An Introduction to Laser Spectroscopy: Second Edition [2 ed.] 978-1-4613-5213-6, 978-1-4615-0727-7

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An Introduction to Laser Spectroscopy Second Edition

An Introduction to Laser Spectroscopy Second Edition Edited by

David L. Andrews University of East Anglia Norwich, UK

and

Andrey A. Demidov LaserSharp Corporation Boston, Massachusetts

Springer Science+Business Media, LLC

ISBN 978-1-4613-5213-6 ISBN 978-1-4615-0727-7 (eBook) DOI 10.1007/978-1-4615-0727-7 ©2002 Springer Science+ Business Media New York Originally published by Kluwer Academic/Plenum Publishers, New York in 2002 Softcover reprint of the hardcover 2ndedition 2002 http://www.wkap.nl/ 10987654321 A C.I.P. record for this book is available from the Library of Congress All rights reserved No part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, microfilming, recording, or otherwise, without writlen permission from the Publisher, with the exception of any material supplied specifically for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work.

Preface

The advantages of spectroscopic measurements based on lasers have long been apparent. In the earliest stages of laser development, appreciation of the potential for collecting spectra with unprecedented resolution was significantly tempered by many real concerns about the practical difficulties. The general consensus was that the associated optical technology was fraught with operational difficulties and required exceptionally painstaking experimentation. True enough; enormous credit is due to those who first showed what could be achieved, using systems so much less reliable and so much more bulky than those available today. It was in the field of Raman measurements, already well-established with arc lamp technology, that analytical applications of laser spectroscopy first became routinely practicable as stable rare gas lasers and associated instrumentation became more widely available and affordable. Many of the other forms of laser spectroscopy which then came to the fore were based on the detection of multiphoton absorption or other nonlinear optical processes. With pulsing methods delivering shorter and shorter pulses, new applications were also found in ultrafast diagnostics. Most recently, the arrival of titanium:sapphire and allied lasers has ushered in a new era in which tunable, high power, ultrashort pulses invite numerous hitherto unrealisable applications. The first edition of this book owed its origins to lecture notes from a Summer School in Laser Spectroscopy held at the University of East Anglia in Norwich, England in 1994. With great success the event was restaged in 1997. Together, the clientele of these meetings represents a cohort of the new laser spectroscopists who are already forging further advances in the field. On the welcome invitation by Kluwer AcademicIPlenum Publishers to produce a new edition to reflect the current state of the art, we opted to

v

vi

Preface

broaden the author base to reflect the international significance of the subject, whilst retaining key elements of the original lecture portfolio. As before, it has been our delight to have acceptances to contribute from so many key players in the field; the result of their collective expertise you have before you. In Chapter I, McCoustra lays out technical details of the main laser systems now employed for laser spectroscopy. In the subsequent chapter, Ashworth establishes the principles and applications of absorption and fluorescence measurements, followed by a description of Raman scattering, given by Jayasooriya and Jenkins. In Chapter 4, Beeby traces the development of pump-probe techniques, and the next two chapters deal more specifically with fluorescence applications. First, Lakowicz et al. describe fluorescence diagnostics for biochemical systems, and then Bain addresses applications to ordered systems. In the next chapter, Bernath discusses the use of tunable infrared lasers for the spectroscopy of transient species. Andrews and Meech introduce the subject of optical nonlinearity in Chapter 8, subsequently focusing on its surface applications. The utilisation of nonlinear optics for tunable ultraviolet generation is described by Lipson et al. in Chapter 9, and Chapter 10 sees a description by Singhal of femtosecond laser ionisation applied in mass spectrometry. The book concludes with a chapter by Demidov on environmental applications of laser remote sensing. It has been our great pleasure to work in such distinguished company on this project. Our hope is that a great many readers will find it a useful and comprehensive resource on the subject.

David L. Andrews, Norwich, UK Andrey A. Demidov, Boston, USA

March 2002

Contributors

David L. Andrews, School of School of Chemical Sciences, University of East Anglia, Norwich, NR4 7TJ, U.K. Stephen H. Ashworth, The School of Chemical Sciences, University of East Anglia, Norwich, NR4 7TJ, U.K. Angus J. Bain, Department of Physics and Astronomy, University College London, Gower Street, London WCIE 6BT, U.K. Andrew Beeby, Department of Chemistry, University of Durham, Durham DHI 3LE, U.K. Peter F. Bernath, Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3Gl, Canada Andrey A. Demidov, LaserSharp Corporation, 86 South Street, Hopkinton, MA, U.S.A. Upali A. Jayasooriya, School of Chemical Sciences, University of East Anglia, Norwich, Norfolk, NR4 7TJ, U.K. Robert D. Jenkins, School of Chemical Sciences, University of East Anglia, Norwich, Norfolk, NR4 7TJ, U.K. Diane Lacey, Department of Chemistry, University of Western Ontario, London, Ontario N6A 5B7, Canada vii

viii

Contributors

Joseph Lakowicz, Center for Fluorescence Spectroscopy, University of Maryland Medical School, 725 W. Lombard St., Baltimore, MD, U.S.A. Robert H. Lipson, Department of Chemistry, University of Western Ontario, London, Ontario N6A 5B7, Canada Martin R. S. McCoustra, School of Chemistry, University of Nottingham, University Park, Nottingham, NG7 2RD, U.K. Stephen R. Meech, School of School of Chemical Sciences, University of East Anglia, Norwich, NR4 7TJ, U.K. Kazimierz Nowaczyk, Center for Fluorescence Spectroscopy, University of Maryland Medical School, 725 W. Lombard St., Baltimore, MD, U.S.A. Yujun J. Shi, Department of Chemistry, University of Western Ontario, London, Ontario N6A 5B7, Canada Ravi P. Singhal, Department of Physics and Astronomy, University of Glasgow, Glasgow G 12 8QQ, Scotland, U.K. Leah Tolosa, Center for Fluorescence Spectroscopy, University of Maryland Medical School, 725 W. Lombard St., Baltimore, MD, U.S.A.

Contents

1.

Sources and Instrumentation for Laser Spectroscopy Martin R. S. McCoustra

2.

Principles of Absorption and Fluorescence Stephen H. Ashworth

3. Introduction to Raman Spectroscopy

1

43 77

Upali A. Jayasooriya and Robert D. Jenkins

4.

Pump-Probe Laser Spectroscopy Andrew Beeby

105

5.

Fluorescence Probes for Biochemical Systems Leah Tolosa, Kazimierz Nowaczyk and Joseph Lakowicz

139

6.

Time-Resolved Polarised Fluorescence Studies of Ordered Molecular Systems Angus J. Bain

171

7.

Infrared Laser Spectroscopy of Transient Species Peter F. Bernath

211

8.

Nonlinear Optics and Surface Applications David L. Andrews and Stephen R. Meech

233

Contents

x

9.

Tunable Short Wavelength Generation and Applications Robert H. Lipson, Yujun J. Shi and Diane Lacey

257

10. Femtosecond Laser Ionisation Mass Spectrometry Ravi P. Singhal

311

11. Laser Remote Sensing Andrey A. Demidov

339

Index

369

An Introduction to Laser Spectroscopy Second Edition

Chapter 1 Sources and Instrumentation for Laser Spectroscopy MARTIN R. S. McCOUSTRA School of Chemistry, University ofNottingham, University Park, Nottingham, NG7 2RD,

u.K.

The maRY spectrv::;copic applications of lasers described in subsequent chapters of this text, in other texts of this type (e.g. Alves, 1988, Andrews, 1992; DemtrOder, 1982) and increasingly in general spectroscopy texts (e.g. Hollas, 1998) demand sophisticated laser systems. While, of course, central to these systems is the laser source itself, ancillary instrumentation for the measurement of optical power and its distribution in frequency, time and space should be considered to be of equal importance. This introductory chapter seeks to address the nature of the laser system through a discussion of simple laser sources and related instrumentation. The purpose of this chapter is not, however, to give an introduction to principles of laser action or describe the detailed operating principles of specific types of laser themselves. Rather the reader is pointed to some of the very many excellent texts both on this subject (e.g. Andrews, 1990; Demtroder, 1982; Hecht, 1992; Mollenauer, 1987; Schaefer, 1990) and to the series of articles entitled Back to Basics published in the journal Laser Focus World for further information.

1.

LASER SOURCES

Since their initial discovery in the early 1960's, almost every laser that has been developed has been applied in a spectroscopic experiment of one form or another. Most of the lasers developed in that early period, such as the ruby laser and rare gas ion lasers, operated only on a fixed frequency, often providing only a single lasing wavelength, or at most lasing on a few narrow atomic or ionic resonances. As such, they were only tunable An Introduction to Laser Spectroscopy, Second Edition. Edited by Andrews and Demidov. Kluwer AcademicIPlenum Publishers. New York. 2002

1

MARTIN R. S. McCOUSTRA

2 (a)

(b)

"T,

"T,

i :RJipid ｎｾｮ､ｩｵｪ｜Ｇｃｬ＠ ｾ＠ Whllc [,.,,1'11 e...c:f,,,1of, 1." SSOMI ＭｌＮＧ｟Ｚｉｾ＠

Ilcc'-I)'

'h

Lun EmiaJiorJ .1694"," A .. 400nrn

..

W."clcngln (A)

'A,

Figure 1. (a) Energetics of the ruby ＨｃｾＫＺａｨＰＳＩ＠ laser. (b) Emission spectrum of a typical laser. Emission occurs only at wavelengths within the fluorescence linewidth of the laser medium that experience gain and also satisfy the standing-wave condition (Andrews, 1990). [Reproduced with the permission ofSpringer-VerIag GmbH.]

across the available optical cavity modes lying within the narrow gain profile of the relevant resonance (Figure I). This, of course, makes such lasers difficult to employ spectroscopically, with the obvious exception of their application in spectroscopic methods such as Raman spectroscopy where excitation on a single narrow frequency band is clearly desirable. It was, therefore, not until the development of broadly tunable lasers such as the organic molecular dye laser that the modem age of laser spectroscopy began to dawn, and it is upon such tunable laser sources that the following discussion will focus. Although spectroscopic measurements of one form or another can be made across the entire electromagnetic spectrum, the majority of studies, and those probably most relevant to our current discussion, are made in the region stretching from the microwave, with wavelengths of around 1 cm, to the vacuum ultraviolet, beyond 200 nm. Each region reflects a particular type of energy level transition and hence spectroscopy; rotational in the microwave and far-IR (ca. 1 cm - 50 ).lm), vibrational in the mid- and near-IR (ca. 50 - 1 ).lm) and electronic in the near-IR through the visible to the UV and VUV and beyond (ca. 1 ).lm - 100 nm). In each of these regions, suitable sources of tunable radiation have existed for a long time; for example microwave generators, monochromatised globars and Nemst filaments in the IR, and monochromatised incandescent and discharge lamps in the near-IR, visible, UV and VUV. Furthermore, such traditional, incoherent sources have proven their utility in many beautifully detailed spectroscopic studies to be found in the literature of both the pre- and postlaser eras. However, in the two regions of the electromagnetic spectrum that this discussion will focus on, the introduction of tunable, coherent radiation sources based on lasers has resulted in a rebirth of high resolution spectroscopy.

3

1. Sources and Instrumentation for Laser Spectroscopy

1.1

Tunable Coherent Radiation Sources in the Visible andUV

The Dye Laser The development of the organic molecular dye laser, some 30 or more years ago, marked the true birth of modem laser spectroscopy in offering the spectroscopist a truly continuously tunable coherent light source. Today, as then, the majority of dye lasers rely on a gain medium comprising of a solution of an organic molecular dye in a suitable solvent. Although recent laboratory studies have revealed that plastic-encapsulated dyes and sol-gel technology thin film dye-based systems may be practical, commercialisation has yet to follow. The variety of dyes ensures that the entire spectrum from around 320 nm to beyond 1000 nm can in theory be covered by a dye laser (Figure 2). The dye molecules themselves are typically complex, conjugated organic molecules which often contain multiple fused aromatic ring systems, as illustrated by the example of Rhodamine B (Figure 3a). Such species are chosen to ensure that the majority of the exciting radiation absorbed by the molecule is returned as fluorescence emission and the processes that deplete the yield of fluorescence, such as non-radiative relaxation, the generation of triplet states and photochemical reactions are minimised. Wavenumber / em· 1 ｾｲＭ 32500

30000

L

27500

-S'l-S'f

f

22500

25000

ｾ＠

0 is to be expected. However for any vibration that distorts the molecule from its

94

UPALI A. JAYASOORIYA and ROBERT D. JENKINS

equilibrium symmetry, a'i = 0 and the depolarisation ratios Pp and Pn take

y.;

their maximum values of ..% and independent of molecular symmetry. 4.2

respectively.

These values are

Solids

In the case of solids such as powders and polycrystalline solids, internal and external reflections and scattering events destroy any residual polarisation information. In order to circumvent these problems it is possible to use absorbing powders such as black copper oxide as matrix materials, reducing the above effects and making limited polarisation information available, even from powder samples. The most detailed polarisation information is obtainable when single crystals are available for study. The ordered structure of crystalline materials allows the expected Raman vibrations to be predicted from a Factor Group approach, as described in many texts (Born and Huang, 1954; Adams and Newton, 1970; Poulet and Mattieu, 1976; Decius and Hexter, 1977; Durman et ai., 1987). Use of a single crystal in a complete polarisation analysis provides full polarisability tensor information for the predicted vibrations. Figure 8 shows the sampling arrangement for a single crystal experiment. Assume a crystal structure of orthorhombic or higher symmetry. This crystal is aligned so that its crystallographic axes a, band c are parallel to the sampling axes x, y and z respectively. For sampling the incident radiation propagates along the x-axis with its polarisation parallel to the y-axis. Analysis occurs along the y-axis with an analyser set to only allow through to the spectrometer scattered light which is polarised along the z-axis. This arrangement may be represented by the symbol x(yz)y, using a notation due to Porto et al. (1966). The latter setting of the analyser also means that only the light emitted by the induced dipole along the z-direction of the sample is measured. Therefore the relevant equation for this process is; z

Analyser Single crystal sample

y

Figure 8. A sampling arrangement for single crystal studies. The illustrated geometry measures the polarisability component ayr.

95

3. Introduction to Raman Spectroscopy

f.J zind

=ax: E x +ayz E y +a zz Ez

.

(35)

However, since the polarisation of the incident radiation is along the ydirection, both Ex and E= are equal to zero, leaving the expression; ind

f.J z

= ayz E y

.

(36)

The measured intensity of scattered light x(yz)y will then result from the polarisability element t:;z, the product of the axes within brackets. It is possible to measure all the polarisability elements individually by appropriate choice of the alignment of the polariser, analyser and the sample crystal.

5.

RESONANCE RAMAN EFFECT

When the wavenumber of the exciting radiation approaches a certain type of electronic transition in a molecule it is possible to find dramatic intensity enhancements for Raman scattering. This useful process is called the resonance Raman effect (RRE) and is used by analytical chemists to gain new information about studied systems. Recalling the quantum mechanical expression for polarisability, given by Eq. 20, it is easy to see the cause of this effect. As nck becomes similar in value to a certain E ri , i.e. the energy imparted by the incoming photon approaches the energy of a real molecular excited state, the denominator in the first term of the polarisability approaches zero. This gives rise to an enhancement of Raman scattering by several orders of magnitude. The situation where Eri ｾ＠ nck is known as resonance and has made Raman spectroscopy a highly valuable tool in the analysis of the chromophoric centres of biological molecules for example. Two broad types of resonance Raman effect can be identified; the preresonance Raman effect (pre-RRE) and the resonance Raman effect itself. Pre-RRE is observed when the exciting frequency approaches the observable vibrational structure of the electronic absorption band involved in the Raman scattering process, but before it falls within the observable vibrational structure. When it falls within the observable vibrational structure of the electronic absorption, RRE is observed. As shown by Eq. 25 it is possible to express the total wavefunctions of the initial and final molecular eigenstates as products of electronic and vibrational wavefunctions, which allows separation of the polarisability tensor derivatives into the following two terms. Using a Taylor series expansion of the transition dipoles in Eq. 20 gives us an explicit form ofEq. 26. It is conventional to write;

96

UPALI A. JAYASOORIYA and ROBERT D. JENKINS

(37) where A is given by;

(38)

(we return to the B tenn below). In Eq. 38, J.l0r is the pure electronic transition moment for the resonant excited state r (evaluated at the equilibrium condition), of which V" is a vibrational level of band width r v", and Vv"i is the frequency for the transition from the ground state vibrational level

I¢6v) =Iv)

I

to the virtual excited state vibrational level ﾢｾＢ＠

). The

factor A therefore becomes enhanced as the denominator becomes smaller (as in resonance conditions) and as J.l0r becomes large (as in charge transfer transitions). The Franck-Condon overlap integrals between the vibrational levels Iv), Iv') and ｉﾢｾＮＩ＠ in the numerator of (38) are of special interest. These might be expected to be zero through orthogonality of the vibrational wavefunctions. However a shift of the equilibrium position, schematically illustrated in Figure 9, removes such orthogonality. Only the nonna1 mode of a totally symmetric vibration can provide such a shift, and therefore enhancement of A is seen only for such vibrations. Such enhancements have been observed for many systems and are often associated with a large number of overtones (Clarke and Mitchell, 1973).

［ｾＮＩ＠

11")

Q

Figure 9. A-tenn resonance Raman enhancement via the Franck-Condon effect.

97

3. Introduction to Raman Spectroscopy

A mechanism for enhancement of the non-totally symmetric vibrations exists via the second term in (37), B, whose expression is as follows; B

ＩＱＭｬＰｲＨｑ･ｾ＠

Ol-ll L; (v'IQq -Qel¢;.·)\¢;.. lv) +\v:I¢;.. )\¢;.·IQq -Qel v) h

.

oQ Q

V v"j -Vi

v

+Uv• (39)

The presence of the normal coordinate Q-dependent vibrational overlap integrals, in addition to the Franck-Condon overlap integrals in the numerator of the B-term, makes this term non-vanishing for vibrations other than totally symmetric modes. The selection rule for the vibrations which are enhanced by the B-term is determined by the vibronic coupling operator, and is given as ('If rio ｑｾ＠ If! 0) :t:. o. Knowing the symmetries of the electronic states 1'1/r ) and 1'1/0)' it is possible to work out the symmetries of the normal modes that make this integral non-zero. The condition is that the direct product of the three irreducible representations within this integral has to contain the totally symmetric irreducible representation. This type of resonance enhancement is observed in many biological compounds containing heme groups and the metalloporphyrin models for such systems (Spiro and Li, 1988). Theory also predicts that the scattering tensor may become asymmetric as the energy of the exciting radiation approaches resonance with an electronic transition. This means llxy may not be equal to Q),x, for example, and in general ras :t:. O. This may give rise to some unusual polarisation effects. Consider the full expression for depolarisation ratios given in Eq. 33. For normal Raman scattering, where the exciting radiation is far from resonance and ras = 0 , the depolarisation ratio cannot be higher than and this is the

.x ,

Pp value shared by all non-totally symmetric modes for which li = O. li has a finite value for totally symmetric modes and Pp then ranges between .% and zero. When the tensor is not index-symmetric, through resonance

conditions, then for non-totally symmetric modes; 3

Pp

Since neither

r!

nor

r;

="4+

2

5ras 4r;

(40)

can be negative, the polarisation must be

anomalous (> .%) if the scattering tensor is asymmetric. Even certain modes which are forbidden in non-resonance Raman scattering may become active under resonance conditions. These are non-

98

UPALI A. JAYASOORIYA and ROBERT D. JENKINS

totally symmetric vibrations for which the scattering tensor is antisymmetric, i.e. aij =-a ji. With these modes both a and Ys are zero while Y.. is fInite, giving a depolarisation ratio Pp

= o. This is called

anomalous or inverse polarisation (ap or ip, respectively).

Again, these effects are best seen in many biological compounds containing heme groups and the metalloporphyrin models for such systems.(Spiro and Li, 1988)

6.

COHERENT RAMAN TECHNIQUES

As the Raman effect is weak, it is advantageous if a technique which utilises the scattering process can be amplifIed in some way. Whilst the resonance Raman effect, discussed in section 5, exploits internal molecular energy levels to achieve its intensity increase, here we discuss the use of optical coherence to secure signal enhancement. When the laser power is strong enough, Stokes photons can also stimulate the emission of further Stokes photons (Bloembergen, 1967). This self-amplifIcation can lead to signal output increases of up to 50%. The stimulated Raman effect and its non-coherent spectroscopic implementation are comprehensively treated elsewhere (McCreery, 2000; Agrawal, 2001; Long, 2001).

6.1

Four-Wave Mixing

With high intensity laser sources incident on a sample, higher order Raman effects can be observed. One example is four-wave mixing (FWM). SpecifIcally FWM occurs when two input laser beam photons of energy h Vi (or ncki ) are assimilated by the sample with two photons (a Stokes and an anti-Stokes) being created. The latter photons have frequencies of Vs = Vi - d V and Vas = Vi + d V respectively. The overall energetics of Raman FWM are shown in the energy level diagram of Figure 10. The species undergoing FWM is automatically returned to its initial state, hence no internal relaxation time is required. Because of this, two more incident photons may immediately interact, signifIcantly improving conversion efficiency. As no net energy is transferred to the sample in FWM, the law of conservation of energy is obeyed automatically. However, the effect is strongest if the law of conservation of momentum is also satisfIed. A photon has momentum nk with the wave-vector magnitude given by;

99

3. Introduction to Raman Spectroscopy

Vas

Figure 10. Energy level diagram for the four-wave Raman effect.

(41) where n( w) is the refractive index. Hence the magnitude depends on both photon frequency and the frequency-dependent refractive index of the medium. To obey the law of conservation of momentum, the condition; (42)

pC Vas Figure 11. Wave-vector matching in FWM.

must be met. As each k is a vector, Eq. 4 may be exhibited pictorially by Figure 11. Tuning a system so that angles f) and ¢ allow the vector-addition condition is known as wave-vector matching or phase matching. Any process which simultaneously fulfils both laws of energy and momentum conservation is known as a coherent parametric process and invariably leads to an increase in photon (here laser-to-Raman) conversion efficiency. It can be seen from Figure 11 that, in order to satisfy wave-vector matching conditions, the Stokes and anti-Stokes photons are created at angles to the incident beam. This allows them to be filtered off spatially and subsequently used as a convenient source of laser radiation. In liquids and pressurised gases FWM offers an efficient means of laser frequency conversion.

100

6.2

UPALI A. JAYASOORIYA and ROBERT D. JENKINS

Coherent Anti-Stokes Raman Spectroscopy

Coherent Anti-Stokes Raman Spectroscopy (CARS) is a further FWM technique, this time using input beams of two frequencies (Harvey, 1981; Clark and Hester, 1988). Essentially the energetics are identical to that of stimulated FWM; however the signal from a single pump laser is split, with one resulting beam yielding the original laser frequency Yj and the other pumping a dye laser, producing a tunable source of frequency \.i. Dependent on the choice of dye VI can take available values below Vi and is itself used to produce the Stokes emission. There are two cases of CARS that can be described, as shown in Figure 12.

Vi

(a)

vi

Vi

Vii

(b)

Figure 12. Energy level diagrams for CARS: (a) non-resonance case, (b) resonance case.

For the general non-resonant case, the generated signal has frequency vsig where clearly; (43)

As phase matching is again achieved (the process is parametric) the signal can be easily separated from other signals with the use of an aperture at a pertinent angle as determined from a vector map. If the dye laser wavelength is such that it leads to a resonance in the sample (Figure 12b) then; (44)

and the signal is coherent anti-Stokes Raman emission, hence CARS. The spectrum has strong lines (104 - 105 times stronger than conventional Raman) when tuned to a resonance within the sample. Importantly the signal intensity is quadratically dependent on the number of molecules per unit volume of the sample. This means that CARS offers excellent opportunities for analysing very dilute samples, if a strong laser source is available. A continuous-wave laser is sufficient for analysis of liquids,

3. Introduction to Raman Spectroscopy

101

whereas typically a frequency-doubled Nd:YAG is needed to produce the vibrational-rotational CARS spectra of gaseous samples. Redemption is found in the fact that CARS signals are only observed from the volume of the intersection of the two beams. Furthermore 3-dimensional flame analysis is possible as a small cross-section volume of the two beams may be moved around inside the flame to determine the temperature profile. The strong signal beam can be detected over the large fluorescence background of combustible samples.

7.

FLUORESCENCE PROBLEM

When the energy of the exciting radiation matches that of an electronic absorption of the system, there is the possibility of exciting sample fluorescence. This has been one of the biggest disadvantages of Raman spectroscopy since the fluorescence process is, in general, much more efficient and would normally swamp any Raman scattering. This problem is commonly found even when attempting to observe non-resonant Raman spectra. However there are several ways to overcome fluorescence problems: (a) Often the interfering fluorescence is due to minor amounts of impurities in the sample. The best cause of action to obviate this is to purify the sample by some chemical or physical methods such as re-crystallisation. In some cases, leaving the sample irradiated in the laser beam for an extended period of time preferentially burns off the impurity thus removing the fluorescence. The latter technique is of course only applicable when the samples are robust. (b) A more sophisticated technique exploits the difference in lifetimes between the virtual molecular state involved in the Raman process and the real excited electronic state of fluorescence. The latter is much longer-lived than the former. In fact the virtual state in non-resonant Raman spectroscopy is considered to have an infinitely short lifetime. Time- resolved measurement of the scattering by nanosecond or faster pulsed lasers, with electronic time-gating, make it possible to discriminate between Raman scattering and fluorescence. (c) A less expensive and a popular way to avoid fluorescence is to use a longer wavelength of excitation. The use of reliable and stable nearinfrared (NIR) lasers as a source in Raman spectroscopy avoids fluorescence from a large majority of compounds. However there are disadvantages in using NIR radiation. In particular, due to Rayleigh's law (see Eq. 21), a large decrease in scattering efficiency is evident on moving to longer wavelengths, leading to detection problems. This and other difficulties have, to a great extent, been overcome by the advent of Fourier Transform (FT) Raman spectroscopy (Laserna, 1996; Hendra et al., 1991). Biological applications of the technique which were

102

UPALI A. JAYASOORIYA and ROBERT D. JENKINS

problematic due to fluorescence have recently been reviewed by Schrader et al. (1999).

8.

CONCLUSION

Due to the new developments in Raman spectroscopic instrumentation (Strommen, 1998; Coates, 1998) during the last two decades or so, initially anticipated applications of this technique as a useful analytical tool are at last being realised. The growth in biological applications explores a variety of sampling strategies, in addition to the established use of resonance Raman spectroscopy in enzymology (Carey, 1998a). It is now possible to use both resonant and non-resonant Raman labels, with the latter making use of highly polarisable molecules, for the study of biological systems (Carey, 1998b). Improvements in instrumentation have made it possible to use Raman difference spectroscopy (RDS) to probe small changes in the vibrational spectrum of protein structures resulting from, for example, isotopic substitution (Callender et a/., 1998). The instrumentation for such an experiment is similar to that presented schematically in Figure 7, but using a split sample cell with the two halves containing the samples to be compared. The cylindrical sample is spun about the vertical axis during spectral accumulation, with the scattering data from the two halves of the cell collected separately using a multichannel analyser. With the inherent advantages of Raman spectroscopy for aqueous samples when compared to IR spectroscopy, RDS is proving to be a powerful method for the study of protein folding and ligand binding. The advent of fast pulsed laser systems has made it possible to study the ultrafast dynamics of molecular systems and excited state structures using Raman spectroscopy (Myers, 1997; Biswas and Umapathy, 1998). It is now possible to study the time-dependence of molecular structure in large macromolecules such as proteins and nucleic acids using Raman spectroscopy (Thomas, 1999). There is also a large increase in current and potential applications to materials and industrial problems, e.g. the analysis of structural phase transitions (Husson, 1999), applications in the semiconductor industry (Torres, 1997), studies of interfaces (Dumas et a/., 1999), and of materials under high pressures (Jayaraman and Sharma, 1998), and monitoring the catalytic activities of heterogeneous catalysts (Knozinger and Mestl, 1999; Wachs, 1999). An interesting application with obvious environmental applications is the in situ investigation of aerosol particles (Schweiger, 1999). Another very active and exciting area of research has been in the use of Raman microscopy. This technique provides the unique advantage of vibrational mapping at around J.lIIl spatial resolution. A number of recent reviews have appeared with the applications of this technique dealing with

3. Introduction to Raman Spectroscopy

103

instrumental aspects (Zhang et al., 1998; Schaeberle et al., 1999), polymer analysis (Williams and Woodcock, 1997; Al Khanbashi et al., 1998; Sammon et al., 1999; Everall and King, 1999) to remote analysis (Hong et al., 1999). Surface enhanced Raman scattering (SERS) and RRE are now challenging fluorescence spectroscopy for sensitivity of detection. There are several reports showing the possibility of observing single molecule vibrational spectroscopy with these techniques (Emory and Nie, 1998; Emory et al., 1998; Kneipp et al., 1999; Klug et al.,1999). Use of fibre optics in Raman spectroscopy has extended the technique's suitability for industrial applications (Lewis and Griffiths, 1996). These and other interesting analytical applications of SERS spectroscopy are reviewed by Mulvaney and Keating (2000). The literature quoted in this chapter together the references sited therein should provide the interested reader with an insight into the vast literature now available on the applications of modem Raman spectroscopy.

