chapter 5

Infrared Molecular Spectroscopy

Unlike electronic transitions that involve the ro-vibrational levels within both the upper and lower electronic states encompassed by the transition, vibrational spectroscopies only involve the ro-vibrational levels within a given electronic (usually ground) state, so they are usually less congested and more information rich. Transitions between the ro-vibrational states occur via emission or absorption of photons in the infrared to radiofrequency regions or via inelastic scattering of photons as in the Raman effect. In condensed phases, rotational motions are hindered (liquids) or inhibited entirely (solids) by neighboring molecules, leading to the collapse of the rotational fine structure. This chapter leads the reader through the historical developments of infrared absorption vibrational molecular spectroscopy methods (Raman and coherent Raman are discussed in Chaps. 6 and 7, respectively) used in dynamic compression research, as well as some seminal research results. It also compares dynamic spectroscopy of some materials to their spectroscopy at static high pressure/high temperatures.

5.1 Infrared Absorption Infrared means “below red,” meaning lower frequency than the red portion of the visible spectrum. The infrared portion of the electromagnetic spectrum is typically divided into three regions: the near-, mid-, and far-infrared. The higher-energy nearinfrared (NIR), approximately 14,000–4000 cm−1 (0.7–2.5 µm wavelengths), is the region of overtones or combinations of molecular vibrations. The mid-infrared, approximately 4000–400 cm−1 (2.5–25 µm), is the region of the fundamental vibrations of molecules. That region is highlighted in Fig. 5.1. The far-infrared, approximately 400–10 cm−1 (25–1000 µm), is typically used for rotational spectroscopy and low-frequency vibrations of molecules or phonon modes of solids.

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 D. S. Moore, Molecular Spectroscopy of Dynamically Compressed Materials, Shock Wave and High Pressure Phenomena, https://doi.org/10.1007/978-981-19-2420-0_5

89

90

5 Infrared Molecular Spectroscopy Wavenumbers (cm -1 ) 4000

2000

1000

100 nm

1 μm

10 μm

500

250

1 nm

10 nm

X-Ray

1018

UV

1017

1016

Visible

Wavelength 0.1 nm

1015

100 μm

1 mm

100 mm

1013

1012

1011

1010

1m

10 m

100 m

Radio

Microwave

Infrared

1014

10 mm

109

108

107

Frequency (Hz)

Fig. 5.1 The electromagnetic spectrum with the mid-infrared region highlighted and expanded. The conversion from wavelength to wavenumbers for this region is also shown in the upper expanded scale

Infrared spectroscopy is used to measure the frequencies of molecular vibrations. Molecular vibrational energies are quantized according to the structure of the molecule, including the bond strengths, atomic distances, and atomic masses involved in the vibration. Light interacts with the vibrations via the change in dipole moment that occurs during that vibration. A table (Table 6.1) of vibrational frequencies for molecular modes of interest to the subject of this book is found in Chap. 6. → The dipole moment vector − μ of a bond in a molecule can be calculated using the equation − → μ =

 − qi → r i

(5.1)

i

where qi is the magnitude of the ith charge, and r i is the vector of the position of the ith charge. The intensity of an infrared transition can be related to the change in dipole moment during the vibration  IIR ∝

dμ dQ

2 (5.2)

where Q is the coordinate of the vibration. Quantum mechanically, the intensity of the infrared transition is related to the transition moment integral   IIR ∝ Ψi | Mˆ Ψ f

(5.3)

where i and f are the initial and final states, Ψ is the molecular wave function and Mˆ is the dipole moment with cartesian coordinates, Mˆ x , Mˆ y , and Mˆ z . (Note: this