REFERENCES Adams, D.M., and Newton, D.C., 1970, J. Chem. Soc. A 2822. Agrawal, G.P., 2001, Application o/Nonlinear Fiber Optics. Academic Press. AI Khanbashi, A., Dhamdhere, M., and Hansen, M., 1998, Appl. Spectrosc. Rev. 33: 115. Andrews, D.L., 2000, In Encyclopedia o/Spectroscopy and Spectrometry. (J.C. Lindon, G.E. Tranter and J.L. Holmes, eds.), Academic, New York p.1993 Banwell, C.N., and McCash, E.M., 1994, Fundamentals 0/ Molecular Spectroscopy. McGraw-Hili. Biswas, N., and Umapathy, S., 1998, Curro Sci. 74:328. Bloembergen, N., 1967, Am. J. Phys. 35:989. Born, M., and Huang, K., 1954, Dynamical theory 0/ Crystal Lattices. Clarendon Press, Oxford. Brillouin, L., 1922, Ann. Phys. (Paris) 88: 17. Cabannes, J., 1928, Compt. Rend. 186:1201. Callender, R., Denig, H., and Gilmanshin, R., 1998, J. Raman Spectrosc. 29: 15. Carey, P.R., 1998a, J. Raman Spectrosc. 29:7. Carey, P.R., 1998b,J. Raman Spectrosc. 29:861. Clark, RJ.H., and Hester, R.E., 1988, Advances in Non-Linear Spectroscopy. John Wiley, Chichester, Chapters ＳＭｾＮ＠ Clarke, R.J.H., and Mitchell, P.D., 1973, J. Amer. Chem. Soc. 95:8300. Coates, J., 1998, Appl. Spectrosc. Rev. 33:267. Cotton, F.A., 1963, Chemical Applications o/Group Theory. Wiley Interscience, New York. Craig, D.P., and Thirunamachandran, T., 1984, Molecular Quantum Electrodynamics. Academic Press, London. Damen, T.C., Porto, S.P.S., and Tell, B., 1966, Phys. Rev. 142:570. Decius, J.C., and Hexter, R.M., 1977, Molecular Vibrations in Crystals. McGraw-Hili, New York. Dirac, P.A.M., 1981, The Principles o/Quantum Mechanics (4'" edition). Clarendon Press, Oxford. Dumas, P., Weldon, M.K., Chabal, Y.J., and Williams, G.P., 1999, Surf. Rev. Lett. 6:225.

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Dunnan, R., Favre, P., Jayasooriya, U.A., and Kettle, S.F.A., 1987,1. Cryst. Spec. Res. 17:431. Emory, S.R., Haskins, W.E., and Nie, S., 1998,1. Am. Chem. Soc. 120:8009. Emory, S.R., and Nie, S., 1998, J. Phys. Chem. B 102:493. Everall, N., and King, B., 1999, Proc. Macromol. Symp. 141:103. Ferraro, J.R., and Nakamoto, K., 1994, Introduction to Raman Spectroscopy. Academic Press. Harvey, A.B., 1981, Chemical Applications of Nonlinear Raman Spectroscopy. Academic Press, London, Chapters 2, 5, 8. Hendra, P., Jones, C., and Wames. G., 1991, Fourier Transform Raman Spectroscopy. Ellis Horwood Ltd. Hong, P.V., Cavagnat, R., and Bruneel, E.J.L., 1999 Spectra Anal. 28:26. Husson, E., 1999, Key Eng. Matter. 155/156: I. Jayaraman, A., and Sharma, S.K., 1998, Curro Sci. 74:308. Kettle, S.F.A., 1985, Symmetry and Structure. Wiley, New York. Klug, J.T., Wang, G.D., Emory, S.R., and Nie, S.J., 1999, J. Am. Chem. Soc. 121:9208. Kneipp, K., Kneipp, H., Itzkam, I., Dasari, R.R., and Feld, M.S., 1999, Chem. Rev. 99:2957. Knozinger, H., and Mesti, G., 1999, Top. Catal. 8:45. Kramers, H.A., and Heisenberg, W., 1925, Z. Phys. 31:681. Landsberg, G.S., and Mande1stam, L.I., 1928, Naturwiss. 16:557. Laserna, J.J., 1996, Modern Techniques in Raman Spectroscopy. John Wiley, Chichester. Lewis, I.R., and Griffiths, P.R., 1966, Appl. Spectrosc. 50: 12A. Long, D.A., 2001, The Raman Effect: A Unified Treatment of the Theory of Raman Scattering. Wiley, Chichester. McCreery, R.L., 2000, Raman Spectroscopy for Chemical Analysis. John Wiley and Sons, Chichester. Mulvaney, S.P., and Keating, C.D., 2000, Anal. Chem. 72: 145R. Myers, A.B., 1997, Acc. Chem. Res. 30:519. Porto, S.P., Giordmaine, J.A., and Damen, T.C., 1966, Phys. Rev. 147:608. Poulet, H., and Mattieu, J.P., 1976, Vibrational Spectra and Symmetry of Crystals. Gordon and Breach, New York. Raman, C.V., and Krishnan, K. S., 1928, Nature 121:501. Raman, C.V., and Krishnan, K.S., 1929, Proc. Roy. Soc. Lond. 122:23. Rayleigh, Lord, 1871, Phil. Mag. XLI:274,447. Rocard, Y., 1928, Compt. Rend., 186:1107. Sammon, C., Hajatdoost, S., Eaton, P., Mura, c., and Yarwood, J., 1999, Proc. Macromol. Symp. 141:247. Schaeberle, M.D., Morris, H.R., Turner, J.F., and Treado, PJ., 1999, Anal. Chem. 71: 175A. Schrader, B., Dipple, B., Erb, I., Keller, S., Lochte, T., Schultz, H., Tatsch, E., and Wessel, S., 1999,1. Mol. Struct. 480/481:21. Schweiger, G., 1999, In Analytical Chemistry of Aerosols. (K.R. Spurney, ed.), Lewis, Boca Raton, p.319. Smekal, A., 1923, Naturwiss. 11:873. Spiro, T.G., and Li, X-Y., 1988, In Biological Applications of Raman Spectroscopy. (T.G. Spiro, ed.), Vol 3. John Wiley, New York. Spiro, T.G., and Strekas, T.C., 1972, Proc. Nat. Acad. Sci. USA 69:2622. Strommen, D.P., 1998, In The Handbook of Instrumental Techniques for Analytical Chemistry. (F.A. Settle, ed.), Prentice Hall, PTR: Upper Saddle River, p. 994. Thomas, G.J., 1999, Annu. Rev. Biophys. Biomol. Struct. 28: I. Torres, C.M.S., 1997, NATO ASI Ser. Ser. E 344:331. Wachs, I.E., 1999, Top. Catal.8:57. Williams, K.P.J., and Woodcock, I.C., 1997, Polym. Test. 3: I. Zhang, S., Franke, F.S., and Niemczyk, T.M., 1998, Mod. Technol. Appl. Spectrosc. 291

Chapter 4

Pump-Probe Laser Spectroscopy

ANDREW BEEBY Department of Chemistry, University of Durham, Durham DHJ 3LE, U.K.

1.

WHY PUMP-PROBE?

Understanding the sequence of events that occur following the absorption of a photon is key to our understanding of photochemical reactions and processes. In many reactions there exist metastable or transient species, for example excited electronic states of radicals. These species are by their very nature short-lived, and under conventional reaction conditions their concentration at any instant is vanishingly small - making them virtually impossible to observe. Prior to the introduction of pump-probe techniques the intermediacy of these species could at best be only inferred. In the late 1940s Porter, then working as a Ph.D. student with Norrish at Cambridge, realised that if a short, intense pulse of light were used to trigger a reaction then there would be a relatively high concentration of the intermediates. These intermediates would be present in such amounts that they could be detected by conventional spectroscopic techniques. The pioneering work of Porter and the effort of subsequent workers has led to the development of a range of experiments based upon pump-probe spectroscopy. In recognition of Porter's pioneering work in this area he and Norrish shared with Eigen the 1967 Nobel Prize for Chemistry 'for their studies of extremely fast chemical reactions, effected by disturbing the equilibrium by means of very short pulses of energy'. It is now just over 50 years since these early experiments, and the area of pump-probe spectroscopy has matured to become the most widely used technique for monitoring fast reactions. The timescale on which experiments can be measured has been reduced from ca. milliseconds in 1950 to tens of An Introduction to Laser Spectroscopy, Second Edition, Edited by Andrews and Demidov, Kluwer Academic/Plenum Publishers, New York, 2002

105

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ANDREW BEEBY

femtoseconds at the tum of the century, allowing an insight into the dynamics of reactions at the most fundamental level. This chapter will briefly review key milestones in the development of pump-probe spectroscopy and discuss the range of spectroscopic techniques that are now available for probing the intermediates. A complete survey of all of the techniques is beyond the scope of this work, but it is hoped that the reader will be made aware of the range of possibilities that are to hand. The experimental methods employed for a pump-probe experiment are largely determined by the time-resolution required for an experiment. 'Slow' measurements, that is the study of reactions with a transient lifetime of > 10 ns, can be studied using 'real-time' measurements in which the pump and probe sources are controlled independently. At shorter timescales there are limitations arising from the uncertainties associated with the triggering of light sources, the response time of detectors and subsequent data-acquisition electronics. For these reasons, measurements on a timescale of < 10 ns tend to use a single light source that acts as source of both the pump and probe.

2.

PUMP-PROBE: THE BASIC EXPERIMENT

In the pump-probe experiment the system under study is pumped with a short, intense pulse of radiation. This generates a high concentration of the transient species, permitting their detection. Here the term short is defined as being significantly less than the lifetime of the transient species we want to study; for example if the transient species has a lifetime of the order of microseconds, then a pump duration of a few nanoseconds duration would be suitable. The system is then allowed to evolve for a period of time during which the transient species decays. In the case of the transient being an excited electronic state it may either revert to the ground state or transform into further transient species or reaction products. After this evolution time the system is probed by some means, allowing information about the identity and/or the concentration of the transient species to be obtained. By systematically varying the evolution period between the pump and probe event the kinetic behaviour of the transient species can be determined.

3.

HISTORY: PORTER'S EARLY EXPERIMENTS

Lord Porter's early experiments that led to the development of pumpprobe spectroscopy, specifically the flash-photolysis experiment, have been described in a recent publication (Porter, 1997) and therefore only a brief summary will be included here. The early experiments used large

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4. Pump-Probe Laser Spectroscopy

flashlamps, filled with rare-gas, as the pump source (Porter, 1950). These lamps produced pulses of the order of milliseconds, and pulse energies of thousands of Joules were not uncommon! Porter chose to probe the entire absorption spectrum of the irradiated sample using a second flashlamp, known as the spectroscopic flash. This second lamp was triggered by an elaborate and mechanically complex system of switches and shutters. Light from this flash was sent through the sample and into a spectrograph equipped with a photographic plate. Thus, the entire absorption spectrum of the sample was probed at a specific delay time, requiring many experiments to be carried out to determine the kinetic behaviour. A schematic diagram of an early flash-photolysis experiment is illustrated in Figure 1. The temporal resolution of these experiments was dictated by the duration of the probe pulse. The detection system, a photographic film, effectively integrated all of the light that fell on it during the probe pulse duration. However, in order to study the kinetic behaviour of the system in this way a number of experiments have to be performed with different time delays between the pump and probe pulses, and if the kinetics are complex, a large number of experiments have to be accurately and reproducibly carried out. An alternative means of investigating the kinetic behaviour of a system is to use a continuous probe beam. Temporal fluctuations in its intensity can be quantitatively determined using a detector such as a photomultiplier, although these devices can monitor only one wavelength or bandwidth of probe beam at a time. Originally, it was not possible to simultaneously monitor the changes in intensity with time across the entire spectrum.

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ANDREW BEEBY

This highlights the phenomenon of information overload, which prevents the experimentalist obtaining both kinetic and spectral data simultaneously. Consider the amount of data that would be generated during a single pumpprobe experiment in which we want to record the entire absorption spectrum of the system as a function of time. For example we may require to record changes in absorbance over the range 250 - 750 nm, with a resolution of perhaps 1 nm: 500 data points per spectrum. For kinetic studies we might like 1000 data points per wavelength and to ensure a good signal to noise ratio and dynamic range in the data we would require 2 bytes per data point after conversion to a digital format. Thus, each lamp-flash experiment would generate a 1MB data set in the space of perhaps a few tens of microseconds; a rate of 104 MB per second. Such rates of data acquisition are beyond the scope of available instrumentation and as a result, experimentalists restrict the amount of data acquired in a single experiment, recording either (a) the absorption at a single wavelength as a function of time or (b) an entire transient absorption spectrum at a certain delay time after the·pump pulse. In terms of equipment, option (a) is the cheaper and most commonly used. Clearly a full array of absorbance/ wavelength! time data can be built up by sequential stepping of either the wavelength or time delay in experiment (a) or (b) respectively. After the early pioneering work, improvements in flashlamp design led to shorter pulse durations, with outputs of the order of microseconds. This was followed by the introduction of the ruby laser in the mid 1960s. With an output pulse of ca. IOns, the laser led to a further leap in the ability to study events on a faster timescale. The use of what was, at the time, a very short pump pulse led to difficulties in generating equally short probe pulses that were perfectly synchronised in time to the pump. In 1968, Porter and Topp found a solution to this problem that meant that once again the timeresolution of the experiment was limited by the pulse duration of the pump source (Porter and Topp, 1968). The past three decades have witnessed great advances in laser technology, with off-the-shelf laser systems now providing pulse durations of 100 fs or less. Equally important developments have led to tunable that provide radiation across the UVnanosecond and ultra-fast laser ｳｯｾｲ｣･＠ vis-NIR spectrum. These developments have ensured that flash photolysis remains an important tool for the study of excited states, transient intermediates and photochemistry.

4. Pump-Probe Laser Spectroscopy

4.

109

EXCITED ELECTRONIC STATES AND TRANSIENT SPECIES

Pump-probe spectroscopy is frequently used to study excited triplet states or metastable isomers of materials which are relatively 'long-lived' and there are a large number of research groups who have instrumentation designed to carry out such measurements. Such instrumentation can probe a number of features of the sample, the most common being the electronic (UV-vis) spectrum, although a wide variety of other techniques can be used including resonance Raman and IR absorption. More detail and examples of these probe techniques will be discussed later. Experiments which demand a time resolution of> 10 ns have a distinct advantage in terms of the relative simplicity and low cost of instrumentation, since they can be carried out using 'real-time' detection. As discussed above, if a suitable continuous or pseudo-continuous probe source can be generated then the intensity of the probe beam transmitted by the sample can be monitored as a function of time using a photomultiplier tube or suitable semi-conducting diode. A typical response time of a PMT detector is of the order of 10 - 20 ns, and its output can be digitised in real time by a digital storage oscilloscope; currently it is not unusual to see oscilloscopes capable of digitising I G samples/so Thus, a trace of intensity of transmitted light as a function of time can be generated. Alternatively two pulsed sources may be used to pump and probe the sample. When considering transient species with lifetimes of > 50 ns it is possible to synchronise the firing of two separate laser systems using an electronic pulse generator. In practice the firing of the lasers is typically controlled by circuitry inside the laser and, due to the nature of the processes that occur in the firing sequence, there is a random fluctuation in the time between an electronic trigger pulse being sent to the laser and the appearance of light. This is referred to as 'jitter' and in a modem laser system it is typically < IOns. Thus, it is possible to synchronise two lasers to within IOns. The synchronisation of two lasers in this fashion is vital for experiments that use two lasers for the pump and probe such as ns-time-resolved resonance Raman spectroscopy.

5.

TRANSIENT UV-VIS ABSORPTION SPECTROSCOPY: LASER FLASH PHOTOLYSIS

In its simplest form this experiment concerns study of the absorption spectra and the kinetic behaviour of a transient species by monitoring the electronic absorption spectra. A Jablonski diagram, illustrated in Figure 2, is

ANDREW BEEBY

110

usually used to represent the energy level diagram of an organic molecule. Let us consider the study of a triplet state of a molecule, T I.

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5. Fluorescence Probes for Biochemical Systems

147

and Grossman, 1983 and Kronick, 1986), flow cytometry (Oi et al., 1982) and single-molecule detection (Nguyen et a!., 1987).

Green Fluorescent Protein The green fluorescent protein (GFP) is one of the most important fluorescent probes for biochemical systems to date. GFP originated from the bioluminescent jellyfish, Aequorea victoria. The GFP structure (Figure 8) is a ｾＭ｢｡ｲ･ｬ＠ with the highly fluorescent chromophore securely protected within its interior (Ormo et al., 1997). The chromophore of GFP is spontaneously formed upon protein folding by the cyclization and oxidation of Ser 65, Tyr 66 and Gly 67. For this reason, GFP becomes inherently fluorescent upon expression without the need for additional cofactors or enzymatic reactions.

Figure 8. Stereo structure of the green fluorescent protein. The chromophore is linked to structure by covalent bonds. Figure provided by R. Tsien and S. Remington. the ｾＭ｢｡ｲ･Ｑ＠

The DNA sequence of GFP can be inserted into the C- or N-terminal of a gene of interest producing a fluorescent fusion protein upon expression. As a result, GFP has been used extensively as a probe for gene expression in cells and tissues (Chalfie et ai., 1994, Kotarsky et ai., 2001 and Tsukamoto et ai., 2000). GFP has also become a convenient marker for the identification of transgenic animals (Hom and Wimmer, 2000 and Chan et ai., 2001). The absorption and emission spectra of wild type GFP and the T2031 mutant are shown in Figure 9 (Ehrig et al., 1995). Mutants have since been designed which display enhanced blue (EBFP), cyan (ECFP), green (EGFP)

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LEAH TOLOSA et al.

and yellow (EYFP) emission (Crameri et al., 1996, Cubbitt et al., 1995 and Heim and Tsien, 1996). With the broader palette of spectral properties, it is now possible for in vivo monitoring of protein-protein interactions (Li et aI., 2001) and other cell activity (Kerr et aI., 2000 and Zacharias et aI., 2000) by fluorescence energy transfer between fluorescent proteins.

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Science.

A highly fluorescent red protein from the coral Discosoma was recently cloned by homology to GFP. This protein, known as DsRed, absorbs at 558 nm and emits at 583 nm upon maturation (Fradkov et al., 2000). Mutation of Lys-83 to Met shifts the emission further to 602 nm (Baird et al., 2000). The red-shifted spectra relative to GFP are attributed to a more extensive conjugated 1t-system in the chromophore (Wall et al., 2000). By all indications, this new protein can be used for the same purposes as GFP. However, unlike GFP, DsRed forms tetramers and takes days to mature into the fluorescent form. Cell biological applications will have to wait until new mutants with suppressed tetramerization and accelerated maturation have been developed.

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5. Fluorescence Probes for Biochemical Systems

3.

EXTRINSIC FLUOROPHORES

In the many instances where the biomolecule of interest is nonfluorescent, or where the intrinsic fluorescence is inadequate, fluorescence is obtained by labelling the molecule with extrinsic probes. The number of fluorophores has increased dramatically in the past decade, and no single source summarizes the diverse spectral properties. Useful information on a wide range of fluorophores can be found in the Molecular Probes catalog (Haugland, 1996).

3.1

Protein-Labelling Reagents

There are a wide variety of fluorescent probes for covalent and noncovalent labelling of proteins. The probes for covalent labelling are designed to react with the amine, sulfhydryl, histidine and in rare cases to alcohol side chains in proteins. Some of the more widely used probes are shown in Figure 10. N-hydroxysuccinimides, isothiocyanates and sulfonyl chlorides are typically used for labelling amines, while iodoacetamides and maleimides are used for labelling sulfhydryl groups (Waggoner, 1995).

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Fluoresceins and rhodamines have found widespread use as extrinsic labels. These dyes have absorption maxima at 480 and 560 nm, respectively,

150

LEAH TOLOSA et al.

with extinction coefficients near 80,000 M-1cm- 1. The long wavelength of absorption and emission and the high quantum yields make these probes suitable for minimizing the background autofluorescence of biological samples. Additionally, these dyes are suitable for quantifying the associations of small, labelled molecules with proteins via changes in fluorescence polarization. One disadvantage of fluorescein is its small Stokes' shift (Figure 11 , top). This leads to a tendency to self-quench. When more than a single fluorescein group is bound to a protein, there can be energy transfer between these groups. This explains the frequently observed non-linear relationship between brightness of fluorescein-labeled proteins and the extent of labelling. Dyes with longer Stokes' shifts are more desirable. Dansyl chloride is an example of these types of probes (Figure 11, middle). However, at higher degrees of labelling, the highly hydrophobic dansyl group causes protein aggregation. More recently developed probes have both a large Stokes' shift ｍｏｾＮ＠ III

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5. Fluorescence Probes for Biochemical Systems

151

and favourable water solubility. An example of this is Cascade Yellow which displays excitation and emission maximum at 409 and 558 nm, respectively (Figure 11, bottom). The charges on the aromatic rings facilitate water solubility.

Solvent-Sensitive Probes Probes based on naphthalene derivatives tend to have environmentsensitive emission spectra. The naphthylamine sulfonic acids (Figure 10), dansyl chloride (DNS-CI), l-anilinonaphthalene-8-sulfonic acid (ANS) and 6-(p-toluidinyl)-naphthalene-2-sulfonic acid (TNS) are examples of these (Slavik, 1982). These dyes are weakly fluorescent in buffer but fluoresce very strongly when bound to proteins or membranes. Prodan (Weber, 1979) and its reactive form, acrylodan (Figure 10), also display high sensitivity to the polarity of the local environment (Pendergast, F. G., 1983). This is due to a charge separation between the amino and carbonyl groups upon excitation.

Red and Near-Infrared (NIR) Dyes The development of probes that absorb at longer wavelengths progressed from the need for higher sensitivity and ease of measurement in biochemical systems. The autofluorescence of biological samples is often the limiting factor in the sensitivity of fluorescence measurements. Long wavelength probes are desirable because with increasing excitation wavelengths, the aurtofluorescence decreases, and detectability over background increases (Thompson, 1994). Another advantage of long wavelength fluorophores is that they can be excited with simple and inexpensive laser sources such as solid-state laser diodes (e.g., laser pointers). Some of the more commonly used long wavelength probes, such as Cy-3, Cy-5 and Cy-7, belong to a group of cyanine dyes (Figure 12). The cyanine dyes typically display small Stokes' shifts as shown for Cy-3 in Figure 12 making them prone to selfquenching. Charged side chains are used to improve the water solubility of the dyes and to prevent self-association, a common cause of self-quenching in these dyes.

Long-Lifetime Probes We mentioned in the previous section that the driving force for the development of new probes is the need for better sensitivity in biological applications. Long-wavelength probes decrease the contribution of shortwavelength autofluorescence. It is also a known fact that this autofluorescence is relatively short-lived, typically in the ns time scales. With a few exceptions, almost all organic fluorophores have lifetimes within this time range. This not only limits sensitivity, but also the dynamic information of fluorescence in complex biological systems.

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In recent years, metal-containing luminophores with forbidden or semiforbidden transitions have been developed as long-lifetime probes. The lanthanides are distinctively fluorescent metals that display emission in aqueous solution and have decay times of 0.5-3 ms (Richardson, 1982 and Sabbatini and Guardig1i, 1993). Because of weak absorption, lanthanides are usually excited indirectly through chelated organic ligands (Figure 13). These ligands absorb the incident light and transfer the excitation to the metaL One of the more favourable properties of lanthanides is that they can be substituted chemically for calcium in many calcium-dependent proteins (Bruno et al., 1992 and Horrocks and Sudnick, 1981). The decay times when bound to proteins can be used to calculate the number of bound water molecules in a calcium binding site (Sabbatini, 1993). For proteins without intrinsic binding sites, lanthanide-chelating reagents have been developed which can be coupled to proteins. Widespread use of lanthanides has been found in "time-resolved" detection immunoassays (Lamture and Wensel, 1993 and, 1995) and high throughput screening (Stenroos, 1998). The term "time-resolved" is to some extent a misnomer since it does not refer to measurement of decay times. It is rather a method that is "time-gated" as shown if Figure 14. The purpose of this strategy is to increase sensitivity by

153

5. Fluorescence Probes for Biochemical Systems

measuring the intensity of the long-lived lanthanide after the short-lived autofluorescence has disappeared.

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4.

DNA PROBES

There are numerous probes that spontaneously bind to DNA and display enhanced emission (Steiner and Kubota, 1983 and Suh and Chaires, 1995). Some of these probes are shown in Figure 20. Perhaps the most widely used of these probes is ethidium bromide (EB). The emission of EB in water is very weak but increases about 30-fold upon binding to DNA. The decay time of EB is about 1.7 ns in water and increases to about 20 ns upon binding to double- stranded DNA. EB binds to DNA by intercalation of the planar aromatic ring between the base pairs of the double helix. Binding affInity to DNA is enhanced when two of these intercalating groups are present as in the EB homodimer (Glazer et al., 1990) and in TOTO-l (Rye et al., 1992). Hoechst 33342 and 4',6-diamino-2-phenylindole (DAPI) bind into the minor groove of DNA.

5.

FLUORESCENCE SENSING

Now that we know the many different kinds of fluorescent probes, it is but fitting to discuss some fluorescence applications. Fluorescence sensing is the determination of chemical and biochemical analytes by fluorescence

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spectroscopy. This is a very active area of research that was initially driven by the need to eliminate or reduce the use of radioactive tracers. Additionally, there is a need for rapid, low-cost and highly sensitive testing methods for a wide range of clinical, bioprocess, and environmental applications. In recent years, we have witnessed the introduction of exciting new methods based on fluorescence detection, including DNA sequencing, high-throughput screening, DNA arrays and a variety of fluorescence immunoassays. Some useful monographs have summarized the numerous analytical applications of fluorescence (Ichnose et al., 1987, Lakowicz, 1994 and Czarnik, 1993).

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5.1

Optical Clinical Chemistry

The value of the high sensitivity of fluorescence is increasingly becoming more apparent with devices that are currently in development that can be used in the doctor's office, at the patient's bedside, or for home health care. A prototype of such a device to detect clinically relevant compounds is shown in Figure 21. The blood sample can be withdrawn into the sensing device, which could be a syringe, capillary tube or self-filling cartridge. The

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sensor would then be placed into an instrument which would determine the analyte concentration. These point-of-care instruments would use solid-state light sources and perform sensing based on fluorescence intensity, polarization, or lifetime. Cappllary Tua Sensor

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Several companies are now vigorously developing the so-called lab-on-achip based on a similar principle but in miniature form. This will require some micro fluidic devices to perform initial sample preparation such as cell lysis and transport of the sample into the fluorescence sensor. These miniature devices are predicted to detect DNA sequences such as genes for cancer, mv or hepatitis and protein markers for diseases by immunoassays.

5.2

Mechanisms of Sensing

Any phenomenon that results in a change of fluorescence intensity, wavelength, anisotropy, or lifetime can be used for sensing. The simplest mechanism for such an effect is collisional quenching. In this case, one identifies a fluorophore which is quenched selectively by the analyte. Collisional quenching results in a decrease in the intensity or lifetime of the fluorophore. Either effect can be used to determine the analyte concentration. An example of this is the sensing of oxygen with ruthenium metal ligand complexes. Figure 22 shows the Stem-Volmer plots for oxygen quenching of [Ru(phen}3i+ and [Ru(Ph2phen}3]2+ (Wolfbeis, 1991). The

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Fluorescence resonance energy transfer (FRET) is perhaps the most general and valuable phenomenon for fluorescence sensing. Any process that brings the donor and acceptor into closer proximity will result in a decrease in the donor intensity and/or decay time. Since energy transfer acts over macromolecular distances, it can be used to detect association of proteins, as occurs in irnmunoassays and receptor-substrate interactions. An elegant example of FRET is the in vivo sensing of Ca2+ using the calcium-binding protein, calmodulin fused to the cyan-mutant of GFP (CFP) (Miyawaki et al., 1997). In the presence ofCa2+, this protein binds the M13 peptide that is fused to GFP. The proximity of the two proteins results in emission of the GFP at 535 nm when CFP is excited at 440 nm (Figure 23). This protein has been used to observe Ca2+ mediated signal transduction in C. elegans in real time (Kerr et al., 2000). Sensing can also be accomplished when the fluorophore can exist in two states, depending on the ana1yte concentration. One form can be nonfluorescent, in which case emission is only seen in the absence or presence of ana1yte, depending on which form is fluorescent. Alternatively, both forms may be fluorescent but differ in quantum yield or emission spectra. This type of behaviour is often seen for pH probes, where ionisation of the probe results in distinct absorption and/or emission spectra. Spectral shifts are also seen for probes which bind specific cations such as calcium. Such probes allow wavelength-ratiometric measurements. Probes which bind specific analytes are often referred to as probes of analyte recognition. There are many other mechanisms that can be used to design probes that exhibit changes in fluorescence in response to analytes. These include

5. Fluorescence Probes for Biochemical Systems

161

twisted intramolecular charge transfer (TIeT) states and photoinduced electron transfer (PET).

Figure 23. Calcium-sensing protein based on RET between two mutant GFPs at opposite ends of calmodulin (CaM). Revised and reprinted with permission from Miyaki, 1997, Copyright © 1997, Macmillan Magazines Limited

5.3

Immunoassays

Immunoassays constitute an enormous and diverse family of assays mostly based on the principles described above, whereas others are based on principles not yet discussed. The basic scheme is to couple the association of antibody (Ab) with antigen (Ag) to some other event that yields an observable spectral change. Various mechanisms are possible, including energy transfer, anisotropy, lanthanide trapping and release, or the use of enzymes to amplify the signal from a limited number of antigens. Several reviews are available (Hemmila, 1192, Ozinskas, 1994 and Vo-Dinh et a!., 1993).

Enzyme-Linked Immunosorbent Assays (ELISA) The ELISA method is probably the most commonly used immunoassay format due to its high sensitivity and applicability to a wide range of antigens. The antibody specific for the antigen of interest is immobilized on a surface (Figure 24). The sample is then incubated with the surface-bound antibody to allow the antibody to capture the antigen. The sample is then exposed to a secondary antibody that is covalently bound to an enzyme, typically alkaline phosphatase (AP), horseradish peroxidase, or /3galactosidase. In the example in Figure 24, the enzyme alkaline phosphatase cleaves nonfluorescent umbelliferyl phosphate (UroP), producing the highly fluorescent umbelliferone. The sensitivity of most ELISA's is in the picogram levels.

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163

DNA TECHNOLOGY

The past several years have witnessed astonishing advances in the use of fluorescence to study DNA. The sequencing and analysis of the human genome, undoubtedly the most remarkable milestone in recent history, progressed with the use of fluorescent probes (The International Human Genome Mapping Consortium, 2001 and Venter et al., 2001). In the postgenomic era, fluorescence techniques are expected to continue to be the method of choice in detecting DNA hybridisation, in uncovering single nucleotide polymorphisms (SNPs) and in the study of genes and their interactions using DNA arrays. With the rapid advancements in DNA technology, it is difficult to give a complete description of this extensive area of molecular biology and diagnostics.

6.1

DNA Sequencing

The first report of a practical DNA sequencing method utilized radioactive 32P-containing dideoxynucleotides (ddNTPs) (Sanger et al., 1977). The technique is based on the random termination of DNA polymerization by these ddNTPs along an unknown template DNA. Four separate reactions are done for each base. Analysis of the mixture of oligonucleotides resulting from these reactions by gel electrophoresis reveals the sequence of bases in the unknown sample. Several years later, fluorescent probes replaced the radioactive tracer. Figure 26 is a schematic of a fluorescently labeled ddNTP that is typically used in fluorescence-based DNA sequencing. A single fluorophore may be used for all bases, in which case, reaction and analysis is done on separate lanes for each base. The same is true when a single fluorescent primer is used and non-fluorescent ddNTPs terminate the reactions (Ansorge et al., 1986). A major improvement is the use of four different fluorophores, one for each base. The fluorophores are typically selected so that all can be excited using the 488-nm line of the argon-ion laser. This can be difficult using single fluorophores. Hence, donor-acceptor pairs have been used to accomplish these requirements. One such set of donor-acceptor pairs (Figure 27) has been used with great success (Ju et al., 1995 and 1996). The donors and acceptors were covalently linked within the Forster distance (Ro) using oligonucleotides or DNA-like sugar polymers as spacers (Figure 28). This strategy allowed for the equalization of intensities and similar absorbance at 488 nm. Additionally, the emission spectra are moderately separated so that the bases can be readily identified by measurement at the four emission wavelengths.

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6.2

DNA Hybridization

Detection of DNA hybridisation is extremely important in molecular biology, cell biology, genetics, and forensics. The simplest method to detect DNA hybridisation is the Southern blot, where a spot of unknown DNA on nitrocellulose paper is identified with a complementary probe DNA. Hybridisation is also the basis of the polymerase chain reaction (peR),

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fluorescence in situ hybridisation (FISH) and DNA arrays. Most of the methods to detect DNA hybridisation rely on energy transfer between donorand acceptor-labeled DNA. Several possible methods are shown schematically in Figure 29. d)

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Fluorescence In Situ Hybridization (FISH) FISH is typically used as a tool for spectral karyotyping of chromosomes, in identifying chromosomal aberrations representing diseased states and more recently, assigning the chromosomal location of clusters of genes. Probes for FISH are prepared by fIrst isolating and purifying the individual chromosomes by flow cytometry or by microdissection. The isolated chromosomes are then digested into smaller fragments and amplifIed by PCR. Labeling with fluorophores is accomplished either during the process of PCR or by nick-translation. To be able to identify all 24 human chromosomes, the DNA probes are labelled with different combinations of fluorescent dyes. The number of useful combinations for N number of dyes is 2N-1, so that for the 24 human chromosomes, 5 fluorescent dyes are needed to distinguish each one. In spectral karyotyping (SKY) the chromosome probes were combinatorially labelled with the fluorophores,

5. Fluorescence Probes for Biochemical Systems

167

Cy2, Spectrum Green, Cy3, Texas Red and Cy5 or combinations thereof (Schrock et al., 1996). Specific colors are assigned for each of the 24 chromosomes based on the dye combinations. The probes are hybridised onto metaphase chromosome spreads of the sample cells and imaged using either an interferometric method (Schrock et al., 1996) or interference filters (Speicher et al., 1996). Chromosomal abnormalities such as translocation of chromosome fragments can be easily identified with this method.

DNA Arrays The sequencing of the genomes of several organisms, including humans has led to a shift from the study of single genes to parallel studies of hundreds to thousands of genes at a single time. The ability to accomplish this feat is made possible by the development of the DNA microarrays. These are essentially matrices of microscopic amounts of DNA fragments that are complementary to know genes. There are two types of arrays that are available at present. High-density oligonucleotide arrays are prepared by synthesizing 25-50 oligomers in situ on glass wafers or nylon sheets by photolithography and combinatorial chemistry (Chee et al., 1996). These can contain oligos that are sequence-specific for thousands of genes and even whole genomes. The other type are the spotted microarrays which are composed of pre-synthesized single-stranded or double-stranded DNA's of known genes or open reading frames (ORF's) printed onto glass slides (Schena et al., 1996). In both cases, mRNA from a test and a reference sample are fluorescently labelled with two distinct fluorophores (typically Cy5 and Cy3). In spotted arrays, the two samples are mixed and hybridised onto the array. The relative gene expression level is then quantified based on the relative intensities of the dyes. In high-density microarrays, the reference and sample are hybridised onto separate arrays. The data is then combined and relative gene expression is calculated using bioinformatics software.

REFERENCES Albisson, B., Kubista, M., Norden, 8., and Thulstrup, E. W., 1989, J. Phys. Chem. 93:6646. Ansorge, W., Sproat, B. S., Stegemeann, J., and Schwager, C., 1986, J. Biochem. Biophys. Methods 13:315. Baird, G. S., Zacharias, D. A., and Tsien, R. Y., 2000, Proc. Natl. Acad. Sci. USA 97: 11984. Brochon, J.-C, Wahl, P., Monnuese-Doublet, M.-O., and Olomucki, A., 1977, Biochemistry 16:4594. Bruno, J. Horrocks, W. DeW., and Zauhar, R. J., 1992, Biochemistry 31:7016. Burstein, E.A., Vedenkina, N.S., and Ivkova, M.N., 1974, Photochem. Photobiol. 18:263. Callis, P.R., 1997, Methods Enzymol. 278: 113. Chalfie, M., Tu, Y., Euskirchen, c., Ward, W. W., and Prasher, D. C., 1994, Science 263:802.

168 Chan, A. W. S., Chong, K. Y., Martinovich,

LEAH TOLOSA et al.

c., Simerly, c., and G.

Schatten, 2001, Science

291:309. Chee, M., Yang, R., Hubbell, E., Berno, A., Huang, X. C., Stern, D., Winkler, J., Lockhart, D. J., Morris, M. S., and Fodor, S. P. A., 1996, Science 274:610. Crameri, A., Whitehorn, E. A., Tate, E., and Stemmer, W. P. C., 1996, Nature Biotechnol.

14:315. Cubitt, A. B., Heim, R, Adams, S. R, Boyd, A. E., Gross, L. A., and Tsien, R Y., 1995, Trends Biochem. Soc. 20:448. Czarnik, A. W. (ed.), 1993, Fluorescent Chemosensors for Ion and Molecule Recognition, American Chemical Society, Washington, D. C. Demas, J. N. and DeGraff, B. A., 1994, Design and applications of highly luminescent transition metal complexes. In Topics in Fluorescence Spectroscopy, Volume 4, Probe Design and Chemical Sensing (J. R. Lakowicz, ed.), Plenum Press, New York, pp. 71-

107. Demchenko, A.P., Apell, H.-J., Sturmer, W., and Fedderson, B., 1993, Biophys. Chem.

48:\35. Eftink, M. R, 1990, Methods Biochem. Anal. 35:117. Eftink, M.R, Selvidge, L.A., Callis, P.R, and Rehms, A. A., 1990, J. Phys. Chem. 94:3469. Ehrig, T., O'Kane, D. J., and Pendergast, F. G., 1995, FEBS Lett. 367:163. Fradkov, A. F., Chen, Y., Ding, L., Barsova, E. V., Matz, M.V., and Lukyanov, S. A., 2000, FEBS Lett. 479: 127. Friedman, A. E., Chambron, J.-C, Sauvage, J.-P., Turro, N. J., and Barton, J. K., 1990, J. Am. Chem. Soc. 112:4960. Gafni, A. and Brand, L., 1976, Biochemistry 15:3165. Glazer, A.N. and Stryer, L., 1984, Trends Biochem. Soc.423. Glazer, A. N., Peck, K., and Matheis, R. A., 1990, Proc. Natl. Acad. Sci. USA 87:3851. Guo, X.-Q., Castellano, F. N., Li, L., and Lakowicz, J. R., 1998, Anal. Chem. 70:632. Haugland, R P., 1996, Handbook of Fluorescent Probes and Research Chemicals, Molecular Probes, Inc., Eugene, Oregon. Heim, R. and Tsien, R Y., 1996, Curro Bioi. 6: 178. Hemmila, I. A., 1992, Applications ofFluorescence in Immunoassays, John Wiley & Sons, New York. Holzwarth, A.R, Wendler, J., and Suter, G.W., 1987, Biophys. J. 51: I. Hom, C. and Wimmer, E. A., 2000, Dev. Genes Evol. 210:630. Horrocks, W. DeW., and Sudnick, D. R, 1981, Acc. Chem. Res. 14:384. Ichnose, N., Schwedt, G., Schnepel, F. M., and Adachi, K., 1987, Fluorometric Analysis in Biomedical Chemistry. John Wiley & Sons, New York. Ju, J., Ruan, C., Fuller, C. W., Glazer, A. N., and Mathies, R. A., 1995, Proc. Natl. Acad. Sci. USA 92:4347. Ju, J., Glazer, A. N., and Mathies, R. A., 1996, Nat. Med. 2:246. Kerr, R, Lev-Ram, V., Baird, G., Vincent, P., Tsien, R. Y., Schafer W. R, 2000, Neuron

26:583. Kinnumen, P. K. J., Koiv, A., and Mustonen, P., 1993, Pyrene-Iabeled lipids as fluorescent probes in studies on biomembrandes and membrane models. In Fluorescence Spectroscopy: New Methods and Applications (0. S. Woltbeis, ed.), Springer-Verlag, New York, pp. 159-171. Kobasyashi, Y., Amitani, K., Watatnabe, G., and Miyai, K., 1979, C/in. Chim. Acta 37: 1924. Kotarsky, K., Owman, C., and Olde, B, 2001, Anal. Biochem. 288:209. Kronick, M.N. and Grossman, P.D., 1983, C/in. Chem. 29:1582. Kronick, M.N., 1986, J.lmmun. Methods 92:1-13.

5. Fluorescence Probes for Biochemical Systems

169

Lakowicz,1. R. (ed.), 1994, Topics in Fluorescence Spectrocopy, Volume 4, Probe Design and Chemical Sensing, Plenum Press, New York. Lakowicz,1. R and Gryczinski, I., 1991, Frequency-domain fluorescence Spectroscopy. In Topics in Fluorescence Spectroscopy, Volume I, Techniques (1. R. Lakowicz. ed.), Plenum Press, New York, pp. 293-355. Lamture, J. B. and Wensel, T. G., 1993, Tetrahedron Lett. 34:4141. Lamture, J. B. and Wensel, T. G., 1995, Bioconjug. Chem. 6:88. Li, H. Y., Ng, E. K., Lee, S. M., Kotaka, M., Tsui, S. K., Lee, C. Y., Fung K.P., and Waye M. M., 2001,1. Cell Biochem. 80:293. Li, M. and Selvin, P. R., 1995, J. Am. Chem. Soc. 117:8132. Longworth, 1. W., 1971, Luminescence of polypeptides and proteins. In Excited States of Proteins and Nucleic Acids (R F. Steiner and I. Weinryb, eds.), Plenum Press, New York, pp. 319-484. MacColl, R. and Guard-Friar, D., 1987, Phycobiliproteins, CRC Press, Boca Raton, Florida. Miyawaki, A., Llopis, 1., Heim, R., McCaffery, 1. M., Adams, 1. A., Ikura, M., and Tsien, R. Y., 1997, Nature 388:882. Nguyen, D. c., Keller, R. A., Jett, 1. H., and Martin, 1. C., 1987, Anal. Chem. 59:2158. Oi, V. T., Glazer, A. N., and Stryer, L., 1982,1. Cell Bioi. 93:981. Ormo, M., Cubitt, A.B., Kallio, K., Gross, L. A., Tsien, R. Y., and Remington, S. 1., 1996, Science 273: 1392. Ozinskas, A. 1., 1994, Principles of fluorescence immunoassay. In Topics in Fluorescence Spectroscopy, Volume 4, CRC Press, Boca Raton, Florida. Pendergast, F. G., Meyer, M., Carlson, G. L., Iida, S., and Potter, 1. D., 1983,1. BioI. Chem. 258:7541. Rayner, D.M. and Szabo, A.G., 1978, Can. 1. Chem. 56:743. Rye, H. S., Yue, S., Wemmer, D. E., Quesada, M. A., Haugland, R. P., Mathies, R. A., and Glazer, A. N., 1992, Nucleic Acids Res. 20:2803. Richardson, F. S., 1982, Chem. Rev. 82:541. Robbins, R1., Fleming, G.R., Beddard, C.S., Robinson, G.W., Thistlethwaite, P.J., and Woolfe, G.J., 1980,1. Am. Chem. Soc. 102:6271. Sabbatini, N. and Guardigli, M., 1993, Coord. Chem. Rev. 123:201. Sanger, F., Nicklen, S., and Coulson, A. R., 1977, Proc. Natl. Acad. Sci. USA 74:5463. Schena, M., Shalon, D., Davis, R. W., Brown, P.O., 1996, Science 270:467. SchrOCk, E., du Manoir, S., Veldman, T., Schoell, B., Wienberg, 1., Ferguson-Smith, M. A., Ning, Y., Ledbetter, D. H., Bar-Am, I., Soenksen, D., Garini, Y., and Reid, T., 1996, Science 273:494. Slavik, 1., 1982, Biochim. Biophys. Acta 694: 1. Speicher, M. R., Ballard, S. G., and Ward, D. C., 1996, Nat. Genet. 12:368. Steiner, R. R. and Kubota, Y., 1983, Fluorescent dye-nucleic acid complexes. In Excited States ofBiopolymers (R. F. Steiner. ed.), Plenum Press, New York, pp. 203-254. Stenroos, K., Hurskainen, P., Eriksson, S., Hemmila, I., Blomberg, K., and Lindqvist, C., 1998, Cytokine 10:495. Suh, D. and Chaires, J. B., 1995, Bioorg. Med. Chem. 3:723-728. Szabo, A.G. and Rayner, D. M., 1980, J. Am. Chem. Soc. 102:554. Szmacinski, H., Terpetschnig, E., and Lakowicz, 1. R, 1996, Biophys. Chem. 62: 109. Terpetschnig, E., Szmacinski, H., and Lakowicz, J. R, 1997, Methods Enzymol. 278:295. The International Human Genome Mapping Consortium, 2001, Nature 409:934. Thompson, R B., 1994, Red and near-infrared fluorometry. In Topics in Fluorescence Spectroscopy, Volume 4, Probe Design and Chemical Sensing (J. R. Lakowicz, ed.), Plenum Press, New York, pp. 151-222.

170

LEAH TOLOSA et al.

Tsukamoto T., Hashiguchi, N., Janicki, S.M., Tumbar, T., Belmont A.S., and Spector D. L., 2000, Nat. Cell Bioi. 2:871. Vaccari, S., Benci, S., Peracchi, A., and Mozzarelli, A., 1997, J. Fluoresc. 7:135S. Velick, S. F., 1958, J. Bioi. Chern., 233: 1455. Venter, J. C., et ai, 2001, Science 291:1304-1351. Vo-Dinh, T., Sepaniak, M. J., Griffin, G. D., and Alarie, J. P., 1993, Immunomethods 38:85. Waggoner, A., 1995, Methods Enzymol. 246:363. Wall, M. A., Socolich, M., and Ranganathan, R. 2000, Nat. Struct. Bioi. 7:1133. Weber, G. and Farris, F. 1, 1979, Biochemistry 18:3075. Woltbies, O. S., 1991, Oxygen sensors. In Fiber Optic Chemical Sensors and Biosensors, Vol. II. (0. S. Woltbeis, ed.), CRC Press, Boca Raton, Florida, pp. 15-93. Xiao, G.-S. and Zhou, l-M., 1996, Biochim. Biophys. Acta 1294: 1. Zacharias, D. A., Baird, G. S., and Tsien, R. Y., 2000, Curro Opin. Neurobiol. 10:416.

Chapter 6 Time-Resolved Polarised Fluorescence Studies of Ordered Molecular Systems ANGUS J. BAIN Department of Physics and Astronomy, University College London. Gower Street. London WCIE 6BT, U.K.

1.

INTRODUCTION

This chapter explains how short-pulsed (picosecond/femtosecond) polarised laser excitation can be used in conjunction with time-resolved fluorescence polarisation measurements to determine both the order and full angular motion of probe molecules in an anisotropic medium. The study of ordered molecular systems is an area of wide scientific interest; the presence of order can have a profound influence on intrinsic optical (Tompkin et al., 1990) and photophysical properties (Kenney Wallace et al., 1975; Andrews et al., 1995; Anfinrud et al., 1986), fundamental molecular motions (Anfinrud et al., 1988) and chemical reactivity (Loesch et al., 1991; Kanusi et al., 1996). In nature, highly ordered molecular environments are found within cell membranes (Yu et al., 1996), the local environments of chromophores in proteins (Narazaki et aI., 1997), the mesophases of liquid crystals (Zannoni et al., 1983; Fontana et al., 1986) and self-assembling molecular structures and aggregates (Chen et al., 1991; Fujita et al., 1994). The existence of steady state order in a molecular population implies a bias in the orientational probability distribution for any given directional molecular property in a laboratory-fixed polar coordinate system (Figure 1) according to; p(e,q,) = exp(- E(e,q,)/kT)/Z

An Introduction to lAser Spectroscopy. Second Edition, Edited by Andrews and Demidov, Kluwer Academic/Plenum Publishers, New York, 2002

(1)

171

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172

Figure 1. Polar coordinate system used to specify molecular orientation in the laboratory axis system.

where Z is the orientational partition function and E(9,4» is the energy of a particular molecular orientation (9,4» (McQuarrie, 1983). The polarisation or anisotropy of molecular fluorescence is a direct measure of the degree of order (alignment) present in a photoexcited molecular population. In an initially isotropic fluid an ordered array of excited state molecules can be created by angle-dependent photoselection using an ultrashort (linearly) polarised laser pulse (Beddard et ai., 1981; O'Connor et aI., 1984; Fleming 1986). This gives rise to an anisotropic distribution of absorption and emission transition dipole moments and a consequent difference in the orthogonally polarised components of the emission intensity. In conventional fluorescence experiments, measurements of these components are made using a right-angled geometry, the Z-axis of which is determined by the polarisation vector of the excitation pulse (Figure 2a); wholly equivalent information can also be obtained in a 180 0 or straight-through configuration (Bain et al., 1984, 1984b, 1996) (Figure 2b) as the photoexcitation has cylindrical symmetry about the laboratory Z-axis. The fluorescence anisotropy R(t) is constructed from; R(t) =(l z (t)-l y (t))/(l z (t)+2I y (t)) == (I z( t) - 1x (t))/( 1z( t) + 21 x (t)) = (a ｾＨ＠

t)) / J5

(2)

where (a. 20 (t)) is the degree of cylindrically symmetric molecular alignment remaining at time t following photo-excitation (Bain, et al., 1996). For symmetric molecules with parallel absorption and emission transition moments along the molecular symmetry axis, Eq. 2 takes on a particularly simple form; (3)

6. Time Resolved Polarised Fluorescence Studies

173

where Y20 is the rate of alignment relaxation due to small step isotropic rotational diffusion (Berne et al., 1976; Tao, 1969). Provided the absorption and emission transition dipole moments are parallel, the initial fluorescence anisotropy R(O) is 0.4. For an angle X between the absorption and emission dipole moments the subsequent angular averaging leads to a modification (reduction) in R(O) given by; (Tao, 1969)

1

R( 0) = 2" (3 cos 2 X-I) ( a ｾ＠ ( 0) )/ J5

(a)

sample

(4)

ｾＮＬｙ＠

fluorescence

(b)

z

Figure 2. (a) Conventional right-angled geometry for performing fluorescence anisotropy experiments, the sample excited by an ultrashort (picosecond) laser pulse whose polarisation vector defines the Z-axis of the measurement system. The Z and Y polarised fluorescence signals are detected at 90 0 to the excitation direction. (b) The 1800 geometry for fluorescence anisotropy experiments, fluorescence collected collinear to the excitation direction. Photose1ection is cylindrically symmetric about the polarisation vector of the incident laser pulse and in an isotropic or cylindrically symmetric medium the fluorescence anisotropy is identical to that measured in (a).

Time-resolved fluorescence anisotropy measurements can be made using a variety of techniques including upconversion via sum and difference frequency mixing (Beddard et al., 1981), phase fluorimetery (Lacowicz, 1983) and time-correlated single photon counting (TCSPC) (O'Connor et ai., 1984; Fleming, 1986). The applicability of each technique is dependent upon the excitation source parameters (e.g. repetition rate), fluorescence emission wavelengths and the required time resolution. In TCSPC the fluorescence decay is built up from repeated measurements of the time separation between excitation and single photon emission events. The Iz(t)

174

ANGUS J. BAIN

soooo (a) ｾＬ＠

'tlO= 2.3 os

i\

40000

i

'tn=4.1os

:

! \

::1:1

=

30000

.i \

20000

"ｾ＠

l

C

U

ＡｾＮ＠ 'I instrument!:

10000 response

'I

function

f.

ｏｾＭﾷ＠

-s

ｾＮ｜＠

\,. \.

\

-, '--(' ｾＧＯＮＺＬ＠

I,c(t)

o

Iz(t)

............

ｾ＠

s

-----10

Time/os (b)

YlO=4.26xlO R(O)=0.37

0.4

8 -I S

8;...

If u c

ｾ＠

".!II

0.2

o

s

10

Time/os Figure 3. (a) Z and X polarised components of rhodamine 6G fluorescence decay in ethylene glycol recorded by time-correlated single photon counting, with the instrument response to scattered laser light (b) Corresponding R(t) calculated from the Z and X polarised decay components, exhibiting single exponential decay with a reorientation rate of 4.26 x lOB S·l (rotational lifetime 'tzo= 2.3 ns).

and Ix(t) decays that are used to construct R(t) are recorded by sequential measurements of Z and X polarised photon arrival times (Chandna, 1995; Bryant, 2000). This approach is particularly suited to the study of aligned molecular systems, in that the initial and relaxed fluorescence anisotropies together with their evolution are directly accessible in a moderately short collection time (Chandna, 1995; Bryant, 2000). Also artefacts (arising for

6. Time Resolved Polarised Fluorescence Studies

175

example from reflections or an accidental polarisation bias in the apparatus) can readily be identified and removed. An example of the Z and X polarised fluorescence components recorded using TCSPC are shown in Figure 3a and the resulting anisotropy decay characteristic of single-axis isotropic rotational diffusion in Figure 3b. In the case of an ordered sample, molecular probe motion is more complex and the existence of intrinsic molecular order will result in a non-zero steady state fluorescence anisotropy; in addition, the interaction of polarised light with an ordered ground state population will yield a more complex initial excited state angular distribution and a departure of R(O) from its isotropic value. Moreover the restricted nature of orientational averaging within an ordered environment will necessarily give rise to a more complex evolution for R(t) than that given by Eq. 3 (Bain et al., 2000, 2000a). It is nonetheless possible in such an environment to obtain detailed information on both the full angular motion and equilibrium order of a molecular probe. This is achieved using the technique of variable photoselection which allows the creation of a range of well- characterized non-equilibrium excited state angular distributions (Bain et al., 1996; Bryant et al., 1998; Bain et al., 2000, 2000a). Each of the corresponding fluorescence anisotropy decays that result highlights specific aspects of both steady state molecular order and anisotropic orientational relaxation. As will be seen later, from an analysis of this set of anisotropy decays the ground and excited state equilibrium order of the probe can be determined together with the rates for both 9 and cI> diffusion in the laboratory frame.

2.

POLARISED MOLECULAR ARRAYS AND FLUORESCENCE POLARISATION

In experiments involving optical probes of molecular order and motion in fluid media it is advantageous to adopt a formalism in which the inherent symmetry of the laser-molecule interaction and those of the experimental observables are fully exploited. To this end an expansion of the molecular orientational distribution function in terms of spherical harmonics has been widely used (Tao, 1969; Szabo et al., 1980; Zannoni et al., 1983). In the polar coordinate system of Figure 1, the probability of finding a molecule with an orientation between the polar angles 9 and 9+d9, cI> and cI> +dcl> is given by;

P(9,cI»

=

L:\ CKQ)YKQ( a,q,) KQ

(5)

where the expansion coefficients \ C KQ ) define the moments of the distribution and can be found from Eq. 5 using the orthogonality of spherical harmonics;

ANGUS J. BAIN

176

2nn

(C KQ ) = J ｊｙｾｑＨ･Ｌ＼ｉﾻｐｻｳｩｮ､ｬ＾＠

(6)

o0

The overall probability of finding a molecule in any orientation on the unit sphere is necessarily unity, thus; 27t7t

I ( CKQ)J J ｙｾｑＨ｡Ｌ＼ｉﾻｳｩｮ､ｬ＾＠ KQ 00

= I(CKQ)fuOKOOQO = 1 KQ

(7)

1/

giving a constant population moment (Coo) = fu. The orientational probability can also be expressed in terms of normalised moments, (a KQ ) = (CKQ)/(C oo );

(8)

As will be seen, the multipole moments (C KQ ) are directly related to measurable experimental quantities; moreover their transformation properties upon a change in coordinate system are particularly suited to fluorescence polarisation experiments. Consider an anisotropic distribution of molecular orientations which can be defmed in two space-fixed axis systems (XbYbZ I) and (X2,Y2,Z2); p(a, l)}[I 2( t) - I3( t)] (46)

If 12(t) = 13(t) the cylindrically asymmetric contributions to w!:(a,cI>,t) vanish;

(47)

This can be written as;

where B(I) is given by the ratio of the input intensities; (48)

and can be varied from 1.0 (I2(t) = 13(t) = 0) to -0.5 (I1(t) = 0). Thus, for an isotropic ground state, (ae:o(O»)/.f5 has a tuning range from 0.4 to 0.2.and R(t) yield a single exponential decay for all values of the initial photoselected alignment (Figure 7). This is contrary to the anisotropy decays using single beam excitation where the presence of an alignment decay in the denominator ofR(t) (see Eq. 23) leads to an apparent change in rotational diffusion rate with J3 (Bryant et al., 1998). In the general case of an anisotropic medium the cylindrically symmetric excitation process gives rise to an excited state distribution whose moments are given by;

6. Time Resolved Polarised Fluorescence Studies

189

In the case of an ordered but axially symmetric ground state, R(O) is given by;

(50)

0.4

0.2

ＮＭｾ＠

(1) '10=1.67115, (2) '1O,"I.72os (4) '1O=l.72os, (5) '1O=1.69Ds

-0.2 ' - - - - ' - - - - - - - - - - - - - - - - - - ' 12 4 2 6 8 10

t(ns) Figure 7. Fluorescence anisotropy decays produced by three beam excitation of resorufin in an isotropic solution of ethylene glycol; variation in the relative intensity of vertical to horizontal intensities allows the initial degree of cylindrically symmetric alignment (i.e. R(O» to be tuned between positive and negative values. The resulting anisotropy decays all follow the form of Eq. 2 and (within experimental error) yield identical orientational relaxation times.