5.1 Infrared Absorption

91

Fig. 5.2 Schematic representation of a simplified infrared spectroscopy system

“Bra-Ket” or Dirac notation is a convenient and common method used to denote quantum states.) A simplified infrared spectroscopy system is shown schematically in Fig. 5.2. Actual instruments usually use mirrors instead of lenses to focus as it is difficult to find lens materials that don’t absorb part of the infrared or have flat enough dispersion to avoid severe chromatic aberration. The spectrometer can be a Fourier transform type or a grating type. Either a scanning Fourier transform spectrometer or a scanning grating type spectrometer with single element infrared detector would be too slow to capture infrared spectra under dynamic compression conditions, but have been used effectively in static high-pressure experiments [344–349]. For single-shot dynamic experiments, a broadband of infrared from the source would be dispersed in a grating type spectrometer and detected using a multipixel detector. Time resolution in the experiments could be either using a pulsed infrared source, or a gated multipixel detector. Examples of both approaches are provided in the sections below. The photon flux from a thermal infrared source (usually a glowbar) can be calculated from the Planck equation (see Chap. 4). In order to maximize the number of photons in the mid-infrared region, the temperature of the glowbar should not be too high, but that choice also restricts the photon flux available. Figure 5.3 illustrates the conundrum, with 1000 K providing a maximum photon flux near the middle of the mid-infrared spectral region. After integrating over a hemisphere and all frequencies (see Chap. 4), the 1000 K glowbar provides 1.5 × 1034 photons s−1 m−2 photon flux integrated over the full emitted wavelength range. That seems like a lot of photons! However, as we would like to use these photons to follow shock-compression processes that occur in nanoseconds or less, that means at most 1.5 × 1025 photons ns−1 m−2 . Again, that 1.5 × 1025 photons number comes from spatially integrating over an entire hemisphere and assumes a square meter of glowbar can be used in the measurement. To see how this situation applies to a dynamic compression experiment, let’s use the example of an f/1.8 lens system to image the glowbar at 1:1 magnification onto our shock-compressed sample, which is of the size typical of a gun flyer target –4 −2 collected by the f/1.8 lens is (~1 ×  10 m ). The fraction of glowbar photons 0.79 4π = 0.063, so that means about 9.5 × 1019 photons/ns on the target. Now we image that target into our infrared spectrometer also using an f/1.8 lens at 1:1

92

5 Infrared Molecular Spectroscopy

Fig. 5.3 Planckian photon flux curves for a blackbody at three temperatures. The 1000 K curve has a maximum in the fingerprint region of molecular vibrational frequencies

imaging through a 0.1 × 2 mm slit. The area of that slit is 2 × 10–7 m2 so that 1.8 × 1013 photons/ns make it into the spectrometer. Let us now assume the spectrometer is also f/1.8 so that all the photons make it to the grating and that the grating is 50% efficient at dispersing the photons onto the detector array. Let us also assume that the detector is limited only by its quantum efficiency (e.g., at unity gain for a quantum efficiency of 90%, which provides a signal to noise ratio (SNR) of 2 for 100 photons of 1.55 µm wavelength—but realize that the actual situation for infrared detector arrays is much more complicated than this analysis and we refer the reader to current detector manufacturer literature). For a hypothetical infrared detector array of 500 pixels of size 0.1 × 2 mm to match the slit and a grating which gives a dispersion of 10 cm−1 per pixel, means an average of 108 electrons to detect per ns. That seems like a sufficiently large number to enable infrared absorption spectroscopy of our shock-compressed sample! However, usually, the temperature of a sample under shock compression is also high. For compressible molecular materials, a typical temperature at shock pressures that might cause some kind of chemistry to occur (which is usually why one would want to use infrared absorption in the first place—to look for evolved species as chemistry occurs behind the shock wave) is at least 1000 K. That means there will be as many or more photons from the sample as from the glowbar and the desired sample infrared absorption spectrum can be obscured by the sample thermal emission. To surmount the above difficulty, we need many more infrared photons from our source available during the time of the experiment (i.e., in a pulse rather than continuous) so that we can time resolve events as they happen. We also need to remind ourselves that the required infrared detector arrays were not available for shock-compression studies until the 2000s. Two of these kinds of methods used in shock-compression research are discussed in the sections below.

5.1 Infrared Absorption

93

Another critical point to be made here is that the infrared absorption coefficients for many molecules are quite large, which means that very thin samples are required so that the ambient sample is not opaque to the probe beam. Typically, thicknesses should be at most a few microns for pure materials. That consideration makes timing between the shock drive and the probe very critical, and the pulse length of the probe or time resolution of the detector system must be extremely short as well. At a shock velocity of 5 km/s, a 5 µm thick film is traversed in 1 ns.