The initial alignment can be tuned between;

(51)

190

ANGUS J. BAIN

and;

(afo(ss»)

J5

Ｈ｡ｾｏﾻＩ＠

J5

｟｛ｾ＠

5

Ｋｾ＠

(afo(ss»)

J5

7

］Ｍｾ＠

1-

Ｈ｡ｾｓ＠

_0

ＫｾＨ｡ｧｓｳﾻＩｬ＠

35

40

(52)

(ss»)

J5

This is illustrated in Figure 8, for excitation from a purely aligned ground state.

ＬＮＭｾ｟＠

0.75

I

.!:.O

:(

＼ｾＨｳＬｭ｡ｸﾻＯＭｦ＠

0.50

0.25

3

r:/l

]

·0

&j

-0.25

-0.50

ＧＭｾ＠

-0.2

o

0.2

0.4

Ground State Alignment Figure 8. Illustration of the dependence of the tuning range in 3-beam photoselection upon ground state alignment. The ground state alignment range limits of -0.2 to 0.4 represent the maximum and minimum values that can be present without the inclusion of higher moments.

3.3

Two-Photon Photoselection

With the development of modem solid-state ultrashort pulse laser systems, the application of two-photon fluorescence has become widespread as a tool in photophysics (Lacowicz et al., 1992; 1996 Volkmer et aI., 1997) and the life sciences (Xu et al., 1996; Yu et al., 1996). Photoselection in twophoton absorption shows a considerably sharper angular dependence than in single photon transitions. The angle-dependent excitation probability for two-photon absorption with Z-polarised light of intensity I and frequency v is given by; (53)

6. Time Resolved Polarised Fluorescence Studies

191

where ('j(2) is the two-photon cross-section and 9 is the angle between Z and the transition dipole moment. Expanding Eq. 53 in terms of spherical harmonics yields;

If the excitation polarisation is rotated through an angle p (as in Figure 4) the transition probability in the laboratory frame becomes;

Two-photon excitation results in the creation of more highly polarised arrays than in the case of single-photon excitation (Bain et al., 1984a); this can be seen from Figure 9, which compares the degrees of initial alignment that can be prepared by single- and two-photon excitation from an isotropic ground state. These predictions are born out experimentally as can be seen from a comparison of the single- and two-photon fluorescence p = 0° anisotropy decays obtained for rhodamine 60 in ethylene glycol (Figure 10). The advantage of using two-photon excitation in conjunction with the single-photon measurements is twofold; firstly the interaction between the radiation and the polarised molecular array now allows the direct contribution of higher moments of the ground state distribution, e.g. (u 60) , to the initial fluorescence anisotropy; (Durbin et al., 1984; Bain et al., 1985c) this allows a more accurate picture of the initial molecular order to be obtained. From the Y4Q(9,cp) term in Eq. 55, it can be seen that the two-photon excitation leads to the direct creation of (U40) terms in the excited state. From simple symmetry considerations, all moments of rank K> 2 cannot contribute to single-photon fluorescence. (Bain et al., 1985, 2000) However in an ordered medium the anisotropy in the diffusion dynamics (cross-relaxation) could in principle permit the contribution of higher moments. The degree of any such cross-relaxation and the degree to which it may contribute depends on the asymmetry of the medium, but moreover on the initial values of the higher alignment moments created following photoselection (Bain et al., 2000). A comparison between singleand two-photon anisotropy decays will help to address such issues.

192

ANGUS J. BAIN ｑＶｲＭｾ＠

Tho8dm

5a

t""""

Q.3

Q

....ｾ＠

= ｾ＠

ｾ＠

,

,,

ｯｲＭｾ＠

ｾ＠

Q ".

ｾｌＭ

_ _ｾ＠

o

_ _ _ _Ｍｾ＠

Q.3

ｻｾＨｏＩｽＡ＠ ｾ＠

e 02 e 1:1

= < .... ..= ｾ＠

ｾ＠ Tho ...... Fwi4I'im

ｾ＠

ｾ＠

ｾ＠

Ql

ｾ＠

Q

0 0

.., ,

,

, ,,

,,

, ,,

,,

,

////l

...'

s.ve ...... ｾＱ＠

...

00

J)

p Figure 9. Variation in the initial degrees of cylindrically symmetric and asymmetric alignment produced in single- and two photon excitation from an isotropic medium.

193

6. Time Resolved Polarised Fluorescence Studies

Two Photon Umlt, R(O)=O.57

0.6

Two photon excitation R(O)=O.52, '20= 2.72 ns

Single Photon limit, R(O)ooO.4

Single photon excitation R(O): 0.35, '20= 2.n ns 0.2

4

8

t(ns) Figure 10. Single- and two-photon excited fluorescence anisotropy decays recorded for rhodamine 6G in ethylene glycol; the initial anisotropies for both processes are close to the theoretical maxima (dashed lines) for excitation from an isotropic ground state.

ｆｌｕｏｒ｜ｅｎｾ＠ ＡＱ ＧＱｲ

ｾｉｦＬＺｮ［ＭＱ＠

_ -

-

·x(t) ANALYSING POLARJSER

x

SAMPLE

nO

Figure II. Single-beam photoselection in a uniaxial medium such as a nematic liquid crystal. The liquid crystal director defines the laboratory Z axis. The Z and X electric field components of the excitation pulse experience the extraordinary (nJ and ordinary (no) refractive indices respectively.

ANGUS 1. BAIN

194

3.4

Photoselection in Highly Anisotropic Media

In highly ordered materials, sample birefringence can influence both the photoselection process and the relative intensities of the fluorescence anisotropy components. This is particularly noticeable in the study of nematically aligned liquid crystal mesophases such as in 5-cyanobiphenyl (5CB) where the difference in the ordinary (n.,) and extraordinary (Ile) refractive indices is significant (- 0.4). Consider single-beam photoselection in a nematic sample, as shown in Figure 11. Polarisation angles other than 0° or 90° will give rise to both Z and X electric field components that experience the refractive indices Ile and Ilo respectively. After passage through a path length / a phase shift 0 is introduced and the excitation probability is given by; (Bain et a/., 2002)

Yoo(e,cj)) + wabs(e,cj),t)=BI(t)I;:iJ ｾ＠

(3cos 2 f3 -1)

.J5

Y20 (e,cj))

I

ＫｃｾＩＲ＠

sin2f3(Y22(e,cj))+Y22(e,cj)))

+ 2sinf3 ｾｦＳｃｏｓ＠

(YZ_1(e,cj)) - Y21 (e,cj))) (56)

where 6=21t(Ile-n.,)//A. and A. is the excitation wavelength. Given axial symmetry for the ground state probe distribution, the moments of the excited state distribution are;

Yoo(e,q,)+

(3cos 2 J3 -1)

.J5

Y20 (e,q,)

1

ＨｃｾｑｏＬｊＳＩ＠

=

(i.)"l sin J3(y (e,q,)+ Y (e,q,)) 10

AL(KQI + K'

2

22

22

+ 2 sin J3 cos J3 coso (y

J10

2-1

(e.l.) _y 21 (e,'I'.1.)\'J ,'I' (57)

With an excitation wavelength of 600 run, over a pathlength of 100 ).lID, 0 will execute approximately 70 full cycles. Calculation of the effect of the induced ellipticity on the excited state distribution would necessitate the

6. Time Resolved Polarised Fluorescence Studies

195

integration of Eq. 57 over I. However in practice this is not necessary; conservation of symmetry ensures that the elliptical contribution to the transition probability gives rise to the creation of excited state moments with Q = ±l; from Eqs. 18 and 21 these make no direct contribution to the anisotropy. This condition is relaxed if the analysing polariser settings are rotated such that an admixture of Z and X polariation components are recorded (Bain et al., 2000a). Misalignment of the detection optics should therefore be avoided to prevent unnecessary complication of the measured anisotropy decay. Sample birefringence does however influence the emission process, together with the transmission of the Z and X polarised components at the sample boundary (Penchev et aI., 1981, 1984). Local field effects (Durbin et al., 1984) modify the intensities of the Z and X polarised components of the emission according to; I z( observed) oc Iz( actual)( n; + 2)

(58)

Ix (observed) oc ｉｸＨ｡｣ｴｵｬＩｮｾ＠

(59)

+2)

Reflection losses at the sample boundary cause differential attenuation of the Z and X polarised components of fluorescence. The transmission coefficients are determined from the Fresnel reflection coefficients; (Lipson et al., 1981) Te = 4nengj(ne + n g)2 To

= 4n Ongj(no

+ngr

(60) (61)

where ng is the refractive index of the adjacent medium at the sample boundary (in the case of liquid crystalline cells a glass or quartz cover slip). The total modification to the Z and X polarised intensities is then; I.{observed)oclz(actual)(n; +2)4nengj(ne +ngf,

(62)

IJobserved) oc I x (actual)( ｮｾ＠

(63)

+ 2)4n ongj(no + n g)2

and the measured fluorescence anisotropy is altered by; R(t)

=

(kI(z, t) - I(x, t}) (kI(z, t) + 2I(x, t})

Here the parameter k is defined through;

(64)

196

ANGUS J. BAIN

(65)

In highly birefringent samples such as nematically aligned 5CB the maximum value of k is - 1.13 (Bryant, 2000), reducing to unity as the sample temperature is increased towards that of the nematic-isotropic phase transition.

4.

RELAXATION DYNAMICS IN ANISOTROPIC MEDIA

For an anisotropic medium the Debye equation must be modified to account for the perturbation brought on by the imposition of order. The general form of the Debye equation in the presence of an applied potential of arbitrary symmetry is as follows;

Here H' denotes the (time-independent) perturbation to the diffusion brought about by the applied potential (obviously DV2» H') and should possess the same symmetry as the equilibrium state of the system. In terms of the moments of pex( 9, cj), t} this becomes;

As (KQIH'I K'Q) is small a first order perturbation solution to Eq. 67 is valid;

6. Time Resolved Polarised Fluorescence Studies

Ｈｃｾｑ＠

t)) -( ｃｾｑＨｳＩ＠

+L

={( ｃｾｑＨｏＩ＠

I[{( ｃｾＧｑＨｏＩ＠

ｾＺ＠

0

where Y KQ

-

-( ｃｾｑＧＨｳＩｽ＠

ＨｃｾｑｳＩｽ＠

197

exp{ -Y KQt)

exp{ -Y K'Q.t')exp{ -Y KQ( t' -t))]dt'

(68)

x (KQIH'I K'Q)

= -(KQIDV2 + H'I KQ).

Without specific knowledge of H',

symmetry considerations can be used to simplify Eq. 68. For example in an axially symmetric system, cross-relaxation between moments of different axial symmetry is forbidden (Bain et al., 2000), yielding;

Ｈｃｾｑ＠

t)) -( ｃｾｑＨｳＩ＠

+

== [( ｃｾｑＨｏＩ＠

L 1[[ ＨｃｾＧｑｏＩ＠ K'"K 0

-( ｃｾｑＨｳＩ｝＠

-( ｃｾＧｑＨｳＩ｝＠

exp{ -Y KQ t)

exp{ -Y K'Qt') exp{ -Y KQ( t' -t))]dt'

x (KQIH'I K'Q) (69)

The conservation of orientational probability, Eq. 5, requires that ＨｃｾＩ＠ remain time-invariant; no such restriction is placed on the other moments. However, to a first approximation, orientational relaxation will be linear and diagonal (i.e. there is no cross-relaxation between the moments of the excited state array) so that;

Q == 0, Ｈｃｾｯ＠ Q

* 0, Ｈｃｾｑ＠

t)) -

ＨｃｾｯｳＩ＠

t)) =ＨｃｾｑｏＩ･ｸｰｻ＠

=[( ｃｾｯＨ＠

0)) - ＨｃｾｯｳＩ｝･ｸｰ＠

- YKOt)

},

(70)

- YKQt)

where; Y KQ == -(KQIDV2 + H'I KQ)

(71)

in an axially symmetric medium. With the assumption of axial symmetry Dxx == Dyy , under these circumstances the YKQ decay rates are given by;

H' must possess the same with Dxx = Dyy == D.L and Dzz == DII • symmetry as the medium; assuming axial symmetry and even parity we can

ANGUS J. BAIN

198 expand H' in terms of spherical tensor operators H ｾ＠ projection M=O as follows;

of rank K and

(73) Truncating the expansion at L=2 and applying the Wigner-Eckart theorem we have (Bain et ai., 2000, 2000a);

For 120 and 122 this yields;

The effect of the isotropic term (H ｾＩ＠ in the expansion of H' is equal for both alignment decays. Without explicit knowledge of the form of the either the expansion coefficient (a 20) or the reduced matrix element (2\1H 2112) , the effect of the quadruopolar (L = 2) term on 120 and 122 can be seen to be equal and opposite. From these simple symmetry considerations it is clear that cI> and e diffusive motions will be unequally influenced by imposition of order. The effect of a pertubation of quadrupolar symmetry as above is to cause a relative constraint to diffusion with respect to either the e or cI> coordinate. In addition the presence of steady state order will, in highly ordered materials such as liquid crystalline mesophases, lead to highly anisotropic friction D.L DII (Dozov et ai., 1989; Ohta et at., 1995).

*"

An important goal of any experimental investigation will therefore be the determination of the relationship between the variation in 120 and 122 with the sign and magnitude of the steady-state alignment. This requires the creation of both cylindrically symmetric and asymmetric molecular arrays using variable photoselection techniques.

199

6. Time Resolved Polarised Fluorescence Studies

5.

MODERATE MOLECULAR ORDERING: JETALIGNED MOLECULES

Molecular order can be imposed in a fluid medium by the imposition of an external force such as an applied electric field (Weber, 1965) or by the shear forces resulting from confined fluid flow (Heller et ai., 1972). The production of significant molecular alignment by these approaches is in general restricted to macromolecules and systems exhibiting a large collective molecular response, such as liquid crystals. The observation of possible molecular alignment in a high pressure free fluid jet was first reported by McCaffery and coworkers (Kenyon et al., 1991, 1991a) who observed a spatial variation in the steady-state fluorescence anisotropy from rhodamine 6G molecules in an ethylene glycol jet produced from a precision sapphire nozzle. Recent time-resolved fluorescence studies of both jetaligned rhodamine 6G and resorufin (Bain et al., 1996, 2000a) have shown that, irrespective of the initial photoselection, the steady state ordering of the emerging molecules corresponds to a marked perpendicular (negative) alignment to the flow ､ｩｲ･｣ｴｯｮＨｬｾｳﾻＩＯｊＵ＠ ｾ＠ -5% (Figure 12). This degree of this alignment is between one to three orders of magnitude greater than that attainable for similar species under favourable Couette or Kerr alignment ·conditions.

5.0

Rhodamine 6G Jet, 18D e 30psi

.. -

2.5

..... •ｾ＠'-' =:'"'"

.-

0

,

. --- .. --- -_

---- ---

-2.5

-5.0 0

2

4

6

8

Downstream Position (mm) Figure 12. Variation in Rss with distance from the nozzle exit.for a rhodamine 6G-ethylene glycol jet. In the immediate vicinity of the nozzle exit, where the molecular ordering corresponds to a net negative alignment of solute molecules perpendicular to the flow axis, is an isotropic region (Rss ... 0); further downstream a small positive degree of steady state alignment is observed together with evidence of cylindrical asymmetry.

200

ANGUS 1. RAIN 0.4

(a) Resorufin Nome Exit Region

0.3 0.2 0.1

ｾ＠

0 -0.1 ｾ＠

-0.2

= 4.6 ± 0.2"1.

-0.3 -0.4 0

2

4

6

8

10

t(ns)

0.4

(b)Resorufm, Isotropic Reference Cell 0.3 0.2 0.1

ｾ＠

0 -0.1

Rss=O -0.2 -0.3 -0.4 0

2

4

6

8

10

t(ns)

Figure J3. (a). R(t, (3) decays for resorufin in the nozzle exit region of the ethylene glycol jet; (b) R(t, (3) decays for an isotropic solution ofresorufin in ethylene glycol.

The study of molecular motion and order in a fluid jet is readily addressed via single beam photoselection (Figure 4). A comparison between the R(t,P) decays obtained for jet-aligned rhodamine 6G and an isotropic reference sample can be made with reference to Figure 13. In the nozzle exit region, irrespective of the initial photoselection, the anisotropy is seen to decay to a constant value in the region of ｾ＠ -4% to -6%. In agreement with the analysis of section 4, the alignment dynamics in the jet are found to be linear. The variation in Y20 and Y22 with the degree of excited state equilibrium alignment is shown in Table 1; it is notable that the e diffusion rate (Y20) for both probe molecules remains constant in both the nozzle exit and isotropic regions. The imposition and subsequent removal of a negative equilibrium alignment is seen to affect solely 4> motion with Y22 approximately 0.86Y20. Models of diffusion in a cylindrically symmetric potential can yield the correct ordering of Y20 and Y22 in the nozzle exit

201

6. Time Resolved Polarised Fluorescence Studies

region (Dozov et al., 1980) but fail to account for the observed invariance in the e diffusion rate in the transition to the isotropic region of the jet. The dominant contribution to the anisotropy in the alignment relaxation rates can be attributed to anisotropic friction, therefore; (77) Similar orientational relaxation dynamics are observed for both probe molecules in spite of the differences in their intrinsic diffusion rates (Y20 rhodamine 6G ::= 0.56y20 resorufm). In the nozzle exit region OJ, ::= 0.8 D1.' A steady rise in OJ, is observed with increasing downstream position whilst (to within experimental error) values for D1. remain constant. At the bottom of the jet, where the side lobes converge, a positive steady-state alignment is observed; here the diffusion anisotropy is seen to reverse with OJ, ::= 1.2D1.' Although in both regions of the jet the degree of steady-state order is not large, the effect on the diffusion dynamics is marked and in both instances motion is greatest in the coordinate perpendicular to that of the preferred alignment. As will be seen, in systems exhibiting a greater degree of steady-state alignment such as liquid crystals, the anisotopy in e and cp diffusion is even more pronounced. Table 1. Variation in the alignment relaxation rates at 30 psi for rhodamine 60 and resorufin in the three regions of a 100 J.1m ethylene glycol jet, together with the e and ｾ＠ relaxation rates (011 and 0.L) and the steady state anisotropy Rss. Rhodamine 6G Nozzle Exit Isotropic Region Bottom of Jet Resorufin Nozzle Exit Isotropic Region Bottom of Jet

6.

Iza x 10' {s·ll 0.36 0.36 0.35

In x 10' {s·ll 0.31 0.36 0.38

D.Lxl08{S·I} 0.6 0.6 0.583

0.65 0.64 0.65

0.56 0.65 0.69

1.083 1.067 1.067

Rss% ｾＬｸｬＰＸｻｓﾷｉｽ＠

0.475 0.6 0.658

-4.2 0 1.6

0.858 1.023 1.117

-4.6 0 0.5

STRONG ORDERING: PROBE MOTION AND ORDER IN NEMATIC LIQUID CRYSTALS

In thermotropic liquid crystals, local orientational and positional order persists over a finite dimension (usually on the order of a few molecular lengths) for a given temperature range. This local ordering gives rise to what are termed domains in which the structure of the fluid, although globally isotropic, is nonetheless crystalline. By application of an external influence, such as contact with an aligned surface or the application of a dc electric field, this local order can be made to persist over macroscopic dimensions. Liquid crystal mesophases have analogues in nature where

202

ANGUS J. BAIN

similar molecular organisation can be found in cell membranes. The nematic mesophase of the widely studied liquid crystal 5-cyanobiphenyl (5CB) gives rise to a cylindrically symmetric arrangement of the 5CB units Alignment measurements have been made about the nematic director using a variety of techniques: Raman scattering (Jen et ai., 1977), NMR (Luckhurst et aI., 1972; Nordio et aI., 1979) and two-photon fluorescence dichroism (Durbin et ai., 1984). In 5CB, retention of a strong degree of molecular order is seen to persist until a few degrees prior to the onset of the nematic-isotropic phase transition temperature (Vertogen et ai., 1988). Recent work has shown that it is possible to incorporate visible fluorescent probes within nematically aligned 5CB (Dean, 1997; Bain et ai., 1998, 1999). In a similar manner to the jet studies a variable (single-photon) excitation polarisation is used to create a range of initial non-equilibrium degrees of excited state alignment with respect to the nematic director (Figure 14). The departure from 'isotropic' relaxation dynamics is found to be considerable. A particularly extreme case is given by the motion of Oxazine 4 (Figure 15). Correcting for the effects of sample birefringence and the depolarisation of fluorescence (Bryant, 2000) it is possible to determine the equilibrium moments, (0'20( ss)) and (0'40( S8)) , of the Oxazine 4 (ground state) orientational distribution function together with the excited state alignment relaxation rates (Bryant, 2000), as displayed in Figure 16.

n.

Figure J4. Single beam photoselection geometry for the study of probe fluorescence in the nematic phase of 5CB, with the excitation polarisation angle varied with respect to the nematic director ii which defines the laboratory Z axis.

203

6. Time Resolved Polarised Fluorescence Studies 0.4 ｾ＠ -. 0.2 c:q. .... .

ct

Oxazine4 in isotropic seB

0

35.3

Rss=O

0 45 54.7

-0.2 90 0.5

ｾ＠ .. .. .

-.

c:q.

,".

. ＮＧｾ＠

".

.

..

Rss=40%

.... '-' 0:::

Oxazine4 in nematic SeB

0.2 0.1

0

2

468

10

12

time (os)

Figure J5. R(t,l3) decays for Oxazine 4 in the isotropic and nematic phases of SeB.

A striking feature in the relaxation dynamics is that, in the approach to the nematic-isotropic phase transition temperature (TNI), while 9 diffusion shows the expected Arrhenius temperature-dependence for liquid crystal diffusion (Ariconi et al., 1987), motion in the c/> coordinate reduces as the system becomes less ordered (Bain et al., 1998, 1999). This is at variance with models of rotational diffusion in a cylindrically symmetric aligning potential which have been used to analyse steady-state fluorescence data in anisotropic media (Dozov et al., 1980). From the values of (aro(ss») and (a:(ss») obtained from the initial fluorescence anisotropy data it is clear that higher moments are necessarily present in the ground state distribution function (Bryant, 2000). From the requirement that the ground state distribution must be physically acceptable (i.e. P(9,c/» ｾ＠ 0) it is possible to estimate the values of the 'missing' moments and to arrive at full angular distribution function for the molecular probe (Durbin et al., 1984). For Oxazine 4 this yields an approximately Gaussian angular distribution centred about a peak polar angle (9 MAX) of - 35°; 9 MAX shows an small (approximate) linear increase with temperature as can be seen from Figure 17. The full width at half maximum of the distribution behaves in a similar fashion with an increase from 33° to 39° over the nematic temperature range. From this it follows that the Oxazine 4 molecules are constrained to move within a 'cone' which retains its structure and orientation to within 0.2

204

ANGUS J. BAIN

°C of TNI' The closeness of SMAX to the polar angle of 38° for the 5CB alkyl tails (Durbin et al., 1984) indicates that these are primarily responsible for anisotropy in the local environment of the Oxazine 4 probe. Local cylindrical symmetry is clearly lost and the decrease in the q, diffusion rate of Oxazine 4 may well be due to increased friction arising from an increasing entanglement of the alkyl tails as the host order decreases (Fontana et al., 1986) .

.... _--------- ......

15

ｔｎｉｾ＠ ｾＲＰ＠

-

!

.....

"

.....

"e_ ____ ,

10

ｾ＠

'e,

.

--

ｯｾＭ＠

--26

---

---

---

---

--- ---

28

-.

.-- --..-. 32

30

Temperature ·C 1.0

ｲＭｾ｟Ｌ＠

T

ｾ＠

.-- --- --- .- --- --- --- ---.-- --. ...

0.5

NI

:

,

:

ｯｾＭ＠

(..:;(0)

-0.5

.----- ....------

-1.0

ＧＭｾＡ＠

26

28

---&...

30

- _ ..... ....6.

A

32

Temperature ·C

Figure 16. Temperature variation in symmetric and asymmetric relaxation times for Oxazine 4 in nematic 5eB.

It is clear that the direct measurement of diffusion dynamics of a probe in the plane orthogonal to the director yields new information over that afforded by conventional fluorescence measurements. The marked difference between S and q, motions across the nematic range shows the inapplicability of standard theories for diffusion in liquid crystals (Ariconi et al., 1987; Dozov et al., 1980, 1989a). The results of this work highlight the importance of measuring the full range of angular observables that are accessible in time-resolved fluorescence anisotropy.

20S

6. Time Resolved Polarised Fluorescence Studies ＵＰｾＭＮ＠

Variation in ｾ＠

for Oxazine 4 in 5CB

45

40

ｾ＠

r:rJ

35 ｾ＠

=

10.4 + 0.877 T

30

25

26

28

30

32

Temperature "C Figure 17. Temperature dependence of the peak angle (SMAX) of the ground state orientational distribution function for Oxazine 4 in the nematic phase of 5CB.

7.

PROBE MOTION IN ISOTROPIC BUT LOCALLY ORDERED MEDIA: LIQUID CRYSTALS AND PROTEINS

In the time-resolved fluorescence anisotropy study of Oxazine 4 diffusion in nematic SCB, the probe molecules were seen to retain their alignment in the laboratory axis system to within approximately 0.2°C of TN1 . Above this temperature the loss of global ordering gives rise to a zero steady-state fluorescence anisotropy, as can be seen in Figure IS. However the fluorescence anisotropy does not yield a single exponential decay as might be expected for the diffusion of a symmetric rotor in an isotropic fluid; instead a two component anisotropy of the following form is observed; (78) This corresponds to a 'fast' (t f :::::: 2.S ns) and essentially temperatureinvariant restricted rotational diffusion, (B =1= 0), mediated in tum by a slower overall rotational diffusive process. This latter decay component exhibits the characteristic Landau-DeGennes behaviour for the temperaturedependence of the correlation time for pseudo-domain rotation (see Figure 18) as TNI is approached from the isotropic phase; (Wong et al., 1974; Deeg et al., 1990, 1990a)

206

ANGUS J. BAIN

't

,,(T)

•

(79)

oc:--

T-T*

Fast and slow isotropic phase rotational correlation times have been identified in transient grating studies of neat SCB and MBBA (Deeg et aI., 1990, 1990a; Stankus et al., 1993); these show a similar temperature dependence to that observed for Oxazine 4. The essential temperatureindependence of the fast correlation time was attributed to relaxation of the local structure within the pseudo-domain. For Oxazine 4, the convergence of the two lifetimes at 8SoC to yield a single exponential decay for R(t) indicates the fmal breakdown of the pseudo-domain or 'cage' structure of SCB, and the onset of truly isotropic rotational diffusion. In addition to the determination of Oxazine 4 and SCB pseudo-domain diffusion times, the anisotropy yields information on the local order of the fluorescent probe in terms of restricted diffusion within a cone of semi-angle a., an approach commonly used in biophysics for the study of fluorescent probe motion within large molecular assemblies such as proteins or membranes (Kinosata et al., 1977; Nishimoto et al., 1998) using the result;

ｾａＭｂ＠

A+B

1 ="2[ cos 0.(1 + cos a.)]

(80)

15

(ns) r 5 f

ｯｾＭ＠

30

40

50

60

70

80

Temperature (OC) Figure 18. Temperature variation in the •fast' and 'slow' rotational relaxation times for Oxazine 4 in the isotropic phase of 5CB

The variation in the Oxazine 4 cone angle with temperature above the nematic istropic phase transition is shown in Figure 19. Up to - 48°C above TNh a. shows an approximately linear increase with temperature; this is followed by a sudden collapse over 4°C with a recovery of isotropic rotational diffusion at S2.9°C above TNI'

6. Time Resolved Polarised Fluorescence Studies

207

Variation in Cone Angle with Temperature Ｍｾ

90

I I I I I I I I I

Isotropic Distribution

:!o

U

50 ｾａ＾＠

......

ｾＭ＠

o

10

./

7'

-

40

Figure 19. Temperature variation in the Oxazine 4 cone angle within the isotropic phase of 5eB

The behaviour of Oxazine 4 within a globally isotropic but locally structured fluid is mirrored in the dynamics of fluorescent chromophores in proteins. It is possible to use anisotropy decays to determine structural changes in proteins following the attachment of physiologically relevant ligands, a recent example provided by the fluorescence anisotropy of the single tryptophan residue (Trp-214) within Human Serum Albumin (HSA) a principal circulatory protein. Figure 20 shows the anisotropy decays recorded for Trp-214 in the bare (defatted) protein and following the attachment of oleic acid (HSA oleate). The fluorescence anisotropy from Trp-214 is sensitive both to the overall diffusion of the protein and to the internal motion of Trp-214. In both instances there is a clear bi-exponential decay in R(t) corresponding to fast (- 200 ps) restricted rotational motion of Trp-214 coupled with the significantly slower (by two orders of magnitude) rotational diffusion of HSA. Binding of oleic acid to HSA causes significant changes to R(t); the diffusion time for HSA increases by 50% whilst the Trp-214 cone angle is seen to decrease from 21° to 16° with an apparent decrease in local viscosity. These results point to a clear conformational change in HSA following ligand binding. The intriguing feature here is that whilst the local order of Trp-2 14 increases (a. decreases) the orientational diffusion time reduces. This parallels the previously unobserved behaviour of 4>-diffusion for probe motion in the nematic phase of 5CB. (Bain et al., 1998, 1999). In an isotropic fluid it is however impossible to distinguish between e and 4> motion within the molecular frame. An important goal is therefore to investigate aligned arrays of systems such as HSA in which it would then be

208

ANGUSJ BAIN

possible to effect orientational photoselection of fluorescent probes within the molecular frame. 0.25 t)=182ps

ｾ＠

USA Oleate

-.

t z=34ns 0.15

Defatted USA

0.10

t z=22ns 0

8

4

12

t (ns)

Defatted HSA

USA Oleate

Figure 20. Time-resolved fluorescence polarisation of Trp-2 14 in HSA. The initial fast decay - 200ps is due to local Trp motion; the slower ns component arises from the overall rotation of HSA. Binding of oleic acid causes a contraction in the trptophan pocket of 5° and a 50% decrease in the HSA diffusion rate.

8.

CONCLUSIONS

This Chapter has shown that time-resolved fluorescence polarisation is a powerful technique for investigating molecular motion and order. It is clear that the development of photos election techniques coupled with the ability to produce aligned molecular arrays now allows a significantly greater degree of information to be obtained on molecular structure and motion than has previously been possible.

ACKNOWLEDGEMENTS I would like to thank my collaborators for their contribution to aspects of the work described in this Chapter: Pamela Chandna and Gillian Butcher for the work on jet-aligned molecular dynamics, Jason Bryant for the

6. Time Resolved Polarised Fluorescence Studies

209

development of the three-pulse photoselection technique and with Richard Dean for the studies of probe motion in liquid crystals; Daven Armoogum, Eugenio Monge and Bojan Obradovic for the two-photon fluorescence anisotropy measurements. The study of trp-214 in HSA was undertaken in collaboration with Paul O'Shea and Neil Chadbom. Financial support of this work by the Engineering and Physical Sciences Research Council is gratefully acknowledged.

REFERENCES Altkom, R., and Zare, R.N., 1984, Ann. Rev. Phys. Chem. 35:265. Ameloot, M., Hendrickx, H., Herreman, W., Pottel, H., Vancauwelaert, F., and Vandermeer W., 1984, Biophys. J. 46:525. Andrews, D.L., and Juzeliunas, G., 1991, J. Chem. Phys. 95:5513. Anfinrud, P.A., and Struve W.S. 1987, J. Chem. Phys. 87:4256. Anfinrud, P.A., Hart, D.E., Hedstrom, 1.F., and Struve, W.S., 1986, J. Phys. Chem. 90:3116. Atkins, P.W., 1983, Molecular Quantum Mechanics. Oxford University Press, Oxford. Arcioni, A., Bertinelli, F., Tarroni, R., and Zannoni, C., 1987, Mol. Phys. 61: 1161. Bain, A.J., and McCaffery, A.1., 1984, Chem. Phys. Lett. 105:477. Bain, A.J., and McCaffery, A.1., 1984a, Chem. Phys. Lett. 108:275. Bain, A.1., and McCaffery, A.1., 1984b, J. Chem. Phys 80:5883. Bain, A.1., and McCaffery, A.1., 1985, J. Chem. Phys 83:2627. Bain, A.1., and McCaffery, A.J., 1985a, J. Chem. Phys 83:2632. Bain, A.J., and McCaffery, A.1., 1985b, J. Chem. Phys 83:2641. Bain, A.1., Bryant, 1., and Dean, R.J., 1998, Opt. Soc. Amer. Tech. Dig. Ser.7:98. Bain, A.1., and Bryant, 1., 1999, Proceedings of QELS 1999, Opt. Soc. Amer. Tech. Dig. Ser. pp.185-186 Bain, A.1., Chandna, P., and Bryant, 1., 2000, J. Chem. Phys. 112: 10418. Bain, A.1., Chandna, P., and Bryant, 1., 2000a, J. Chem. Phys. 112: I 0435. Bain, A.1., Bryant, J and Monge E., 2002, (in preparation). Beddard, G.S., Doust, T., and Porter, G., 1981, Chem. Phys. 61:17. Berne, B., and Pecora, R., 1976, Dynamic Light Scattering. John Wiley, New York Brink, D.M., and Satchler, G.R., 1993, Angular Momentum. Clarendon Press, Oxford Bryant, 1., and Bain, A.1., 1998, Chem. Phys Lett. 286:121. Bryant, 1., 2000, PhD Thesis. University of Essex, England Buckingham, A. D., 1976, In Molecular Electro-Optics, part I, (C. T. O'Konski, ed.) Marcel Dekker, New York Chadbom, N., Bryant, 1., Bain, A.1., and O'Shea, P., 1999, Biophys. J. 76:2198. Chandna. P., 1995, PhD Thesis. University of Essex, England. Chen, S.H., and Frank, C.W., 1991, Langmuir 7:1719. Chu, K.-S., arId Moroi, D.S., 1975, J. de Phys. 36:99. Debye, P., 1929, Polar Molecules. Dover, New York. Dean, R. J., 1997 PhD Thesis. University of Essex, England. Deeg, F.W., and Fayer, M.D., 1990, Chem. Phys. Lett. 167:527. Deeg, F.W., Greenfield, S.R., Stankus, J.1., Newell, V.1., and Fayer, M.D., I 990a, J. Chem. Phys. 93:3503. Dozov, LN., and Penchev, LL, 1980, J. Lumin. 22:69. Dozov, LN., and Kirov, N., 1989, J. Chem. Phys. 90: I 099. Dozov, LN., Kirov, N., Fontana, M.P., Manfredi, M., Rossi, B. and Cywinski, R., I 989a, Liquid Cryst. 4:241 (and references therein). Durbin, S.D., and Shen, Y.R., 1984, Phys Rev. A 30:1419. Fleming, G.R., 1986, Chemical Applications of Utrafast Spectroscopy. Clarendon Press, Oxford. Fontana, M.P., Rosi, B., and Kirov, N., 1986, Phys. Rev. Lett. 56: 1708.

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Fujita, K., Kimura, S., and Imanishi, Y., 1994, Biochirn. Biophys. Acta-Biornernbranes 1195: 157. Heller, W., Tabiban, R., Nakagaki, M., and Papazian, L., 1970, J. Chern. Phys. 52:4294. Jen, S., Clark, N.A., Pershan, P.S., and Priestly, E.B., 1977, J. Chern. Phys. 66:4635. Kansui, H., Hiraoka, S., and Kunieda, T., 1996, J. Arner. Chern. Soc. 118:5346. Kenney Wallace, G.A., Flint, J.H., and Wallace, S.C., 1975, Chern. Phys. Lett. 32:71. Kenyon, A, McCaffery, A.J., and Quintella, C.M., 1991, Mol. Phys. 72:965. Kenyon, A, McCaffery, A.J., Quintella, C.M., and Winkel, J.F., 1991, Mol. Phys. 74:871. Kinosata, K., Jr., Kawato, S., and Ikegami, A.,1977, Biophys.. J. 20:289. Lacowicz, J.R., 1986, Principles offluorescence spectroscopy. Plenum, New York. Lacowicz, J.R., and Gryczynski, 1., 1992, Biophys. Chern 45: 1. Lakowicz, J.R., Gryczynski, 1., Malik H., and Gryczynski, Z., J. Phys . Chern. 100: 19406. Lipson, S.G., and Lipson, H., 1981, Optical Physics (200 ed.). Cambridge University Press, Cambridge. Loesch, H.J., Stenzel, E., and Wustenbecker 8., 1991, J. Chern. Phys. 95:3841. Luckhurst, J.R., and Sansom, AR., 1972, Mol. Phys. 24:1297. McQuarrie, D., 1983, Statistical Mechanics. Academic Press, New York. Nishimoto, E., Yamashita, S., Szabo, AS., and Imoto, T., 1998, Biochern. 37:5599. Nordio, P.L., Rigatti, G., and Segre, U., 1972, J. Chern. Phys. 56:2117. Nordio, P.L., and Segre, U., 1979, In The Molecular Physics of Liquid Crystals (G.R. Luckhurst and G.W. Gray, eds.), Academic Press, London, pp. 23-284. Nunzi, J.M., Charra, F., Fiorini, c., and Zyss, J., 1994, Chern. Phys. Lett. 219:349. O'Connor, D.V., and Philips, D., 1984, Tirne Correlated Single Photon Counting. Academic Press, London. Ohta, K., Terazima, M., and Hirota, N., 1995, Bull. Chern. Soc. Jpn., 68:2809. Paparo, D., Marrucci, L., Abate, G., Santamato, E., Bartolini, P., and Torre, R., 1996, Mol. Cryst. Liq. Cryst.282:461. Penchev, I.I., and Dozov, LN., 1981, Mol. Cryst. Liq. Cryst. 73:267. Penchev, 1., Dozov, 1., and Kirov, N., 1984, J. Mol. Liquids 29: 147. Sengupta, A, Fayer, M.D., 1995, J. Chern. Phys. 102:4193. Stankus, J.J., Torre, R., and Fayer, M.D., 1993, J. Phys. Chern. 97:9478. Tao, T., 1969, Biopolyrners 8:609. Tompkin, W.R., Ma1cut, M.S., Boyd, R.W., and Sipe, J.E., 1989, J. Opt. Soc. Arner. 6:757. Vertogen, G., and de Jeu, W.H., 1988, Thermotropic Liquid Crystals, Springer-Verlag, Berlin. Volkmer, A, Hatrick, D., and Birch, D.S., 1997, Meas. Sci. Technol. 8: 1339. Weber, G., 1965, J. Chern. Phys. 43:521. Wong, G.K.L., and Shen, Y.R., 1974, Phys. Rev. A 10:1277. Xu, C., Zipfel, W., Shear, J.B., Williams, R.M., and Webb, W.W., 1996, Proc. Nat!. Acad. Sci. 93:10673. Yu, W.M., So, P.T.C., French, T., and Gratton, E., 1996, Biophys. J. 70:626. Zannoni, c., Arcioni, A., and Cavatorta P., 1983, Chernistry and Physics ofLipids 32: 179.

Chapter 7 Infrared Laser Spectroscopy of Transient Species

PETER F. BERNATH Department o/Chemistry, University o/Waterloo, Waterloo, ON N2L 3GJ Canada

1.

INTRODUCTION

The infrared region (arbitrarily taken as 100 - 10,000 cm- I ) provides a unique window through which to view molecules (Bernath, 1990, 1995 and 1996). Molecules have their vibrational spectra in the infrared region, although some light species such as H20 and HF have infrared pure rotational transitions and many free radicals (e.g., C2) have infrared electronic transitions. Vibrational transitions have characteristic frequencies and carry chemical group information in a way that rotational and electronic transitions do not. All molecules, except homonuclear diatomics, have at least one allowed vibrational transition. Interestingly, even homonuclear molecules such as H2 and N2 have very weak electric quadrupole transitions. Vibrationally-excited emission of H2 can be seen in shocked regions of molecular clouds in space (Scoville et al., 1982) and the vibration-rotation lines of N2 can be seen in absorption through the earth's atmosphere using the sun as a source (Demoulin et al., 1991). Vibrational transitions are thus a universal monitor of chemical composition. Infrared transitions, however, have some disadvantages. The most serious is that vibrational transitions tend to be relatively weak compared to pure rotational and electronic transitions. The development of infrared lasers has helped to overcome this problem by increasing the sensitivity of infrared spectroscopy. Unfortunately there is no infrared equivalent to the visible dye laser or the microwave oscillator that provides widely tunable radiation. There has been recent progress in infrared laser technology and, for example, An Introduction to Laser Spectroscopy, Second Edition, Edited by Andrews and Demidov, Kluwer AcademicIPlenum Publishers, New York, 2002

211

212

PETER F. BERNATH

pulsed optical parametric oscillators now can furnish high-power, tunable radiation. In this chapter, I will summarize the different sources of infrared laser radiation and illustrate their use with a number of spectroscopic examples using transient molecules. Infrared spectroscopy of stable molecules is, in general, more easily carried out with Fourier transform spectrometers than with lasers except for very low concentrations or very high resolutions. Free radicals and other transient molecules have intrinsically low concentrations because of their great reactivity. The power of modem infrared laser spectroscopy is thus well illustrated with the study of transient species such as ions, free radicals and van der Waals molecules.

2.

SEMICONDUCTOR DIODE LASERS

The use of infrared diode lasers for spectroscopy has been reviewed recently several times and these articles should be consulted for details (Winnewisser et al., 1999; Werle, 1998; Davies, 1999; Feher and Martin, 1995). The basic operating principle of a diode laser is simple. The laser operates by forward bias of a diode made of a layer of an n-type semiconductor in contact with a p-type material. The n-type semiconductor injects electrons into the junction region, while the p-type injects holes. The electrons and holes recombine to release a photon. The photon energy is determined by the band gap of the material. Simple p-n junctions do not confine the radiation much in the lateral direction and have been replaced by more sophisticated designs ("heterojunctions") for improved performance (Demtrooer, 1996). Diode lasers have high gain and so are typically small devices «1 mm), which lead to a cavity mode spacing, v = c/2nd (n is the refractive index and d is the cavity length) of say 50 GHz (1-2 em-I). The laser usually operates on several longitudinal modes that can cause problems for high resolution spectroscopy. With some loss of power, mode selection is carried out with a monochromator (Figure 1). Typical single mode powers are less than 1 m W for cw (continuous wave) operation. The basic range of the diode laser is chosen by selection of the materials that are used to make the p-n junction. The first infrared diode lasers were made from Pb salts such as Pb1_xSnxTe with 0 ｾ＠ x ｾ＠ 1. The entire infrared and near infrared region (Demtroder, 1996) can be covered with various semiconductors starting at the long wavelength limit of about 25 JlIll (400 cm- I ), although anyone laser of a particular composition will cover less than 100 em-I. The band gap and the refractive index of the laser depend on temperature so they can be tuned by selecting a particular heat sink

7. Infrared Laser Spectroscopy of Transient Species

213

temperature and then adjusting the diode current. The diode current changes the junction temperature and tunes the laser over a maximum range of about 1 cm". To cover the full range of the diode, the heat sink temperature must be changed some lO's of degrees Celsius, and even then there will be gaps in the frequency coverage. These gaps and the cryogenic temperatures required for operation constitute the major drawback for mid infrared diode lasers. In the near infrared the performance of diode lasers is much better than in the mid infrared, driven in part by the needs of the fiberoptics communications industry. Communications diodes made of InGaAsP/InP operating near 1.5 JlIIl (6600 cm") have superb performance (Werle et aI., 1998). These diodes can operate at room temperature and have a diffraction grating as an integral part of the diode structure (distributed feedback, DFB). DFB lasers can operate on a single longitudinal mode over a range of as much as 100 cm" without any gaps in coverage. Unfortunately, the near infrared region contains only relatively weak overtone bands of molecules. Nevertheless the high quality and ease of use of these communications diodes allows, for example, the design of a sensitive humidity sensor (Hovde et al., 2001). Single mode operation of a diode laser can be enforced with either an internal diffraction grating (DFB laser) or an external grating. For an external cavity diode laser, one face of the laser is antireflection-coated and a grating used as an end mirror ("Littrow configuration") to the laser cavity (Demtroder, 1996). Only one wavelength is selected by the grating and sent back into the laser for amplification. This external cavity design is very flexible and offers good spectroscopic performance but is only available in the near infrared and visible regions. The most promising new development in the mid-infrared region is the quantum cascade laser (Capasso et aI., 1999). These lasers operate near room temperature in quasi-cw mode (long pulse) with high optical powers (up to 100 mW) (Sonnen et al., 2001). The lasers can be made with DFB structures so that they operate naturally on a single longitudinal mode. Quantum cascade lasers are made by the deposition of numerous layers of, for example, AlInAs and GaInAs by molecular beam epitaxy. When a voltage is applied across the semiconductor superlattice structure, lasing occurs between the different "quantum wells" (i.e., intersubband transitions in the conduction band) rather than simply across the bandgap. This means that the lasing wavelength is determined by the quantum well structure rather than by the semiconductor composition. Fine tuning is, as usual, carried out by changing the heat sink temperature and the current. Diode lasers are generally used for high resolution absorption spectroscopy (Figure 1), although they can also be used when the vibrationrotation lines are not resolved (McNesby et aI., 2001). Typically the diode

214

PETER F. BERNATH

laser is frequency modulated by adding a sinewave signal to the current ramp that scans the laser. The collimated laser beam is mode-selected with a monochromator and then sent through the sample cell. An infrared detector (typically InSb or HgCdTe) then detects the laser beam using a lock-in amplifier, resulting in a short segment of spectrum (Figure 2). Wavenumber calibration is carried out with the known spectrum of a reference gas, interpolated with fringes from an etalon (Figure 1). There are a large number of variations on this basic experimental design using sophisticated modulation schemes (e.g., two tone (Werle, 1998» or the co-addition of rapid spectral scans (Jennings et al., 1998). Sensitivities for MIl can approach values of 10.7 consistent with shot-noise limited performance, if care is taken.

Heat pipe oven

Frequency modulation

Lock In amplifier

Computer

Figure J. A typical experimental diagram for a diode laser spectrometer. The switch allows the selection of amplitude modulation with the chopper for alignment or frequency modulation for increased sensitivity. The reference gas cell is in the same laser beam path as the hot sample in the oven simply to avoid using a third lock-in amplifier.

A typical diode laser spectrum recorded (Brazier et al., 1989) with the apparatus shown in Figure 1 is presented in Figure 2. The heat pipe oven is a tube furnace loaded with LiBr solid and heated to 800°C. The spectrum is very dense because LiBr has four isotopomers, 6Li81 Br, 7Li81 Br, 6Li79Br and ＷｌｩｾｲＮ＠ The lines appear as second derivatives because the lock-in amplifier was set to detect at twice the modulation frequency to help flatten the baseline.

215

7. Infrared Laser Spectroscopy of Transient Species

7Li 8'Br

tr4

R(29\

7Li l1Br

6-4

6-4 2-0 R(28) P(24)

7LI 711Br

R;64) I Lj 711Br 3-1 4-2 P(51) P(45)

tr4

R(65\

i

1070·2

1070'4

em..!

Figure 2. A portion of the LiBr vibration-rotation spectrum (Brazier et al., 1989) recorded with a diode laser spectrometer as depicted in Figure l. The lines are in the flv = 2 overtone region. [Reproduced with the pennission of Academic Press.]

By replacing the hot cell in Figure 1 with a plasma source created with an electrical discharge, free radicals and ions can be detected. Molecular sources such as a pulsed-jet expansion can be used to detect van der Waals molecules (Sharpe et ai., 1988) or carbon-silicon chain molecules (Van Orden et ai., 1994). Using a UV photolysis laser, free radicals such as HCCO can be made by photochemistry and detected by transient diode laser absorption (Unfried and Curl, 1991). Perhaps the most interesting recent experiments detect the rotation-vibration spectra of molecules such as OCS inside He clusters using a diode laser (Grebenev et ai., 2000). The high sensitivity possible with diode lasers is responsible for their widespread use in spectroscopy. Mid-infrared diode lasers are, however, far from ideal sources, even when compared to visible and near infrared diode lasers. Quantum cascade lasers offer considerable promise in making midinfrared lasers more convenient to use without sacrificing sensitivity.

3.

CO2, N20 AND CO LASER SPECTROSCOPY

There are a large number of line-tunable lasers in the infrared and far infrared regions (Demtroder, 1996; Svelto, 1998). The best known are the

216

PETER F. BERNATH

CO2 and N20 lasers at 10 microns, the CO laser at 5 microns and the CO2 laser-pumped methanol laser in the far infrared. These gas lasers are relatively cheap and have high output powers for both cw and pulsed operation. The CO2, N20 and CO lasers are driven by an electrical discharge that results in a population inversion. Lasing occurs between excited vibration-rotation energy levels. In the case of optically-pumped far infrared lasers, the CO2 laser has a co-incidence with a few rovibrationallines of, for example, CH30H and the population inversion is between excited rotational levels. The major drawback for spectroscopy with these lasers is that they are not continuously tunable: the lasers operate only on a few discrete vibration-rotation transitions. Apart from accidental line co-incidences, in order to be useful for spectroscopy, either the molecule must be tuned into resonance with the fixed-frequency laser or the laser must be adapted to make it more tunable. The CO2 laser can be made somewhat more tunable by operating the laser at high pressure to broaden the lasing transitions. Confining the gas discharge to a narrow waveguide also improves performance. Nearly complete tunability in the 10 micron region can be achieved by non-linear mixing of CO2 laser radiation with tunable microwave radiation. The mixing of the laser (VL) and microwave (vm) radiation in a non-linear material such as CdTe or GaAs produces "sidebands" on the laser output, i.e., the output from the mixer is VL ± Vm, in which Vm is tunable (Cheo, 1984). A grating monochromator is then used to separate a single sideband from the unshifted laser and the other sideband 2vm away in frequency. The main disadvantage of the tunable sideband laser, apart from complexity, is the relatively low power «1 mW) inherent in the non-linear mixing process, at least for cw sources. Other options for making fixed-frequency lasers tunable include the use of the spin-flip Raman process (Demtroder, 1996). In this case, a CO or CO2 laser pumps a material such as JnSb in a magnetic field. The energy levels occupied by the electrons in the conduction band are split into two by the Zeeman effect. The stimulated Raman effect causes the fixed frequency laser (VL) to be shifted to v = VL± g*J..I.aBIJi, where g* is the effective g value of the electrons, J..I.B is the Bohr magneton and B is the magnetic field strength. By adjusting the magD,etic field, the spin-flip Raman laser can be tuned. Third order non-linear processes can also be used to make tunable radiation (Inguscio, 1988). For example, two different CO2 laser lines (VLl and Vu) can be mixed together along with microwave radiation (vm> with metal-insulator-metal (MJM) diode made with a pointed W wire on an oxide-covered nickel post to give far infrared radiation (Inguscio, 1988; Jennings, 1989), i.e., v = VLl-VU ± Vm• Evenson and co-workers have

217

7. Infrared Laser Spectroscopy of Transient Species

pioneered this type of device, which they call a TuFIR, for tunable far infrared spectrometer. Rather than modify a line-tunable laser, it is simpler to tune a molecular transition into resonance with a fixed frequency laser using electric or magnetic fields. The electric field case is called laser Stark spectroscopy (Landsberg et at., 1977) (or laser electric resonance) and the magnetic case is called laser magnetic resonance, LMR (Davies, 1981). The basic concept uses a cell containing the molecule of interest inside a laser cavity (Evenson, 1996) (Figure 3) and the molecular transition frequency is tuned by application of an electric or magnetic field. When the molecular transition is in resonance with the fixed-frequency laser there is a sharp drop in output power because of the extra loss inside the laser cavity.

1---------

"'.Oem

Pyrex Tube (5 em lDJ

FIXed

Mirror

40 fLm to 1000 fLm LMR Spectrometer Figure 3. Far infrared laser magnetic resonance spectrometer of K. Evenson (1996). The far infrared laser cavity contains both the optically-pumped gaseous laser medium on the left and the sample cell between the pole faces of a magnet on the right. [Atomic, Molecular and Optical Physics Handbook, "Laser Spectroscopy in the Submillimeter and Far-Infrared Region", Evenson, K.M., p. 477, Figure 40.2, 1996, Copyright Springer-Verlag. Reproduced by permission.]

Figure 3 shows a typical far infrared LMR apparatus, as designed by K. Evenson (1996). To the left is the far infrared laser medium, e.g., CH30H, pumped from the top with a CO 2 pump laser beam through a ZnSe window. A beam splitter at Brewster's angle confines the laser gas to the left part of the cavity. To the right is the intracavity discharge cell between the pole faces of a 2 T magnet, with coils for magnetic field modulation. To the far right is the high reflecting end mirror of the laser. In principle, laser electric resonance can be applied to all polar molecules, but the technique is largely confined to stable species. The problem is that the electrical discharges that are often used to make ions and

PETER F. BERNATH

218

free radicals are not compatible with high electric fields. The presence of ions and electrons makes the operation of Stark plates difficult because of electrical breakdown. Landsberg et ai. (1977), however, succeeded in recording the laser Stark spectrum of the HCO free radical. Laser magnetic resonance relies on the Zeeman effect to tune the transitions of paramagnetic molecules into resonance with the laser. The technique is very sensitive and is ideal for the study of free radicals, which by definition are all paramagnetic. The main difficulty with LMR spectroscopy is the interpretation of very complex spectra (Figure 4). Shown is the far infrared LMR spectrum of SeH (v = 0, X2rr312 ) made by the reaction of H atoms with a Se film (Ashworth and Brown, 1990). The rotational transition is J = 7/2 - 5/2 and the dMj = ± 1 transitions are displayed for six isotopomers. The doubling of the lines is due to Adoubling and 77SeH (1 = 112) displays an additional hyperfine doubling.

LlI

" IlIll 1"11

I

o

1'"

I

o·

11,11 \I

_ _IT

III r HI

I

2·0

Figure 4. The far infrared laser magnetic resonance (LMR) spectrum of the SeH free radical (Ashworth and Brown, 1990). [Reproduced by permission of the Royal Society of Chemistry.]

Urban and co-workers (Hinz et ai., 1985) have modified the normal intracavity (as in Figure 3) arrangement of an LMR spectrometer. They

7. Infrared Laser Spectroscopy o/Transient Species

219

mounted the discharge cell external to a CO laser cavity and used the Faraday effect (rotation of linear polarization by a longitudinal magnetic field) to detect molecular transitions. In this way they improved the sensitivity by more than two orders of magnitude. In addition to electric and magnetic fields, the Doppler effect can be used to tune ions into resonance with a fixed-frequency laser. A. Carrington (1979) has pioneered this type of spectroscopy by co-propagating an ion beam with an infrared laser beam. On resonance, Carrington (1986) and coworkers simply monitor the appearance of a new ion (e.g., W from HeH+) by dissociation using a mass spectrometer.

4.