5.1.1 Time-Resolved Infrared Spectral Photography Before the advent of good infrared detector arrays, several groups sought solutions to measuring short pulse infrared spectra, one of which involved a pyroelectric vidicon [350], and another utilized upconversion of the infrared to the visible by nonlinear sum-frequency mixing [351]. A third solution, which was ultimately adapted to measuring infrared absorption spectra of shock-compressed materials, was invented by the Sorokin group at the IBM Thomas Watson Research Center. They found a methodology that would allow them to downshift the output of a pulsed broadband dye laser (BBDL) from the visible into the infrared, pass the infrared through a sample, and then upshift the infrared frequencies back into the visible for detection using a diode array [352, 353]. They named their approach time-resolved infrared spectral photography or TRISP. The downshifting process utilized stimulated electronic Raman scattering via near resonance of the broadband laser output to a large oscillator strength transition in atomic vapors. Figure 5.4 (left side) schematically represents the stimulated electronic Raman scattering process. Table 5.1 presents the potentially utilizable electronic transitions in Cs, Rb, and K vapors and the resulting infrared frequencies at the transition listed. Note that these are all in the blue and near UV regions, so that the 355 nm third harmonic of a Nd:YAG laser or the 308 nm output of a XeCl excimer laser was needed to be used to pump the BBDL. Note also that the method uses near resonance rather than exact resonance, so that the band of infrared frequencies produced is slightly lower than the resonance frequency when the process uses a broadband dye laser with wavelengths just longer than the resonance wavelength. The conversion efficiency drops off quickly as the difference between laser and atomic resonance wavelengths increases, so care must be taken to match the atom in the heat pipe and the broadband dye laser used to achieve the infrared frequencies desired. To upconvert back to the UV–visible region, we can again use stimulated electronic Raman scattering, but this time in a four-wave coherent mixing process that uses a narrow band dye laser (NBDL) to provide a coherent beam at the Stokes frequency, ωS , that mixes with the broadband infrared to produce the upshifted broadband UV–visible beam at ωU . This process is shown schematically in Fig. 5.4 (right side). The complete apparatus is depicted schematically in Fig. 5.5. That latter figure shows laser drive being used for the shock compression, which could be replaced by

94

5 Infrared Molecular Spectroscopy

Fig. 5.4 The stimulated electronic Raman scattering process illustrated using the energy levels of neutral Rb atoms. ωbbdl is the broadband dye laser (BBDL) and ωIR is the down converted broadband infrared; (right): The coherent four-wave mixing upconversion process that uses the narrow band UV–visible laser at ωlaser to produce a Stokes shifted beam at ωS coherently interacting with the broadband IR to produce broadband upconverted light at ωU Table 5.1 Energy levels of Rb, Cs, and K atoms and corresponding infrared frequencies at the transitions listed Atom Cs

Rb

K

Resonance line

Infrared transition

Transition

Wavelength (nm)

Transition

Frequency (cm−1 )

6 s → 7p3/2

455.7

7p3/2 → 7 s

3411

6 s → 7p1/2

459.4

7p1/2 → 7 s

3230

6 s → 8p3/2

387.7

8p3/2 → 8 s

1475

6 s → 8p1/2

389.0

8p1/2 → 8 s

1392

6 s → 9p3/2

361.2

9p3/2 → 9 s

771

6 s → 9p1/2

361.8

9p1/2 → 9 s

727

5 s → 6p3/2

420.3

6p3/2 → 6 s

3659

5 s → 6p1/2

421.7

6p1/2 → 6 s

3582

5 s → 7p3/2

358.8

7p3/2 → 7 s

1559

5 s → 8p3/2

335.0

8p3/2 → 8 s

807

4 s → 5p3/2

404.5

5p3/2 → 5 s

3693

4 s → 5p1/2

404.8

5p1/2 → 5 s

3674

5.1 Infrared Absorption

95

Fig. 5.5 Schematic diagram of a time-resolved infrared spectral photography apparatus applied to a dynamic compression experiment in reflection. The drive laser dynamic compression method could be replaced by any of the methods in Chap. 3