COLOUR CENTER LASERS

Colour center lasers use an alkali halide crystal with anion defects as the laser medium (Demtroder, 1996). If an electron is trapped in a lattice vacancy left by the removal of an anion, then the transparent crystal becomes coloured. The electronic energy levels are those of an electron trapped in a box (the vacant site), and associated with each electronic state are the lattice phonons (crystal vibrations). These colour centers (or F-centers, for Farbe, German for colour) come in a number of varieties depending on the parent alkali halide crystal as well as the morphology and electron occupation of the defect site. Colour center lasers can be pumped by flashlamps or other lasers such as Nd:YAG, argon ion and krypton ion lasers in either pulsed or cw mode. Colour center lasers cover the range from about 0.5 - 3.4 microns, but only the long wavelength end of the range is useful because of the availability of dye lasers and Ti:sapphire lasers to shorter wavelengths. Other types of solid state lasers do extend into the infrared region but are not widely used for spectroscopy. For example, CO:MgF2 or Co:KZnF3 are vibronic lasers like Ti:Sapphire and are tunable in the 1.6 - 2.07 micron region (Demtroder, 1996). These infrared vibronic lasers use the energy levels of C02+ impurity ions in crystals and are widely tunable because of the extensive involvement of lattice phonons in the electronic transition. In the same way that an electronic transition in a dye molecule involves vibrational modes, an electronic transition of a metal impurity ion can involve crystal vibrations, particularly if there is a change in structure (c.f., Franck-Condon principle). Such vibronic lasers and colour center lasers are thus solid state analogues of dye lasers. The main drawback of these lasers for infrared work is that they do not cover the long wavelength region, e.g., longer than 3.4 microns for colour center lasers.

220

PETER F. BERNATH

Colour center lasers can be used for simple absorption spectroscopy but they can also be used in a more sensitive magnetic rotation arrangement (Figure 5). The colour center laser (made by Burleigh) is pumped by an argon or krypton ion laser (Carrick et ai., 1983). After some of the laser radiation is split off for calibration and stabilization, the radiation is sent through a White-type multipass cell. The linearly polarized light is nearly blocked by a crossed polarizer and then focussed into an InSb infrared detector. The cell is surrounded by a solenoid to provide a modulated magnetic field. The laser is scanned and when the radiation is in resonance with a transition of a free radical then the polarization of the laser is rotated by the Faraday effect and a signal appears at the magnetic modulation frequency. Kr+ or Ar+ LASER

H.V.

30 mA A.C. NEON SIGN TRANSFORMER

"WHITE" CELL

4000 Hz 30 Amp I--_....J 200Gouss TO COMPUTER

(RMS)

Figure 5. The experimental arrangement of a colour center laser spectrometer (Carrick et al., 1983). The linearly polarized laser beam is blocked (or nearly blocked) by a crossed polarizer in front of the detector. The laser beam makes several passes through a White-type ceil filled with plasma excited by a 60 Hz voltage from a neon sign transformer. On resonance with a transition of a paramagnetic molecule, the polarization is rotated and a signal appears on the detector. [Reproduced with the permission of American Institute of Physics]

A typical magnetic rotation spectrum recorded by Carrick, Merer and Curl11983) is displayed as Figure 6. The lines are due to a vibronic band of the A 2 n -X 2 L + electronic transition of the CCH free radical. The CCH molecule was made in an 60 Hz electrical discharge of argon gas above a polyacetylene deposit made in a prior discharge of acetylene gas.

221

7. Infrared Laser Spectroscopy of Transient Species

y

dolL

._......

.L

"""T'

J 0

R,

Q

4108.0

"V

'1

ｾ＠

I III

I,

.... ,....

li.

....

'T

'T

!"'"

-

C2 H2

ｾ＠

u..

. r ﾷＮＬｾｲ＠

.Ji. ..

J 'I

T

ｾ＠

Fl

2

2

21

.11

4

7

41 06 .0

8 I

8

7

3456

9

9 0

10

II

12

If

14

15

I

41 04 .0

FAEQ[l/CMJ Figure 6. Magnetic rotation spectrum of the A2rr -x2r infrared electronic transition (Carrick et al., 1983) of CCH recorded with the apparatus of Figure 5. Notice the Q-branch band head near 4106 ern-I. The C2H2 lines at the top are for wavenumber calibration. [Reproduced with the permission of American Institute of Physics.]

The detection of infrared absorption can be indirect as illustrated in experiments on molecules trapped in He droplets (Grebenev et al., 2000; Nauta and Miller, 1999) (Figure 7). He gas at 20 K is expanded into vacuum to make droplets with about 4000 He atoms. These droplets pass through a pick-up cell in which one or more HCN molecules are added to the He clusters (Nauta and Miller, 1999). When the HCN molecules trapped in a He droplet absorb a photon from a colour center laser, several hundred He atoms evaporate. These smaller droplets result in a change in the flux of energy incident on a cryogenic bolometer, which measures temperature changes. Figure 8 shows a spectrum of linear (HCN)n van der Waals molecules inside a He droplet (Nauta and Miller, 1999). The strong electric field shown in Figure 7 is used to orient the molecules and prevent their free rotational motion. The collapse of the rotational structure leads to a strong single peak for each of the HCN oligomers.

222

PETER F. BERNATH

-v Pick-up Cell

Cryostat

Multi-pass! Stark Cells Figure 7. Schematic diagram of the colour center molecular beam spectrometer (Nauta and Miller, 1999). High pressure He gas (50 bar) at 20 K is expanded into vacuum to form He clusters. The superfluid He clusters contain about 4000 atoms and have a temperature of 0.37 K. As the clusters traverse a pick-up cell containing HCN vapour, they entrain one or more HCN molecules. The Stark cell allows the application of a high electric field to orient the linear (HCN)n n = I, 2, 3, ... van der Waals molecules trapped inside the He clusters. The absorption of colour center laser radiation is detected by loss of He atoms that evaporate from the cluster to remove the excess energy deposited by a photon. The energy of the cluster beam (which depends on cluster size) is monitored with a sensitive bolometer that measures minute changes in temperature. [Reprinted Figure I with the permission from Nauta, K., and Miller, R.E., Nonequilibrium self-assembly of long chains of polar molecules in superjluid helium, 1999, Science, v. 283, pp.1895-1897. Copyright 1999 American Association for the Advancement of Science.]

.......

Trimer

Dimer

I

Tetramer

3305

ｾ＠

; 65

3306

3307

3308

Frequency (cm .1)

Figure 8. A spectrum of the linear (HCN)n n = 2,3, ... van der Waals molecules trapped in He clusters (Nauta and Miller, 1999). The HCN oligomers are excited in the region of the free (not hydrogen-bonded) C-H stretching mode with a colour center laser. A large electric field (Figure 7) is used to orient the molecules and collapse the rotational structure into a single sharp line. [Reprinted Figure 2 with the permission from Nauta, K., and Miller, R.E., Nonequilibrium self-assembly of long chains of polar molecules in superjluid helium, 1999, Science, v. 283, pp.1895-1897. Copyright 1999 American Association for the Advancement of Science.]

7. Infrared Laser Spectroscopy o/Transient Species

5.

223

NON-LINEAR FREQUENCY SHIFTING OF VISIBLE LASERS

The most versatile and popular tunable laser is the visible dye laser. The active medium is a large organic dye molecule dissolved in a solvent such as methanol or ethylene glycol. Unfortunately, the long wavelength limit of dye lasers is about 1 micron for the IR 140 laser dye (Demtroder, 1996). High power pulsed dye lasers, however, can easily be used in the near infrared by Raman shifting their output (Demtroder, 1996). If the laser (vd is focussed into a cell containing, for example, high pressure hydrogen gas then above a certain threshold the stimulated Raman effect will generate a new laser beam at VL - Vyib (Vyib the fundamental vibrational frequency of H2) and with decreasing efficiency, the frequency VL - Vyib can also drive a Raman transition to generate radiation at VL - 2Vyib (and so on). Hydrogen gas is used because it has a high vibrational frequency of about 4160 cm- I and generates a correspondingly large Raman shift. A more complicated but more versatile method of shifting a tunable visible laser into the infrared is based on difference frequency mixing of two lasers (Demtroder, 1996; Simon and Tittel, 1995). Difference frequency mixing (unlike Raman shifting) works for both cw and pulsed lasers. Two visible or near infrared lasers are focussed into a non-linear crystal such as LiNb0 3, which generates VIR = VI - V2, the frequency difference between the lasers. By tuning one or both of the lasers, tunable infrared radiation is available for spectroscopy. The main trick in efficient difference frequency generation is to achieve phase matching, i.e., klR = kl - k2 in which Ikl=21t/A. and the direction of k is in the laser beam propagation direction. The phase matching condition is equivalent to the conservation of momentum (p = lik) of the interacting photons at the same time energy is conserved (hVIR = hVl - hV2). Phase matching can be achieved by having kl cross at an angle to k2 or, for better efficiency, by using crystal birefringence. For co-linear beams, the phase matching equation can be written as n1Rv1R = n1v I -n 2v2 in which n is a refractive index. By polarizing VI at 90 0 to V2 and sending the beams into a birefringent crystal it is possible to fulfill the phase matching condition by adjusting the temperature or the frequencies VI and V2. Difference frequency spectrometers based on lithium niobate, LiNb0 3, have been available for many years now to cover the 2.2 - 4.2 micron (2400 - 4500 cm- I) region (Pine, 1976). Recently materials such as AgGaS2 and AgGaSe2 extended the coverage of difference frequency spectrometers into the mid infrared (Simon and Tittel, 1995; Simon et al., 1993). Figure 9 shows a difference frequency spectrometer (Petrov et al., 1998) for the 6.8 12.5 micron (800 - 1500 cm- I) region based on AgGaS2. Two diode lasers

PETER F. BERNATH

224

are polarized at 90° to each other and focussed into the crystal. Filters are used to separate the pump laser beams from the difference frequency radiation. The major drawback for cw difference frequency spectrometers is the low output power of typically 1 ,...W for 0.5 W laser input powers.

766-786nm

830-868nm

tunable

tunable

laser

laser

diode

diode

11

12

FI

F2

Figure 9. An infrared difference frequency spectrometer based on mixing two high power (0.5 W) diode lasers in a AgGaS2crystai (Petrov et al., 1998). 1I and 12 are optical isolators to prevent optical feedback (back reflections) from making the lasers unstable. PR is a polarization rotator to ensure that the laser beams have the orthogonal linear polarization needed for phase matching. OM is a dichroic filter to combine the two laser beams, which are focussed (L I) into the AgGaS2 crystal. F I and F2 are Ge filters to remove the high power visible laser beams before they strike the HgCdTe infrared detector. [Reproduced with the permission of Optical Society of America.]

The most famous spectrum discovered with a difference frequency spectrometer is that of by T. Oka (1980). has now been seen in emission from the aurora of the giant planets (Tennyson, 1995) and in absorption in clouds of gas in the interstellar medium (McCall et al., 1999). H; can be made in an electrical discharge as shown in Figure 10 (Amano, 1985). In this diagram a large hollow cathode was used to make H2D+ by T. Amano and molecule concentration modulation was used to enhance the sensitivity. By switching the discharge on and off, the concentration of H2D+ varied and the infrared absorption was synchronously detected with a lock-in amplifier (Amano, 1985). Figure 11 compares a glow discharge absorption signal ofH2D+ with that ofthe hollow cathode source. Another very successful molecule modulation scheme is based on velocity modulation of molecular ions (Gudeman and Saykally, 1984). In this case an AC voltage is used to create a glow discharge and the ions in the plasma try to follow the alternating electric field. This motion results in a

H;

H;

225

7. Infrared Laser Spectroscopy of Transient Species

I>

TRANSfOIIMER

AUDIO ....p.

CHAIn' RECORDER

Figure 10. A large hollow cathode discharge cell used to make transient molecules (Amano, 1985). The Iiquid-nitrogen-cooled cathode is driven by the output of a high power (800 W) audio amplifier at 5 kHz. The discharge is only on when the cathode is negative relative to the anode and this modulates the concentrations of the molecules fonned in the plasma. A difference frequency laser beam is passed several times through the cell (White-type mirrors) before synchronous detection. [Reproduced with the permission of Optical Society of America.]

3069.0

Figure 11. The 212 -

306a.7cm- i

III transition of the VI vibrational band of the H2D+ molecule measured (Amano, 1985) with the apparatus of Figure 10. The top trace was recorded with a simple glow discharge cell, while the lower trace with a large hollow cathode cell. The improvement in the signal-to-noise ratio is evident from the traces of the lines. [Reproduced with the permission of Optical Society of America.]

226

PETERF. BERNATH

Doppler shift in the transition frequency relative to the laser beam. The small synchronous frequency shifts can be detected using absorption from a laser beam with an IR detector and a lock-in amplifier. An additional method of frequency shifting visible lasers into the infrared is based on an optical parametric oscillator, OPO (Demtroder, 1996; Svelto, 1998). An OPO can be viewed a "photon splitter" because pump radiation vp is split into two photons VI and V2 (v = VI + V2). The higher frequency VI is called the signal and the lower frequency beam at V2 is called the idler. A non-linear material such as LiNb03 is placed in a resonator and pumped with a laser beam at vp. Again phase matching (npvp = nlvl + n2v2) is crucial and determines the operating frequency of the OPO. For example, adjusting the angle of a birefringent non-linear crystal changes the wavelength for which the phase matching condition holds and allows the OPO to be scanned in wavelength. OPOs typically operate with pulsed lasers (although cw operation is possible) and use the same non-linear materials as difference frequency spectrometers. The operating range of an OPO is determined by the pump wavelength and the range over which phase matching can occur, e.g., 1.4 - 4.3 microns for LiNb03 and 1.2 - 9 microns for AgGaS2 both pumped at 1.064 microns from aNd: YAG laser. There are many ways to detect infrared laser absorption but one of the most sensitive is based on measuring the ringdown time of an optical cavity (Scherer et aI., 1997) (Figure 12). An infrared laser based on Raman shifting a pulsed dye laser (VL - 3Vvib) was sent into an optical cavity constructed with highly reflective mirrors (typically> 99.9%) (paul et al., 1997). The pulse of light is reflected repeatedly by the mirrors and is trapped in the cavity. Effective path lengths in excess of a kilometer are possible. The decay of radiation in the empty cavity is determined by the transmission losses through the mirrors. Any absorber in the cavity adds additional loss and shortens the cavity lifetime. An absorption spectrum is thus recorded by scanning the laser and measuring the cavity lifetime for each laser pulse. In Figure 13, water clusters are made by free jet expansion into vacuum (Paul et al., 1997). The cavity ringdown spectrum of the van der Waals clusters (H20)n for n = 2, 3, '" can be seen in Figure 13. Cavity ringdown spectroscopy has several virtues, including very high sensitivity. In addition, low resolution pulsed lasers can be used and the technique is insensitive to pulse-to-pulse amplitude fluctuations. Infrared spectra of molecules can also be recorded by infrared-ultraviolet double resonance spectroscopy. In this case the tunable infrared radiation generated by difference frequency mixing (Figure 14) is used to populate an excited vibrational state (Omi et al., 1996). If the UV laser is tuned just

227

7. Infrared Laser Spectroscopy of Transient Species

Figure J2. An infrared cavity ringdown apparatus designed to record spectra of molecules formed in a free jet expansion (Paul et al., 1997). Tunable infrared radiation (laser) is based on Raman shifting a pulsed visible dye laser into the 3 micron region. The molecules are formed in a slit jet expansion of He with entrained water vapour. A transient digitizer is used to measure the exponential decay of infrared radiation in the cavity both with the expansion present and without the expansion gas. The additional loss in the cavity due to molecular absorption causes the cavity lifetime to shorten. [Reprinted with the permission of American Chemical Society, Copyright 1997.] H 20)n n· _..::>r6'-T6'--_-.;5i'-'T-4_ _"f3---i2

I I

1

I I

I I

FreeO-H banda

300

I l

i

200

100

3000

3200

3400

3600

" " " - (em-' )

Figure J3. The infrared absorption spectrum of water clusters (H20),. n = 1, 2, 3, ... as recorded by the cavity ringdown method (Figure 12) (Paul et al., 1997). The three different traces correspond to different He gas backing pressures in the slit jet expansion into vacuum that forms the van der Waals molecules. [Reprinted with the permission of American Chemical Society, Copyright 1997.]

PETER F. BERNATH

228

below the ionization threshold, then no ion signal will be observed except for those molecules that have been excited by the infrared laser. As the infrared laser is scanned, parent ions will be detected when the laser is in resonance. Figure 15 shows the infrared spectrum of phenol (Omi et ai., 1996) recorded in this way. The phenol sample was entrained in He carrier gas and expanded into vacuum.

a) IR Dip (Previous)

b) NID-IR (This Work) ion

ion

,,-fl.flr+

IP'

vuv vuv

S,

S, vuv

v..

VIa

ｾｵｶ＠

V

So

1

v..

•

tJ

e"".""

"-

vuv

V

v..

So

j BMtutcetl 10.. C,.".,.,

Figure 14. Energy level diagram for non-resonant ionization detected infrared spectroscopy (NID-IR) as compared to infrared ion-dip (lR-Dip) spectroscopy (Omi et al., 1996). In ion dip spectroscopy (a "V"-type of double resonance) the ion signal of a UV laser-ionized molecule is monitored as a strong pulsed infrared laser is tuned. Resonance with a vibrational transition is thus detected by a decrease in the ion signal caused by depopulation of the ground state of the neutral molecule. In contrast, for NID-IR there is no ion signal when the infrared laser is not in resonance because the UV laser is tuned just below the 2-photon ionization threshold. When the infrared laser populates a vibrationally-excited state, then enough energy is available to ionize the molecule and an ion signal appears. [Reprinted from Chemical Physics Letters, v. 252, Omi, T., Shitomi, H., Sekiya, N., Takazawa, K., and Fujii, M., Nonresonant ionization detected lR spectroscopy for the vibrational study in a supersonic jet, pp. 287-293, Copyright 1996, with permission from Excerpta Meida Inc.]

Infrared spectroscopy can also be carried out by shifting infrared radiation into the visible region for detection. Indeed the early popularity of Raman spectroscopy in the 1930' s over infrared spectroscopy was based on the use of visible light sources and detectors to extract vibrational information. Sum frequency mixing (Shen, 1984; 1989) uses a pulsed source of infrared radiation and a visible laser to shift the detection frequency (vs) into the visible, Vs = VIR + Vvis. By tuning VIR and measuring V s• and infrared spectrum can be recorded. The main application of infraredvisible sum frequency mixing is in surface and interface science. Liquids

229

7. Infrared Laser Spectroscopy of Transient Species

CHVib.

ｾ＠......

...

@

1I0H

fi

0

-a

1I0H+7OH

2500

3500 IR Wavenumber/em· l

3000

4000

Figure 15. The non-resonant ionization-detected infrared spectrum of jet-cooled phenol in the CH and OH stretching regions (Omi et al., 1996) (Figure 14b). Tunable infrared radiation was created by pulsed difference frequency mixing in LiNb03 with a visible dye laser and a Nd:YAG laser.. [Reprinted from Chemical Physics Letters, v. 252, Omi, T., Shitomi, H., Sekiya, N., Takazawa, K., and Fujii, M., Nonresonant ionization detected fR spectroscopy for the vibrational study in a supersonic jet, pp. 287-293, Copyright 1996, with permission from Excerpta Meida Inc.]

and solid media that have a center of symmetry do not permit sum frequency generation, but interfaces lack a centre of symmetry. Sum frequency generation is thus a surface sensitive technique.

6.

FREE ELECTRON LASERS

It is possible to construct a laser which does not use an atom or molecule as the lasing medium (Demtroder, 1996). The free electron laser (FEL) uses a relativistic electron beam at an accelerator. The electron beam is deflected through an optical resonator and through a magnetic "wiggler", which periodically accelerates the electrons. The accelerated electrons emit radiation coherently and the light builds up in the optical cavity to make a laser. The beauty of a FEL is that it provides widely tunable infrared radiation at high power. For example, FELIX (Free Electron Laser for Infrared Experiments) is continuously tunable from 5 to 25 JlIIl (400 - 2000 cm'l) with 5 f.ls long macropulses of up to 100 mJ/pulse at 10 Hz (Oomens et al., 2000). The laser linewidth is typically a few cm,l, making FELIX ideal for large molecule spectroscopy. G. Meijer and co-workers have used a FEL laser to record gas phase spectra of polycyclic aromatic hydrocarbon ions (Oomens et al., 2000).

PETER F. BERNATH

230

uv ＮｾｔｱｆＺ＠

I Ｎｾ＠

MCP

Front view Figure 16. An ion trap used to record infrared spectra of polycyclic aromatic hydrocarbon (PAH) molecules (Oomens et al., 2000). The PAHs are vaporized thermally and then ionized by focussed UV radiation from an ArF excimer laser. The PAH ions are held in a quadrupole ion trap. The trapped ions are irradiated with high-power tunable infrared from a free electron laser. On resonance, the ions are dissociated by multiphoton processes. After some time, the trapping voltages are removed and the contents of the trap are examined with a time-of-flight (TOF) mass spectrometer with a multichannel plate (MCP) detector. [Reproduced with the permission of American Astronomical Society and the authors.]

0.2

0.1

Phenanthrene

0:0

0.0

ｾ＠

0.4

'$,

c 0

0.2

E

0.0

I...

= f

II.

0.04

0.02

0.00 800

1200

1400

1600

1800

Wavenumbers (em· l )

Figure 17. The infrared spectra of the singly-ionized polycyclic aromatic hydrocarbons, phenanthrene, anthracene and pyrene in an ion trap recorded with a free electron laser (Figure 16) (Oomens et al., 2000). The lower trace in each panel is an ab initio prediction of the vibrational spectrum. [Reproduced with the permission of American Astronomical Society and the authors.]

7. Infrared Laser Spectroscopy of Transient Species

231

molecules are thought to occur in various astronomical sources. The cations were made by multiphoton ionization of the neutrals with an ArF laser and held in an ion trap (Figure 16). The FEL laser beam is sent into the ion trap and scanned in wavelength. On resonance, the powerful FEL dissociates the parent ions. In this way gas phase spectra of several ions (Figure 17) including phenanthrene, anthracene and pyrene were recorded (Oomens et ai., 2000).

7.

CONCLUSIONS

Infrared laser technology is making rapid progress and devices such as optical parametric oscillators are now available commercially from several sources. Infrared lasers are crucial for the detection of transient molecules because species such as ions, free radicals and van der Waals molecules have low concentrations.

REFERENCES Amano, T., 1985, J. Opt. Soc. Arn. B 2:790. Ashworth, S.H. and Brown, J.M., 1990, J. Chern. Soc. Faraday Trans. 86: 1995. Bernath, P.F., 1990, Annu. Rev. Phys. Chern. 41:91. Bernath, P.F., 1995, Spectra ofAtoms and Molecules, Oxford University Press, New York. Bernath, P.F., 1996, Chern. Soc. Rev. 25: III. Brazier, c.R., Oliphant, N.H., and Bernath, P.F., 1989, J. Mol. Spectrosc. 134:421. Capasso, F., Tredicucci, A., Gmachl, c., Sivco, D.L., Hutchinson, AL., Cho, A.Y., and Scamarcio, G., 1999, IEEE J. Sel. Top. Quanturn Electron. 5:792. Carrick, P.G., Merer, AJ., and Curl, R.F., 1983, J. Chern. Phys. 78:3652. Carrington, A, 1979, Proc. R. Soc. London A 367:433. Carrington, A., 1986,J. Chern. Soc. Faraday Trans. 2, 82:1089. Cheo, P.K., 1984, IEEE J. Quanturn Electron. QE-20:700. Davies, P.B., 1981, J. Phys. Chern. 85:2599. Davies, P.B., 1999, Spectrochirn. Acta A 55:1987. Demoulin, P., Farmer, C.B., Rinsland, c.P., and Zander, R., 1991,J. Geophys. Res. 96: 13003. DemtrOder, W., 1996, Laser Spectroscopy, 2nd edition, Springer, Berlin. Evenson, K.M., 1996, in Atornic, Molecular and Optical Physics Handbook (G.W.F. Drake, ed.), pg. 473, Am. Inst. Phys., Woodbury, N.Y. Feher, M. and Martin, P.A., 1995, Spectrochirn. Acta A 51:1579. Grebenev, S., Hartmann, M., Havenith, M., Sartakov, B., Toennies, J.P., and Vilesov, A.F., 2000, J. Chern. Phys. 112:4485. Gudeman, C.S. and Saykally, R.J., 1984, Ann. Rev. Phys. Chern. 35:387. Hinz, A., Zeitz, D., Bohle, W., and Urban, W., 1985, Appl. Phys. B 36: 1. Hovde, D.C., Hodges, J.T., Scace, O.E., and Silver, lA, 2001, Appl. Opt. 40:829. Inguscio, M., 1988, Physica Scripta 37:699. Jennings, D.A, 1989, Appl. Phys. B 48:311 .

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Jennings, D.E., 1988, J. Quant. Spectrosc. Roo. Transfer 40:221. Landsberg, B.M., Merer, AJ., and Oka, T., 1977, J. Mol. Spectrosc. 67:459. McCall, BJ., Geballe, T.R., Hinkle, K.H., and Oka, T., 1999, Astrophys. J. 522:338. McNesby, K.L., Wainner, R.T., Daniel, R.G., Skaggs, R.R., Morris, lB., Miziolek, A.W., Jackson, W.M., and McLaren, LA., 200 I, Appl. Opt. 40:840. Nauta, K. and Miller, R.E., 1999, Science 283: 1895. Oka, T., 1980, Phys. Rev. Lett. 45:531. Omi, T., Shitomi, H., Sekiya, N., Takazawa, K., and Fujii, M., 1996, Chern. Phys. Lett. 252:287. Oomens, J., van Roij, A.J.A., Meijer G., and von Heiden, G., 2000, Astrophys. J. 542:404. Paul, lB., Collier, C.P., Saykally, RJ., Scherer, JJ., and O'Keefe, A., 1997, J. Phys. Chern. A, 101:5211. Petrov, V., Rempel, c., Stolberg, K.-P., and Schade, W., 1998, App/. Opt. 37:4925. Pine, A.S., 1976, J. Opt. Soc. Arn. 66:97. Scherer, 1.1., Paul, J.B., O'Keefe, A., and Saykally, RJ., 1997, Chern. Rev. 97:25. Scoville, N.Z., Hall, D.N.B., Kleinman, S.G., and Ridgway, S.T., 1982, Astrophys. J. 253:136. Sharpe, S.W., Sheeks, R., Wittig, c., and Baudet, R.A., 1988, Chern. Phys. Lett. 151:267. Shen, Y.R., 1984, Principles ofNonlinear Optics, Wiley, N.Y. Shen, Y.R., 1989, Nature 337:519. Simon, U. and Tittel, F.K., 1995, Infrared Phys. Techno/. 36:427. Simon, U., Benko, Z., Sigrist, M.W., Curl, R.F., and Tittel, F.K., 1993, Appl. Opt. 32:6650. Sonnen, D.M., Rawlins, W.T., Allen, M.G., Gmachl, c., Capasso, F., Hutchinson, A.L., Sivco, D.L., Baillargeon, 1.N., and Cho, A.Y., 2001, Appl. Opt. 40:812. Svelto, 0.,1998, Principles of Lasers, 4th ed., Plenum, N.Y. Tennyson, J., 1995, Rep. Prog. Phys. 57:421. Unfiied, K.G. and Curl, R.F., 1991,J. Mol. Spectrosc. 150:86. Van Orden, A., Giesen, T.F., Provencal, R.A., Huang, HJ., and Saykally, RJ., 1994, J. Chern. Phys. 101:10237. Werle, P., 1998, Spectrochirn. Acta A 54:197. Werle, P., Miicke, R., D' Amato, F., and Lancia, T., 1998, App/. Phys. B 67:307. Winnewisser, G., Drascher, T., Giesen, T., Pak, I., Schmiilling, F., and Schieder, R., 1999, Spectrochirn. Acta A 55:2121.

Chapter 8 Nonlinear Optics and Surface Applications DAVID L. ANDREWS, and STEPHEN R. MEECH School of School of Chemical Sciences. University of East Anglia. Norwich. NR4 7TJ, U.K.

1.