any of the dynamic compression methods discussed in Chap. 3. Note the dramatically increased experimental complexity compared to the UV–visible apparatus of Fig. 4.6. TRISP was first applied to measure the time-resolved emergence of products in the CO2 laser initiated gas-phase explosion of HN3 by the IBM group [354]. Adding HCl to the gas mixture allowed them to measure the temperature of the HN3 detonation using the HCl infrared ro-vibrational line intensities. TRISP was applied by Renlund et al. for measurement of the infrared absorption spectrum of shock-compressed CS2 using a XeCl excimer laser-pumped DMQ (2methyl-5-t-butyl-p-quaterphenyl) broadband dye laser centered at 360 nm and the 5 s–7 s levels of Rb vapor in the heat pipe to produce broadband infrared from 6 to 8.5 µm wavelengths and about 1 ns pulse length [21]. The narrow band laser was tuned to 423 nm using bis-MSB (p-bis(o-methylstyryl)-benzene) dye. Careful polarization control was necessary to remove residual unwanted wavelength regions before measurement of the upconverted infrared in the spectrometer/intensified diode array detector. Renlund et al. used an RP-1 detonator to drive the shock into a 0.2 mm thick CS2 sample and the spectrum was obtained in reflection mode. The strong ambient sample infrared absorption still allowed some measurable infrared light at the longwavelength edge of the CS2 absorption. However, no clearly identifiable spectral changes during the transit of the shock through the CS2 sample were discovered. Possible reasons are discussed below. The above result of Renlund et al. is a clear example of why very thin samples are needed to do infrared absorption spectroscopy of shock-compressed materials. Plyler and Humphreys, back in the 1940s, measured the infrared absorption spectra

96

5 Infrared Molecular Spectroscopy

of liquid CS2 and showed that the v3 antisymmetric stretch mode “is very intense and showed zero transmission from 6.5 to 6.7 µm with a 0.05 mm thick cell.” Note that Renlund et al. used a 0.2 mm thick CS2 cell [355]. The results from Renlund et al. are then very illustrative of what happens when a sample is too thick. Figures 5.6 illustrates the situation. Figure 5.6 (Top) is the Renlund et al. TRISP data, which only shows an apparent transmission peak at 1400 cm−1 , when the CS2 v3 antisymmetric stretch P and R branch absorption peaks are at 1510 and 1540 cm−1 . Figure 5.6 (Bottom) is a segment of the liquid CS2 infrared transmission spectrum from Plyler and Humphreys, adjusted in scale and orientation to approximately match the wavelength scale of the Renlund et al. figure. The narrower of the two Plyler and Humphreys curves is for a 0.1 mm thick sample.

Fig. 5.6 (Top) TRISP data from Renlund et al.; (Bottom) Liquid CS2 infrared transmission data from Plyler and Humphreys, cropped, scaled, and oriented to approximately match the wavelength scale of the Renlund et al. data. Top Figure adapted with permission from Ref. [21] Copyright 1986 Plenum Press. Bottom Figure adapted from Ref. [355] (open source)

5.1 Infrared Absorption

97

From this alignment of the two data sets, it can be seen that the Renlund et al. TRISP data are just the tail of the CS2 absorption that is not absorbed by the 0.2 mm thick sample. If a researcher is interested in the behavior of the absorption peaks of a given sample, the opacity of a too thick sample prevents that data from being recorded. The appropriate thickness of a given material can be estimated from its tabulated infrared absorptivities given in resources such as the NIST Webbook [356]. Renlund and Trott then used TRISP to measure early gas-phase products from detonations at 0.5 mm distances above the explosive free surface [357]. They observed what appear to be water absorption features in the 2.75 to 3.15 µm wavelength region for HMX and, to a lesser extent, PETN detonations. While TRISP as a concept showed promise to measure infrared absorption spectra of dynamically compressed materials, there were some experimental difficulties that prevented its wide adaptation. Foremost among these were pulse length and timing issues as illustrated above, combined with the small thicknesses of condensed phase samples needed. What was really needed to further this avenue of research were advances in laser and detector technology, which we discuss in the next section.

5.1.2 Ultrafast Laser Methods The 1980s saw a revolution in methods used to generate stable ultrafast laser pulses, starting with the development of the colliding pulse mode-locked (CPM) ring dye laser by Shank and Fork et al. at Bell Laboratories [358]. This system provided stable, nearly bandwidth limited