INTRODUCTION

In this chapter some of the key ideas of optical nonlinearity are introduced and reviewed. The well-established uses of nonlinear optical components for frequency conversion are then discussed, followed by a survey of other more adventurous and more recent applications in surface science. To begin, we consider the conceptual heritage of the subject area. The fIrst observations of weak frequency doubling (Franken et al., 1961) heralding the birth of nonlinear optics forty years ago concerned the conversion in quartz of 694 nm radiation from a ruby laser to its second harmonic at 357 nm (twice the frequency, half the wavelength). Most of the simple concepts subsequently introduced in the infancy of the subject, to explain these and other more exotic laser phenomena, were adaptations of the principles of classical optics. On the whole the resultant framework for the subject is a good support and its own principles are a good guide; but not invariably so. We have learned enough in the intervening period to recognise that the theory would be dealt with quite differently if it were to be created from scratch today. In fact there are a number of distinct approaches to the theoretical representation of any optical process. At the microscopic level, proper description of the optical properties of any material entails representing its· electronic behaviour under the influence of electromagnetic radiation, and such electronic behaviour generally calls for a full quantum mechanical treatment. Nonetheless a broadly classical formulation is common, and if one is not concerned about the detailed nature of the material response, it is often adequate. Those who need to quantify that response, for example in the actual design of nonlinear optical materials, will recognise the need to delve into the quantum mechanics. For the radiation itself, there is again the possibility of either employing a An Introduction to Laser Spectroscopy. Second Edition, Edited by Andrews and Demidov, Kluwer Academic/Plenum Publishers, New York, 2002

233

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DAVID L. ANDREWS and STEPHEN R. MEECH

classical description, or of treating it quantum mechanically. The former choice has the strength of relating clearly to classical electrodynamics, and it is this traditional and more familiar approach that is to be followed in detail below. Before doing so, it is therefore worth briefly outlining the insights that a photonic perspective can offer. The procedure in which both matter and radiation are treated quantum mechanically, known as quantum electrodynamics (QED), has strikingly different concepts involved in its formulation; interested readers can find a general overview of its application in nonlinear optics elsewhere (Andrews, 1993; Andrews and Allcock, 2001). Here it may simply be remarked that QED is the only theory in which one can consistently and meaningfully employ the useful and well-proven concept of photons. Incidental use of the photon concept is to be found either explicitly or often implicitly in most classical theories, yet only on a strictly illegitimate basis. It is a strength of quantum electrodynamics that the description it provides at the microscopic or molecular level is conceptually simple. In terms of photons, for example, second harmonic generation (SHG) is described as a three-photon process in the course of which two frequency 0) photons of incident light (the pump radiation) are annihilated, and a frequency 20) photon carrying the sum of their energies is created. There is overall energy conservation by both the matter involved in the process and by the radiation field; the parametric interaction may in this sense be viewed as a transition between states of the radiation field, supported by the medium. To illustrate just where the photon concept alone can lead us, without intricate mathematics, consider what it immediately tells us about the process of SHG. Remember that this is a process requiring a very intense source of light for its observation; the effect was not experimentally demonstrated until the first pulsed lasers arrived on the scene. It is not hard to understand why this is so. For a single harmonic conversion it is necessary for two photons to pass essentially simultaneously through the region of space occupied by one molecule (or more generally any site or region within which energy conservation is enforceable) and the likelihood of this depends quadratically on the intensity of light (Andrews, 1997).

2.

CLASSICAL PRINCIPLES

2.1

Linear Response

The starting point for the classical description of optical response is invariably introduction of the electric polarisation P. Although not an observable as such, this quantity is meant to represent the relative displacement of positive and negative charges within a medium on application of an electric field, associated with the appearance of an induced

8. Nonlinear Optics and Surface Applications

235

electric dipole moment. For bulk media at common electric field strengths this is often described by the constitutive equation (1)

where E is the applied field, Z (chi) is the electric susceptibility of the medium, and GO is the vacuum permittivity. As so defined, P essentially represents the induced dipole per unit volume, and is known as the polarisation of the medium. (In SI units, Z is dimensionless and P has units of C m·2). Whilst this equation correctly applies to an isotropic system, any reduction in full symmetry, as for example in an axial crystal, can lead to the electric susceptibility having a direction dependence. This simply reflects the fact that the charges within such a structured system are more easily displaced in certain directions than in others. In any such case, the polarisation need not necessarily be induced in the same direction as the field; for generality Eq. 1 should then be re-cast in vector form as; (2) with 'X. acquiring the status of a second rank tensor (representable as a 3 x 3 matrix, each row and each column relating to x, y or z). For the polarisation component in a particular direction i, we thus have

Pi =

L

EO%ijE j

(3)

j=x,y,z

or more concisely, using the convention of implied summation over repeated indices, (4) Eq. 4 represents the classical linear response of a medium to an applied electric field. The signal f it generates, the actual observable, is then expressible as (5)

-

where E is the electric field vector of the output radiation. For a medium comprising molecules or other sites with a distinct electronic integrity (such as chromophore groups in large molecules, or lanthanide ions in doped crystals), Eq. 5 can be written as;

DAVID L. ANDREWS and STEPHEN R. MEECH

236

(6)

to explicitly identify the net response as originating from each of the N sites (molecules). In describing interactions with electromagnetic radiation, where the applied field is time-dependent, then for a given point r within the medium, at a given time t, we can generally write for the field associated with a given radiation mode, (7)

What is meant by mode here is an optical wave, whose electric field oscillates in the plane defined by its amplitude vector Eo and its wave-vector k, the latter pointing in the direction of propagation and having magnitude (8)

In turn, the circular frequency (f)

(i)

(radians per unit time) is given by

=27tV =21!c/ ;., ,

(9)

where A. is the wavelength and n(// is the refractive index of the medium at frequency (i). For later reference, it is worth noting that Eq. 7 may also be written as a superposition of two traveling waves; (10)

with (11) (12)

these forms prove useful in dealing with the quantum mechanics of radiation-matter interactions, since it transpires that E(+) is involved in photon annihilation and E(-) in creation. Clearly the electric polarisation given by Eqs. 2 and 4 oscillates in phase with the radiation with the same circular frequency (f). The classical picture of light scattering depicts the radiation of this fluctuating dipole as the source of emergent light with the same frequency as the incident light; the

8. Nonlinear Optics and Surface Applications

237

tensor character of Eq. 2 allows for light to emerge in directions other than the direction of incidence, so producing the effect of elastic (Rayleigh) scattering. A significant but often overlooked difference between the applications of Eq. 2 to static and radiative electric fields is that the equation is based on the concept of a dipolar description of charge distribution, which takes no account of any spatial variation in the field itself. Whereas electrostatic fields may well be at least approximately spatially homogeneous, the same is not true for the electromagnetic fields described by Eq. 7. Only over regions of physical dimension much smaller than the optical wavelength is the spatial variation negligible, and the dipole approximation is thus not always strictly appropriate for bulk media at optical frequencies, nor indeed can higher order multipolar correction terms readily be incorporated (Landau and Lifshitz, 1960). The dipole approximation is an assumption underlying essentially the whole of classical optics, and it is good to be reminded that in certain cases dipole response is forbidden.

2.2

Nonlinear Response

Having established the classical picture of conventional light scattering, we now consider the case of nonlinear optical response. At high field strengths the simple description given above fails since it provides only for Rayleigh scattering at frequency OJ (or, through coupling with molecular transitions, at Raman frequencies m ± 11m ). The origin of harmonic emission is generally understood through an extension of the theory as follows. Even within the dipole approximation, Eq. 2 must be recognised as only an approximation based on the expectation of a direct proportionality (linear response) to the applied electric field. In general, however, the response of a material may be more accurately represented in terms of a power series in E, with the right-hand side of (4) as the leading term. Thus in general we may write; (13)

with X now designated the first-order electric susceptibility X(I). When the electric field is not too large, the correction terms are negligible and the response is accurately given by the leading linear term. However at the high field strengths provided by intense lasers, the higher-order terms often cannot be ignored. The second term in Eq. 13 represents a correction due to quadratic coupling with the electric field through the second-order susceptibility, the third term a cubic response and so on. In SI units X(n) has units of (mNy-l. Significant levels of second order optical nonlinearity are usually observed in materials where the corresponding susceptibility X(2) takes values of 10 pm V-lor above - though considerably smaller values

DAVID L. ANDREWS and STEPHEN R. MEECH

238

arise in the surface studies to be examined later in this chapter. As a guide, the ratio of each successive tenn in Eg. 13 to its predecessor is a factor typically in the region of (IIIM)Y" where I is the laser intensity and 1M the level of intensity that would lead to ionisation or dissociation, typically around 10)8±4 W m-2 (Eberly et al., 1987). With an electric field as described by Eq. 7, the response given by Eq. 13 may be expressed as follows;

P;(t) = c o[ ｘｾｉＩ＠

t X ｾＡｊ＠

EOj COStUt +t ｘｾ［Ｉ＠

EojEOk (cos2tUt + 1) (14)

EOj EOk Eo/ (COStUt + 3costUt)+ ... ]

evaluated at r = 0 for convenience. From this it transpires that it is the quadratic tenn involving the second-order susceptibility that constitutes a source at the second harmonic frequency 2w, higher-order harmonics being associated with the following tenns in the series. Thus harmonic generation in general represents a fonn of light emission that depends on nonlinear optical response to the electric field of intense (laser) radiation. Notice one failing of the classical picture at this point; it leads to the obviously false result that a system exposed to even one photon can, through quadratic interaction mediated by a second order susceptibility, weakly generate a second harmonic output - thereby violating energy conservation! Other fonns of nonlinear optical interaction can also be understood on the basis of Eq. 13. For example if two beams with the same linear polarisation but different frequencies cq and ｾ＠ are brought together in a medium exhibiting second-order optical nonlinearity, a similar development reveals that signals at the sum and difference frequencies (w) ± ｾＩ＠ are produced. In this sense second harmonic generation can be considered a special case of sum-frequency generation (SFG); whilst SHG delivers an output of wavelength »2, SFG delivers (1/A)+ 1/ｾｲＩＮ＠ To complete the classical description, we note that there is a microscopic counterpart to Eq. 13 directly expressed in molecular tenns, and represented in SI units by;

where Jl ind is the induced molecular dipole moment, a. is the molecular polarisability (J m2 y-2), f3 the hyperpolarisability (J m3 y-3), and y the second hyperpolarisability (J m4 y-4) etc. The only significant differences from the macroscopic fonnulation at this point are firstly that noncentrosymmetric molecules may additionally possess an intrinsic (pennanent) electric dipole moment, so that the total molecular dipole should be represented as

239

8. Nonlinear Optics and Surface Applications

(16) and secondly, that in Eq. 15 the power series is expressed not in terms ofE but d, where d is the electric displacement field; this is the local electric field actually experienced by each molecule, as modified by the electrical influence (polarisation field) of neighbouring molecules: (17) The use of lower-case symbols in (15) and (17) here signifies that both the field and material parameters refer to the optical response at the local microscopic level; the electric field e is the counterpart of the macroscopic field E, whilst the local polarisation p in any isotropic (or cubic) medium is related to the macroscopic polarisation P by P = P (Jackson, 1975). It is worth noting that, because the polarisation is itself involved in the displacement field, it is not possible to directly equate terms of the same order in Eqs. 13 and 15. The correct procedure for disentangling the intricate relationships between microscopic and macroscopic susceptibilities in classical terms is due to Bedeaux (1973).

-t

2.3

Coherence and Wave-Vector Matching

As indicated earlier, the quantum mechanical picture of any linear or nonlinear optical interaction generally makes use of the electric field expressions (11) and (12), since the former proves to be involved in each photon absorption and the latter in each photon emission. An important consequence is that in a process of harmonic conversion, for example, a phase factor arises that differs from molecule to molecule, and in which the time variable disappears (the exclusion of any other terms is what is known as the rotating wave approximation). Let us take a specific example. In the production of a second harmonic by a molecule positioned at r = RI;' we have a product of two phase factors e,ikoR;-ltJI) associated with the two incoming photons of wave-vector k and frequency 00, and one phase factor

e -,i ｫＧｯｒｾ＠

-2ltJ1) associated with the output photon of wave-vector k' and frequency 200. Their product gives an overall phase factor ･ｩ､ｫｯｒｾ＠ , where in this particular case of SHG, the wave-vector mismatch ｾｫ＠ is defined by (2k - k'). With these considerations, the upshot of Eqs. 6 and 10-13 is in general an expression for the nonlinear optical signal of the following form;

240

DAVID L. ANDREWS and STEPHEN R. MEECH N

f

OC

"ｾ＠

2

X

(n) "k ... lj

E- . E O' E Ok ••• e i.:\k·R·< ; .

(18)

J

I

ｾ＠

As this represents the collective response of N sites or molecules, and the summation over the molecules is inside a modulus squared, the signal contains N 2 contributions, of which N are diagonal (single-centre) and the other (N 2 - N) off-diagonal (two-centre). Since they lead to quite different results, it is convenient to separate them, rewriting (18) as;

f

ｾ＠

1

(n)

OC L. Xijk ...

E- E E j

OJ

Ok··· e

ｪ｡ｫﾷｒｾ＠

12

-"I N

-

(n) E- E E 12 L. Xijk ... j OJ ok"i ｾ＠

ｾ＠

ｾ＠

ｾ＠

N

(n)

+ L.L.Xijk ...

ｾ＠

N

E- E E j

OJ

jak·R, -In)

Ok·"e

• Xlmn ...

E-;: :; E- EI

Om

On··· e

-jak·R,.

•

ｾＧ＠

_

" " (n) E- E E -In) E- E- Eｪ｡ｫﾷＨｒｾＭＮＩ＠ + L.L.Xijk ... j OJ Ok",Xlmn.- I Om On"£ ｾ＠

ｾＧ＠

(19) where the substitution of lmn for ijk in the conjugate term reflects a rule that no index appears more than twice; since there is implied summation, all are dummy indices and there is no physical significance. Here there are several important things to note. In the top line of Eq. 19, each contributor to the first (single-centre) term carries a phase factor which disappears on taking the modulus square, as shown in the line below. So, these contributions are ultimately independent of any directive phase factors, and as such they reflect an incoherent signal. These terms differ markedly from the twocentre terms, dependent as they are on the relative molecular displacements R ｾ＠ - ｒｾＮ＠ . In most homogeneous media the effect of the phase factor involving this separation, at the end of the second line in Eq. 19, would be to result in a net destructive interference between the signals associated with each site pair. However for these terms, experimental achievement of wavevector matching ｾｫ＠ ;; 0 leads to each phase factor disappearing; the (N 2 N ) contributions then all add constructively to give a coherent signal that greatly outweighs the incoherent signal. We note the emergence here of several other important principles of nonlinear optics. It turns out that the relatively weak incoherent signal is always present, provided the material has the right symmetry to support the appropriate level of nonlinear susceptibility. In the second harmonic case, for example, this incoherent signal termed hyper-Rayleigh scattering gives rise to a weak, essentially non-directed (though not isotropic) signal in most media comprising non-centro symmetric molecules. The reason for the symmetry constraint on the molecules themselves is that only noncentrosymmetric molecules can support the necessary hyperpolarisability.

8. Nonlinear Optics and Surface Applications

241

The achievement of wave-vector matching, however, produces highly directed second harmonic emission. If refractive effects were ignored, the twin requirements of energy conservation, ro = 2ro, and wave-vector matching, k' = 2k, would ensure emission exactly in the direction of the fundamental (pump beam) throughput. In practice, refractive effects in solids generally mean choosing conditions to ensure that the refractive indices at the fundamental and harmonic frequencies match (indexmatching), as follows from Eq. 8. It is here that another feature emerges. The bulk symmetry of the medium plays its own part in determining the sustainability of the coherent signal; in particular it is lost in any centro symmetric or isotropic medium (such as most fluids). For this reason any directed second harmonic signal emerging from such a medium has to originate at its surface where bulk symmetry no longer holds. The surfacespecificity of the technique thereby affords it a number of important analytical applications, as will be discussed below.

3.

SURFACE APPLICATIONS OF SECOND ORDER .NONLINEARITY The second term in the nonlinear response Eq. 14, represents a source at

2m, the second harmonic frequency. It is evident from (14) that, in the case

of a medium possessing inversion symmetry, i 2l is zero. Consider the case in which measurements are made with the signs of the electric fields inverted. On symmetry grounds one expects inversion of p;(2) , while Eq. 14 leaves it unchanged. The only escape from this contradiction is for

zW to

be zero. This is an important result, which forms the basis for a family of methods of surface analysis through second harmonic generation and sumfrequency generation. The topic has been extensively reviewed (Andrews, 1993; Com, 1991; Com and Higgens, 1994; Eisenthal, 1992, 1993, 1996; Heinz, 1991; Meech, 1993; Shen, 1986, 1989). Consider the intense laser irradiation of an interface between two media with macroscopic inversion symmetry, which might be a gas and a single crystal metal, two liquids, two solids, etc. Neither medium will itself yield any signal at the second harmonic frequency. However, at the interface, inversion symmetry is necessarily absent, so i 2l is non-zero and a second harmonic signal can be observed. Thus the second term of Eq. 14 leads directly to a surface-specific signal, a feature distinguishing it from linear optical spectroscopy. This signal is entirely optical in nature, and therefore does not require one of the media to be a vacuum and the other a metal (a feature which distinguishes it from the family of 'traditional' surface science methods, often based on electron or ion scattering). This SHG (and SFG) method of surface analysis was pioneered by Shen and co-workers in

DAVID L. ANDREWS and STEPHEN R. MEECH

242

the 1980s. The very wide range of interfaces that may be studied with these methods has led to a dramatic and continuing expansion in their application. In the remainder of this chapter the signal strength of SHG will be discussed. This will be followed by a description of the data contained in polarization-resolved SHG measurements. The major applications of the surface SHG signal are in the study of adsorbate orientation, surface kinetics and surface spectroscopy. The underlying principles of each are briefly outlined, and a number of examples are cited by way of introduction to the literature.

3.1

Practicalities

Since surface SHG measurements constitute nonlinear optical measurements on essentially a monolayer of material, strong signals cannot be expected. By combining the equations for radiation from a polarised slab with an expression for the nonlinear polarisation the following general expression for the intensity of sum-frequency generation has been obtained: (Heinz, 1991; Shen, 1989; Mizrahi and Sipe, 1988)

in which the two incident frequencies are cq and ｾＬ＠ the sum frequency is n, the incident intensities are I( w), the illuminated area is A, the collinear angle of incidence is ｾＬ＠ and .s( Wi) are the frequency-dependent dielectric constants of the medium through which the beams propagate. The frequencydependent factors e are products of polarisation vectors and Fresnel coefficients (Jenkins and White, 1981), representing a modification of the incident electric field so that the polarisation is expressed in a form appropriate for an irradiated surface. Finally the second order surface nonlinear susceptibility is ｚｾＲＩＮ＠ Eq. 20 applies to both SFG and SHG, in the classical limit. The SFG case is of great importance when one of the incident frequencies is in the infra-red region, and the other in the visible. In this situation the vibrational spectra and dynamics of adsorbates may be monitored (see below) with detection in the visible, where sensitive detection methods are available. In the following the focus will be on SHG (WI = ｾＩＬ＠ but it should be borne in mind that the methods are readily extended to the case of SFG. For the purpose of estimating the signal strength it is convenient to rewrite Eq. 20 as an expression for the number of second-harmonic photons generated per second: (Shen, 1989; Heinz and Reider, 1989)

8. Nonlinear Optics and Surface Applications

S(2m) =(47l"2w)sec2Bjp21 3 Co Ii c

r

R

243 (2)

12

(21)

XS,qJ'

in which P is the laser power, ,the laser pulsewidth and R the laser repetition frequency. The effective surface second-order nonlinear incorporates the geometry-dependent Fresnel factors. susceptibility, ｘｾ［ｦＧ＠ Eq. 21 shows that a 'typical' laboratory laser source, for example a 10Hz, 1 W Nd:YAG laser with a 10 ns pulsewidth operating at the fundamental of 4 frequency, irradiating a I cm2 area of surface characterised by a ｘｾｦ＠ x 10.21 m V-I, will yield a signal of 10 second harmonic photons per second. (For consistency the SI unit of nonlinear susceptibility is given here, although in the literature the more common practice is to report the value in electrostatic units; the confusing picture with units has been addressed usefully in Svirko and Zheludev (1998.) Such small signals, even at the level of I count per second, are readily measured as the second harmonic signal is monochromatic and highly directional (see above), allowing all the value given is appropriate for a photons to be detected. The ｘｾＺ［ｦ＠ monolayer of weakly nonlinear molecules that are non-resonant with either the fundamental or second harmonic frequencies. If the surface is covered with molecules of larger hyperpolarisability, or resonance enhancements are exploited, or the surface itself is highly polarisable (as for a metal), then the harmonic signal may be many orders of magnitude larger. In such cases it is very easily detected.

Det.

Figure J. Typical experimental geometry for surface SHG measurement. Polarisation of the laser at frequency 0) is adjustable by a half-wave plate (AJ2). The beam is routed to the surface, through a long wavelength pass filter (LPF) to remove SHG generated by the optics, and impinges at angle ｾＮ＠ Reflected SHG at 20) appears at angle il; if the medium is dispersive, n?' OJ, since sinO/sinn = n2Jn.,. The SHG, separated from the fundamental by short pass filter SPF, is resolved into s- and p- polarised components by polariser P prior to detection.

DAVID L. ANDREWS and STEPHEN R. MEECH

244

Eq. 21 is also useful in the design of surface SHG (or SFG) experiments. The basic experimental set-up is very straightforward (Figure 1) and in using such a system the main choices for the experimentalist are the nature of the laser and the detection system. From (21) it follows that the largest signal would be generated if all of the available laser power were to be concentrated in a single ultrashort pulse, focused to the minimum area. Of course the limitation here is surface damage. Much early work of high quality was performed using sources with the parameters of the 10Hz Nd: YAG laser in the example above. However, pulse-to-pulse stability in such systems is poor, requiring long averaging times, and the associated damage problems are quite severe. Subsequently mode-locked quasi-cw Nd: YAG lasers were employed. These provide a high repetition rate (R = 100 MHz), high power (several Watts), short pulses (r = 100 ps) and high pulse-to-pulse stability. Such sources permit the use of photon counting detection and also the advantage, when used in conjunction with a chopper, of lock-in detection. This set-up permits the rapid acquisition of signals, which in turn allows the dynamics of chemically intricate surface systems to be studied, such as those that occur at the electrode during cyclic voltammetry, for example. The principal disadvantage with these quasi-cw systems is the possibility of significant surface heating. Most recently, stable ultrafast laser systems such as the self mode-locked titanium:sapphire laser have been used. These deliver extremely short (50 fs) pulses with good pulse-to-pulse and long term stability, again at high repetition rates. The low power « 1 W) further reduces the problem of surface heating, and is more than compensated for by the nonlinear signal enhancement associated with the reduction in pulsewidth.

3.2

The Elements of ｘｾＲＩ＠

The second rank tensor ｘｾＲＩ＠

comprises 27 elements, ｘｾＩＮ＠

Fortunately the

number of non-zero elements generally reduces by symmetry (ultimately to zero, in the case of an isotropic medium). For SHG the index symmetry relation X ｾＩ＠ = X ［ｾＩ＠ applies and the number of distinct elements reduces to 18. If the symmetry of the interface is known, or may be assumed, many of the remaining 18 elements may be zero, or turn out to be equal or opposite to other elements. The non-zero elements for several different interface symmetries have been tabulated by Heinz (1991). A particularly common symmetry is one with 2D isotropy in the surface plane (azimuthal isotropy). This is expected for adsorbates at liquid/vapour and non-crystalline solid / isotropic fluid interfaces. Such symmetry can be assumed if the signal is observed to be invariant to rotation of the sample about the surface normal. Application of the relevant symmetry operations leaves only three non-zero independent elements, and these may be separately determined by

245

8. Nonlinear Optics and Surface Applications

polarization-resolved experiment (Dick, 1985; Heinz, 1991). The relevant polarisations Pi are calculated from:

＠ｾ Py

=&0

ｾ＠

0 0

0 0

0 0

Z= Z= Zzzz

0

Z.= 0

Z:= 0 0

ｾ＠

E2x E2y E2z 2E E y 2Ez E x

(22)

Z

2ExEy

Eq. 22 makes possible a number of useful predictions. If the fundamental beam is s- polarised, P x = P y = 0 and P z = XzxxE/ so that 1(2(1) is purely ppolarised, and dependent on a single tensor component. Similarly a ppolarised fundamental beam also leads to a purely p-polarised /(2(1). An spolarised signal will only be observed, for this surface symmetry, with a mixed input polarisation (a. =I:- 0° or 90°). Furthermore, all non-zero elements of Z 2 ) contain a z index, dictating that for normal incidence the

1

harmonic signal intensity 1(2(1) = 0; this is an important consideration when the weak second harmonic is to be isolated from the intense fundamental by means of a bandpass filter. Such polarisation selection rules prove generally useful in characterising the SHG signal. In Eq. 22 the fields Ei are those experienced in the interfacial layer, which may be calculated from the incident field as a function of polarisation angle;

Ex acosa Ey = csina Eo, Ez bcosa

(23)

where the factors a, b and c are the effective Fresnel coefficients for transmission of the fundamental beam from the incident medium into the surface. These coefficients have been tabulated for a three-layer slab geometry (Shen, 1989), for the simpler single interface system (Mizrahi and Sipe, 1988), for more complex interface structures (Grudzkov and Parmon, 1993) and for a total internal reflection SHG geometry (Mizrahi and Sipe, 1988). Eqs. 22 and 23 may be combined, using Eq. 14, to obtain Pi' which on substitution into the equations of Bloembergen and Peshan (1962) (see also Heinz, 1991, and Zhang et al., 1990), yields the following results for the p- and s-polarisation resolved second harmonic intensity, measured as a

DAVID L. ANDREWS and STEPHEN R. MEECH

246 function of a:

(24) and (25) where the constants A and B are given by;

A =a

'V(2)

l.ILxa

B=a

+ a2.IL= 'V(2) + a 'V(2) 3.IL= ,

'V(2)

4.IL= '

C=a

'V(2)

5.ILxa '

(26) (27) (28)

and the a j ( i = 1 - 5) are combinations of the (generally complex) Fresnel factors determined by the angle of incidence and the refractive indices of the interface and substrate. Explicit expressions for the Fresnel factors have been derived elsewhere (Heinz, 1991; Shen, 1989; Mizrahi and Sipe, 1988). By evaluating these and fitting the polarization-resolved data to (24) and (25) the relative values of the three independent elements of Z ｾＲＩ＠ may be obtained. An example of such a fit is shown in Figure 2, exhibiting the variation of second harmonic intensity obtained from a protonated dye on an acidic sub-phase as a function of surface pressure (Lin and Meech, 2000). Note the failure of the p-polarised data in Figure 2(b) to return to the baseline. This is indicative of the existence of a nonzero (or 1t) value for the relative phase, , between A and B. This may arise from either complex Fresnel coefficients or from contributions of more than one element of the molecular hyperpolarisability (see below) to the signal. A major problem with this type of measurement lies in the value assumed for the dielectric constant of the interfacial region, which in the (usual) case of a resonantly enhanced signal will be complex. Different model assumptions have been discussed by Grudzkov and Parmon (1993): nonetheless it should be borne in mind that the unknown value of the dielectric constant is one factor that limits the reliability of the determination of Z ｾＩ＠ .

247

8. Nonlinear Optics and Surface Applications

(a)

1F1.0mNlm

Jt=13.7mN1m

o

60

(b)

120

180

240

300

360

Polarisation angle, y(deg.) Figure 2. The p (e) and s (0) components ofSHG intensity, measured as a function of input polarisation angle, a. Data are shown at pressures well below (1.0 mNm- l , a) and above (13.7 mNm· ' , b) a phase transition detected in surface pressure - area measurements. Data have been fitted to Eqs. 24 and 25. Note the failure of the p-polarised SHG signal at 13.7 mNm- 1 to return to the baseline for any value of a, indicating the importance of the phase tenn in Eq. 24 (Lin and Meech, 2000).

3.3

Applications in Surface Science

In most instances one is interested in extracting the microscopic (molecular) infonnation contained in the macroscopic measurement of surface susceptibility. The key relationship between the measured, macroscopic X ｾｩＩ＠ and the molecular parameters of interest is (Heinz, 1991; Meech, 1993; Corn and Higgins, 1994; Eisentha1, 1992, 1996):

DAVID L. ANDREWS and STEPHEN R. MEECH

248

(29) where the /3iJ'k' represent the 27 elements of the molecular hyperpolarisability tensor. The prime indicates the molecular frame coordinate system, the z' axis being taken as the principal symmetry axis of the adsorbate. The matrix (TA).,,) transforms between the surface and molecular frames, and the number density of adsorbates is N's' This expression, (29), suggests the possibility of exploiting the surface SHG method to secure several kinds of information: adsorbate orientation, through the connection of the molecular to the laboratory frame; adsorbate kinetics, through the dependence on N s; adsorbate spectroscopy, through resonances in the molecular hyperpolarisability. We discuss each of these in tum below. Orientation The transformation between the molecular frame and the laboratory frame, in which the polarisation resolved measurements of Eqs. 24 and 25 are defined, is achieved through the product of three 3 x 3 Euler angle matrices (Goldstein, 1969). The three angles required are: (), the angle between the molecular z' axis and the surface normal; If, the angle through which the molecule is twisted about its z' axis; and ¢, the angle between the y axis and the projection of z' onto the surface. Thus the measured X ｾＩ＠ may be expressed in terms of these angles and the molecular

hyperpolarisability, /3ijk' As we have seen with the susceptibility, the number of finite elements in !3ijk also generally reduced by symmetry. For a molecule with C2v symmetry, seven elements survive. Most molecules that have been studied by SHG are planar aromatics, for which the dominant terms in the hyperpolarisability entail in-plane components. In such cases, elements containing the y' index may be set equal to zero. With this approximation three independent elements survive, /3=, /3=, and /3=. With the further approximation that the angle ¢ is randomly distributed in the plane, three relatively simple and compact expressions are obtained for the measured nonlinear susceptibilities in terms of the molecular hyperpolarisability and the angles () and If (Heinz 1991);

ｸｾ＠

ｘｾＺＡ＠

= Ns L(cos3 0) fJz·z·z· + (cosO sin 2 0 sin 2 'II) (ftz·x·x· + 2fJx·x·z·) J =

ｾ＠

L

N, (cosOsin 2 0) pz,z'x' +( coso) Pz'x'x' -

(30)

(coso sin 2 0 sin 2 If/)(Pz'x'x' + 2Px'x'z') J

ｸｾ＠

(31)

=ｾ＠ N, L(coso sin 2 0) Pz'z'x' + (cosO) Px, xg) » ｘｾＲＩＮ＠ (Robinson and Richmond, 1990) For time-resolved measurements on timescales longer than a few seconds it is sufficient to simply monitor the time-dependent SHG intensity. Measurements of adsorption isotherms (Heinz et ai., 1983), molecular adsorption (Shen, 1986), dissociative adsorption (Reider and Heinz, 1991), adsorbate reorientation during film compression (Lin and Meech, 2000) and temperature-programmed desorption (Xhu et at., 1985) have all been reported. For measurements on microsecond timescales it is essential to employ mode-locked quasi-cw laser sources with sensitive detection, and to have some means of repetitively cycling the system. Several such studies of the kinetics of electrodes have been made (Richmond et at., 1988; Com and Higgens, 1994). For measurements on an ultrafast timescale more complex experimental geometries are required. Typically a conventional pum{}-probe geometry (Fleming, 1986) is modified such that the pump pulse perturbs the surface, and the second harmonic signal generated by a time-delayed probe pulse is used to monitor the recovery of the surface. The method was initially used to measure excited state relaxation dynamics at liquid-air (Sitzmann and Eisenthal, 1988), solid-air and solid-liquid interfaces (Meech and Yoshihara, 1989, 1990). For example, the dynamics of excited state torsional motion for an adsorbate at a surface has been reported (Morgenthaler and Meech, 1992, 1993). Experiments with better time resolution and signal to noise have allowed time- and polarization-resolved measurements to be performed, permitting the observation of orientational

8. Nonlinear Optics and Surface Applications

253

motion in the interface (Zimdars et al., 1999). Very recently the method has been extended to permit the measurement of ultrafast solvation dynamics at liquid interfaces (Shang et al., 2001). The method is not restricted to the study of excited state dynamics. For analysing the vibrational dynamics of adsorbates the ir + visible SFG analogue of SHG can be used. The method was fIrst described by Harris et al. (1991) and used to measure the population relaxation of adsorbate CH modes. It has recently been extended to investigate the reaction induced by vibrational excitation of CO adsorbed on ruthenium (Bonn et al., 2000). Measurements of the dephasing dynamics of adsorbate vibrational transitions were also reported, using ir photon echoes with SFG to probe the echo intensity (Guyot-Sionnest, 1991). Spectroscopy It is well known that the molecular polarisability exhibits resonances when the applied electromagnetic field is of the same frequency as a molecular excitation. The same is true for the hyperpolarisability except that additional resonances also arise if the applied fIeld equates to the second harmonic. The key relationship between molecular hyperpolarisability and electronic structure was given by Ward (1965); in an undamped, simplifIed form for resonant SHG the result is

in which Llrn is the dipole moment of the resonant excited state, rng is the transition dipole moment of the resonant transition and mng is the transition frequency. It is easy to see from Eq. 35 how resonance enhancement can make one or two elements of the hyperpolarisability dominant; for example if the dipole moment and transition dipole lie along the molecular z' axis, then pz.z.z. will be dominant. In addition, from Eq. 35 it is evident that a molecule with a large excited state dipole moment and an intense transition will lead to a large nonlinear signal. These facts have been exploited in numerous efforts to develop highly efficient organic nonlinear optical materials. As seen above, the SHG signal will be resonantly enhanced whenever (i) or 2m matches the molecular transition frequency, mng• In practice, for most adsorbates on insulator substrates the signal is indeed too weak to be useful unless a resonance enhancement is exploited. If the incident frequency is scanned, the electronic spectrum of the adsorbate can be obtained. Such measurements have been made, but generally the electronic spectra of adsorbates are too broad to be highly informative.

DAVID L. ANDREWS and STEPHEN R. MEECH

254

More detailed spectroscopic information can be obtained from ir + visible SFG, in which case the vibrational frequencies of the adsorbate can be determined. It is instructive to rewrite Eq. 35 for SFG as; (36)

in which liJq are the frequencies of the adsorbate vibrational transitions, {i)1T the frequency of the incident radiation, .dp the population difference between the ground and vibrationally excited state, and the Fq term signifies explicit inclusion of damping. The terms Ak and M;j are respectively the ir and Raman transition probabilities; this introduces an SFG selection rule: to be observed as a resonance in SFG, the vibrational transition must be both ir and Raman allowed. The strongest SFG signal will be generated when both terms are large, though in general a transition with large ir moment tends to have a weak Raman excitation probability, and vice versa. The SFG spectra may be generated using a fixed-frequency visible laser and a tunable source of ir, the latter usually utilising optical parametric amplification or difference frequency generation in a nonlinear crystal. More recently, broad-band ir sources have been used in conjunction with narrow-band visible lasers to yield the entire SFG spectrum, which is then dispersed and detected by a charge-coupled device (CCD) (van der Ham et ai., 1996). In either case it should be recalled that both the substrate and adsorbate contribute to the signal, and that signal components from these two sources may have a phase shift between them. Thus, particularly on metal surfaces where the non-resonant background is strong, the vibrational spectra may have an odd appearance due to the interference term, and in some cases will appear as a decrease in SFG intensity (Harris et ai., 1987). In spite of these difficulties and complications (and, at least before the advent of reliable high-repetition rate solid-state lasers, the considerable difficulties in generating intense coherent infra-red) the SFG method has proved a powerful one in the determination of adsorbate vibrational spectra. Adsorbate CH modes, particularly of adsorbates possessing long alkyl chains, have been widely studied, both on solid surfaces (Hunt et ai., 1987) where orientational data have been recorded (Ward et ai., 1993) and on liquid surfaces (McKenna et ai, 2000). The SFG spectra of reactive systems on metal surfaces have also been reported (Miragliotta et ai., 1990) and the spectra of the surface of pure liquids have been obtained (Superfine et ai., 1991).

4.

SUMMARY From its inception in the observation of almost impossibly weak optical

8. Nonlinear Optics and Surface Applications

255

hannonics, the field of nonlinear optics has grown and developed with the progressive availability of highly intense laser sources and powerful detection systems. As a fully-fledged discipline, it now finds applications ranging from routine laser frequency conversion to highly advanced methods of materials analysis. In studies on a great number of surface and interface systems, with all their associated physical and chemical intricacy, second hannonic generation in particular has proved a technique that can deliver information inaccessible by other methods. As our ability to interpret results on the basis of sensible physical mechanisms has also progressed, the precision and quality of that information has improved. It appears that surface nonlinear optics is a discipline with a secure future.

REFERENCES Andrews, D.L., 1993, Modern Nonlinear Optics, Part 2. (M.W. Evans and S. Kielich, eds.), Adv. Chem. Phys. 85:545. Andrews, D.L., 1997, Lasers in Chemistry, 3rd edn.. Springer-Verlag, Berlin, Heidelberg, New York. Andrews, D.L., and Allcock, P., 2001, Modern Nonlinear OptiCS, Part 1. (M.W. Evans, ed.), Adv. Chem. Phys. 119:603. Bedeaux, D., 1973, Physica 67:23. Bloembergen, N., and Pershan, P.S., 1962, Phys. Rev. 128:606. Bonn, M., Hess, C., Funk, S., Miners, 1. H., Persson, B. N. 1., Wolf, M., and Ertl, G., 2000, Phys. Rev. Lett., 84:4653. Byers, J. D., Vee, H.I., Petralli-Mallow, T., and Hicks, 1. M., I 994a, Phys. Rev. B 49:14643. Byers, J. D., Vee, H. I., and Hicks, 1. M., I 994b, J. Chem. Phys. 101: 6233. Com, R.M., 1991, Anal. Chem. 63:4285. Com, R.M., and Higgins, D.A., 1994, Chem. Rev. 94:107. Crawford, M. J., Haslam, S., Probert, 1. M., Grudzkov, Y. A, and Frey, A. G., 1994, Chem. Phys. Lett., 229:260. Dick, B., 1985, Chem. Phys., 96:199. Eberly 1.H., Maine P., Strickland D., and Mourou, G., 1987, Laser Focus 23(10):84. Eisenthal, K.8., 1993, Ace. Chem. Res. 26:636. Eisenthal, K.B., 1992, Ann. Rev. Phys. Chem. 43:627. Eisenthal, K. 8., 1996, Chem. Rev., 96: 1343. Fischer, P., and Buckingham, A. D., 1998, J. Opt. Soc. Am., B, 15:2951. Fleming, G. R., 1986, Chemical Applications of Ultrafast Spectroscopy. Oxford University Press. Franken, P.A., Hill, A.E., Peters, C.W., and Weinrich, G., 1961, Phys. Rev. Lett. 7:118. Goldstein, H., 1969, Classical Mechanics, sixth edition. Addison-Wesley, New York Grudzkov, Y.A, and Pannon, V.N., 1993, J. Chem. Soc. Faraday Trans. 89:4017. Guyot-Sionnest, P., 1991, Phys. Rev. Lett. 66:1489. van der Ham, E. W. M., Vrehen, Q. H. F., and Eliel, E. R., 1996, Opt. Lett., 21:1448. Harris, A.L., Chidsey, C.E.D., Levinos, N. J., and Loiacono, D.N., 1987, Chem. Phys. Lett. 141:350. Harris, AL., Rothberg, L., Dhar, L., Levinos, N. 1., and Dubois, L.H., 1991, J. Chem. Phys. 94:2438. Hecht, L., and Barron, L. D., 1996, Mol. Phys., 89:61. Heinz, T.F., 1991, Second order nonlinear optical effects at surfaces and interfaces. In Nonlinear Electromagnetic Phenomena (H.E.Ponath and G.E. Stegeman, eds.), Elsevier, Amsterdam.

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Heinz, T. F., Tom, H. W. K., and Shen, Y. R., 1983, Phys. Rev. A., 28:1883. Heinz, T.F., Loy, M.M.T., and Thomson, W.A., 1985, Phys. Rev. Lett. 54:63. Heinz, T. F., and Reider, G. A, 1989, Trends in Analytical Chemistry 8:235. Hunt, 1. H., Guyot-Sionnest, P., and Shen, Y.R., 1987, Chem. Phys. Lett. 133:189. Jackson, 1.D., 1975, Classical Electrodynamics. Wiley, New York. Jenkins, F.A., and White, H.F., 1981, Fundamentals of Optics, fourth edition. McGraw-Hill, New York. Kaurenen, M., Verbiest, T., Maid, J. 1., and Persoons, A, 1994, J. Chem. Phys., 101:8193. Landau, L.D., and Lifshitz, E.M., 1960, Electrodynamics in Continuous Media. Pergamon, New York. Lin, S., and Meech, S. R., 2000, Langmuir 16:2893. McKenna, C. E., Knock, M. M., and Bain, C. D., 2000, Langmuir 16:5853. Meech, S.R., and Yoshihara, K., 1989, Chem. Phys. Lett. 154:20. Meech, S.R., and Yoshihara, K., 1990, Chem. Phys. Lett. 174:423. Meech, S.R., 1993, Kinetic applications of surface nonlinear optical signals. In Advances in Multiphoton Processes and Spectroscopy (S.H. Lin, A Villaey and Y. Fujimura, eds.), World Scientific, Singapore. Miragliotta, J., Polizzotti, R. S., Rabibowitz, P., Cameron, S. D., and Hall, R. B., 1990, Chem. Phys., 143:123.

Mizrahi, V., and Sipe, J.E., 1988, J. Opt. Soc. Am. 85:660. Morgenthaler, MJ.E., and Meech, S.R., 1992, Ultrafast torsional dynamics in adsorbates. In Ultrafast Phenomena V/II (J-L. Martin et al., eds.), Springer Series in Chemical Physics 55:606. Morgenthaler, MJ.E., and Meech, S.R., 1993, Chem. Phys. Lett. 202:57. Petersen, E. S., and Harris, C. B., 1989, J. Chem. Phys. 91:2683. Petralli-Mallow, T., Wong, T. M., Byers, 1. D., Vee, H.I., and Hicks, J. M., 1993,J. Phys. Chem.97:1383.

Reider, G. A., and Heinz, T. F., 1991, J. Chem. Phys. 94:4080. Richmond, G.L., Robinson, 1.M., and Shannon, V.L., 1988, Progress in Surface Science 29:1. Robinson, J. M., and Richmond, G.L., 1990, Chem. Phys. 141: 175. Simpson G. J., and Rowlen, K. L., 1999, J. Amer. Chem. Soc. 121:2635. Simpson G. J., Westerbuhr, S. G., and Rowlen, K. L., 2000, Anal. Chem. 72:887. Simpson G. J., and Rowlen, K. L., 2000, Anal. Chem. 72:3399. Singhal, R., 1995, Nonlinear Optics. In An Introduction to Laser Spectroscopy, 1st edn. (D.L.Andrews and AA Demidov, ed.), Plenum, New York. Sitzmann, E. V., and Eisenthal, K.B., 1988, J. Phys. Chem.92:4579. Shang, X., Benderskii, A. V., and Eisenthal, K. B., 2001, J. Phys. Chem., B, (in press). Shen, Y.R., 1986, Ann. Rev. Mater. Sci. 16:69. Shen, Y.R., 1989, Ann. Rev. Phys. Chem.40:327. Superfine, R., Huang, lY., and Shen, Y.R., 1991, Phys. Rev. Lett. 66:1066. Svirko, Y. P. and Zheludev, N. I., 1998, In Polarisation ofLight in Nonlinear Optics. Wiley, Chichester. Ward, IF., 1965, Rev. Mod. Phys. 37: 1. Ward, R. N., Davies, P. B., Bain, C. D., 1993,J. Phys. Chem. 97:7141. Yariv, A, 1989, Quantum Electronics, 3rd edn.. Wiley, New York. Zhang, T.G., Zhang, C.H., and Wong, G.K., 1990,J. Opt. Soc. Am. 87:902. Zhu, X. D., Shen, Y. R., and Carr, R., 1985, Surf. Sci. 163:114. Zimdars, D., Dadap, 1 I., Eisenthal, K. B., and Heinz, T. F., 1999, J. Phys. Chem. B, 103:3525.

Chapter 9 Tunable Short Wavelength Generation and Applications

ROBERT H. LIPSON, YUJUN J. SHI, and DIANE LACEY Department a/Chemistry, University a/Western Ontario, London, Ontario N6A 5B7, Canada

1.

INTRODUCTION

The worldwide demand for lasers in 2001 makes up an approximately twelve billion dollar market, and represents a remarkable growth of 375% since 1997. While it should come as no swprise that nearly 80% of these sales involves diode lasers, because of their importance to the telecommunications industry, the remaining non-diode based laser economy certainly cannot be ignored. The dominant applications for lasers in the latter category are materials processing (> $1.5 billion), medical applications (> $0.6 billion) and basic research Ｈｾ＠ $0.1 billion) (Anderson, 2001; Sander, 2000). Motivations to generate coherent short wavelength light abound. For example, within a lithography and materials processing context, lasers with outputs in the ultraviolet (UV, 200 nm ｾ＠ A. < 400 nm) and vacuum ultraviolet (VUV, 100 nm ｾ＠ A. < 200 nm) can produce smaller features than those possible with visible and infrared radiation, not only due to the shorter wavelengths involved, but also because of their ability to photoablate surfaces without thermal transformations or damage (Endert et at., 1999). Einstein was the fIrst to consider the case of a two-level system, (levels 11), degeneracy g\, and 12), degeneracy, ｾＩ＠ in radiative thermal equilibrium, coupled by an electric dipole allowed transition. The following relationships between the now so-called Einstein coefficients, B ......2 and B2...... (S.1. units C2 m 2 ]"2 S·2) and A2...... (s·\ proportional to the rates of absorption, stimulated emission and spontaneous emission, respectively, can be derived through a An Introduction to lAser Spectroscopy, Second Edition, Edited by Andrews and Demidov, K1uwer Academic/Plenum Publishers, New York, 2002

257

ROBERT H. LIPSON et al.

258 straightforward kinetic analysis;

(1) and; (2)

The factor of V in Eq. 2 represents a profound limitation at high frequencies (short wavelengths) on how long a population inversion can be maintained for laser action before the excited state population decays by spontaneous emission. Similarly, when the ratio of the stimulated to spontaneous emission rate is considered;

(3)

where u( v) is the photon energy density at temperature T, k is the Boltzmann is the photon energy, it becomes clear that stimulated constant, and h ｾＭＫｬ＠ emission is only important for temperatures where the thermal energy kT is ｾ＠ ｨｾＭＫｬＮ＠ This temperature equals 33 500 K for optical energies in the visible (::::: 2.5 eV), a condition which typically can only be achieved within stars (Silfvast, 1996) - and it is even higher for transitions in the UV and VUV spectral regions. It follows therefore, that lasers are almost never thermally pumped. The input power densities (optical or electrical) required to achieve a population inversion with given gain per unit length scale as VIl v, where v and Il v are the lasing frequency and linewidth, respectively (Hooker and Webb, 1994). Since Doppler linewidths (for gases) scale as v, the required pump power density scales as v4• Consequently, pumping rates are an order of magnitude larger in the VUV than the UV. There, pump densities of - 1 MW cm-3 are not unusual, "and therefore VUV lasers, almost without exception, operate in a pulsed mode.

1.1

UV and VUV Sources Based on Stimulated Emission

There are few genuine three- and four-level lasers operating in the UV and VUV, and typically their tuning ranges are quite limited (Sauerbey, 2000). Table 1 gives a listing of some of the better known pulsed sources.

259

9. Tunable Short Wavelength Generation and Applications

Table 1. Some UV and VUV laser sources based on stimulated emission

Lasing Medium

Electronic Transition

N2

e 3fl u ｾ＠

B3fl g

Lasing Wavelength (nm) 337.1

Reference Dreyfuss 1972a

and

Hodgson,

Patterson et al., 1972 eo H2

ａｬｦｾｸｉｌＫ＠

elflu ｾ＠

XILg+

18l.0 - 197.0

Hodgson, 1971

109.8 - 161.3

Wayant, 1972 Dreyfuss 1972b

and

Hodgson,

Dreyfuss and Hodgson, 1974 Benerofe et al., 1991 F2

D,3fl2g ｾ＠

A,3fl 2u

157

Ohwa and Obara, 1987 Yamada et al., 1989

Xe2

B (A) I (3)Lu+ ｾ＠

XI!;g+

173 ±2

Basov et al., 1971 Koehler et al., 1972 Gerado and Johnson, 1973

Kr2

B (A) I (3)L/ ｾ＠

XI!;g+

145.7 ± I

Hoff et al., 1973

Ar2

B (A) I (3)L/

ｾ＠

XILg+

126.1 ± 0.3

Hughes et al., 1974 Uehara et al., 1985

XeCI

B2L+ ｾｘＲｌＫ＠

KrF

B2L+ ｾ＠

ArF

B2L+ ｾ＠

308

Ewing and Brau, 1975

X2!;+

248

Ewing and Brau, 1975

X2L+

193

Hoffinan et al., 1976

Of the many rare-gas monohalide excimer lasers known (Brau, 1984), the XeCl and KrF lasers are arguably the most important commercially since their outputs can be used to pump tunable dye lasers (Tomin et al., 1978; Uchino et al., 1979; Telle et al., 1981; Bos, 1981). Complete UV coverage is possible with excimer-pumped dye lasers for all wavelengths 2: 312 nm. The upper lasing level of an RgX rare gas halide dissociates to Rg+(np 5; 2p) + X (n'p6; ISO). In the absence of configurational mixing, the ground state, made from Rg (np6; ISO) + X (n'p5; 2p ), is repulsive except for a possible shallow van der Waals minimum. Since doubly-ionized alkali metals, A2+, are isoelectronic with Rg+, it is expected that the A2+X- 4 A+ + X transitions should also exhibit gain. The energies of the upper ion-pair

260

ROBERT H. LIPSON et al.

states however, are strongly dependent on the ionization potential of the species yielding the cation upon excitation. As shown in Table 2 these arguments have led researchers to propose the alkali mono-halide ions as gain media for VUV and :xuv lasers (Sauerbrey and Langhoff, 1985). Table 2. Possible VUV and XUV Lasing Wavelengths for the Alkali Monohalide ions

Excimer

Lasing Wavelength Inm

K2+F

86 - 110

Rb2+P-

104 - 126

Cs2+F

154 - 195

cs2+cr

131 - 205

Cs2+Br-

117 -190

Lastly, VUV lasers operating between 165 run and 260 run have been suggested based on solid state materials such Nd3+, Er3+, and Tm3+-doped LaF3, YF 3, LiYF4 and LuF3 as gain media, pumped with either an electron beam, an H2 laser (Yang and DeLuca, 1976) or a F2 laser (Dubinskii et al., 1992).

1.2

Nonlinear Optical Sources

Today, the generation of tunable coherent UV and VUV radiation by nonlinear optical schemes is commonplace. Here, the short wavelength optical signals originate from an oscillating nonlinear polarization, P( (0), (dipole/unit volume), which can be induced in a material by the strong external electric fields, E((O), of one or more incident laser beams (typically E ｾ＠ 103 V cm- I ; light intensities, ｉｾ＠ 106 W cm-2). The high frequency light originates from anhannonic oscillations of the electrons and nuclei within a nonlinear medium at high incident light intensities. Under moderate laser powers, P( (0) can be written as a converging Taylor series in E( (0);

P((O) = N{l) .E((O) + %(2) : E((O)E((O) + %(3) :E((O )E((O )E((O) + ...} ,

(4)

where N, il), and iO), n ｾ＠ 2 in Eq. 4 are the number density, linear susceptibility, and the nth order nonlinear susceptibilities tensors of the medium, respectively. At the molecular level Eq. 4 can be rewritten as;

9. Tunable Short Wavelength Generation and Applications

261

were Pi is the molecular polarization, aij is the linear polarizability, Pijk is the second-order hyperpolarizability, Yijkl is the third-order hyperpolarizability, and the jkl subscripts label the directions of the electric fields E. While higher order tenns in Eqs. 4 and 5 have been exploited, the main discussion here will be limited to those proportional to 2) (j3ijk) and 3) (Yijkl)

i

1.3

1

Ultraviolet Generation

The first experimental demonstration of nonlinear optics, involving the nonlinear polarization term proportional to i 2) in Eq. 4, occurred when Franken and co-workers (1962) doubled the red output of a ruby laser by second harmonic generation (SHG) in crystalline quartz. Today, frequency doubling is routinely carried out using a wide array of inorganic and organic crystals. While it is generally recognized that the more polarizable the medium the greater its optical nonlinearity, asymmetry is also an essential requirement. It can be shown that all even hyperpolarizabilities (and susceptibilities) including Pijk (i2) ) vanish in centrosymmetric materials regardless of their polarizability. Although second harmonic generation in isotropic atomic media has also been demonstrated, it is now generally accepted that this phenomenon is a third order nonlinear effect involving two input laser photons and a third radial DC electric field produced by atom ionization (Jamroz et al., 1982; Mullin et al., 1985).

2.

FREQUENCY DOUBLING

2.1

Inorganic Crystals

Wavelengths down to :s 200 nm with a frequency bandwidth :s 0.5 cm- I are routinely generated in many spectroscopy labs by SHG of pulsed fundamentals in inorganic crystals (Bordui and Feyer, 1993). A central concept for nonlinear optics in general, and for SHG in particular, is phasematching. This, for SHG, can be summarized as; (6) where k(CO) is the wave-vector of the polarization inducing field at fundamental frequency lOJ, and k(2lOJ) is the wave-vector of the polarization at the doubled frequency 2lOJ. Equation (6) is simply a statement of the conservation of momentum for the three photons involved in the SHG process, and it implies that the phase velocities of the input and output waves

262

ROBERT H. LIPSON et al.

involved must be equal. Dispersive anisotropic crystals are characterized by two different indices of refraction, no( liJ), and ne( liJ). A material is termed positively birefringent if ne ｾ＠ no (where the subscripts denote the so-called extraordinary (e-) and ordinary (0-) waves), and negatively birefringent if the reverse scenario holds. The two indices are equal along a specific direction in the crystal called the optical axis. However, both are functions of the angle 8 formed between the propagation direction, k, and the optical axis. Physically, this means that an electromagnetic wave at a fixed frequency propagating in an arbitrary direction through the anisotropic material will experience two indices of refraction and as a result will exhibit a phase change, upon leaving the crystal, whose magnitude depends on the distance of propagation and the difference between ne and no. The phase-matching requirement for SHG can be fulfilled if the input waves and the generated harmonic signal wave propagate at an angle 8 with respect to the optical axis where n(2liJ) = n(liJ). Type-I phase matching corresponds to the situation when waves 1 and 2 at liJ) propagate as e- (0-) waves and the second harmonic at 2liJ) travels as an o-(e-) wave in a positively (negatively) birefringent material. Type-II phase matching happens when input wave 1 and output polarization wave 3 travel as 0-( e-) waves and the input wave 2 travels as an e-(0-) wave in a positively (negatively) birefringent sample. It can be shown that the generated intensity of the second harmonic wave over a propagation length, L, I(2liJ,L), is proportional to the following expression; (7)

where Me = I k(2liJ) - 2k(liJ)1 is the wave-vector mismatch between the SHG and input fundamentals. When the length L exceeds the coherence length Le defined by; (8)

the fundamental and SHG waves become out of phase and the second harmonic intensity decreases. Thus, the index difference n(2liJ) - n(liJ) should be sufficiently small that Le is greater than L. Many of the inorganic crystals listed in Table 3 (Demtroder, 1996; Lin, 1990) for SHG into the visible and UV are now commercially available.

263

9. Tunable Short Wavelength Generation and Applications

Table 3. Common Nonlinear Crystals used for Second Harmonic Generation Crystal

Non-linear coefficientb)

ｒ｡ｮｧ･ＨｾｭＩ＠

Damage Threshold (GWcm-2t)

KDP") (Bilt et al., 1977; Massey and Johnson, 1976)

0.2 - 1.5

0.4

BBOd) (Miyazaki et al., 1986; Kato, 1986; Miischenbom et al., 1990; Miikenheim et al., 1988)

0.19 - 3.0

3 -5

4.4

Lil03 (Nath and Haussiihl, 1969)

0.3 - 5.5

0.01 - 0.05

12

KTpe) (Bolt et al., 1985)

0.35 - 0.45

0.5 - I

II - 15

LiNb03 (Kitamoto et al., 1995)

0.4 - 5.0

0.01 - 0.04

10

Transparency

KNb03 0.4 - 5.5 a) 10 ns pulse at 1064 nm; b) Relative to d36(KDP) BaB20 4; e) KTiOP04•

=

37 -47 0.2 - 0.4 1.04 x 10-9 esu; c) KD2P04 ; d)

13-

Crystals such as KDP are also routinely used to frequency-double the output of the powerful neodymium-doped yttrium aluminium garnet (Nd:YAG ) infrared laser, operating at a wavelength of 1.06 /Jlll, to green light at 532 nm. The 532 run and 1.06 /Jlll beams can then be sum-mixed in a second nonlinear crystal to generate the UV third harmonic at 355 run. Similarly, the 532 run light can be frequency-doubled to generate the fourth harmonic at 266 nm. These harmonic frequencies are valuable for pumping tunable dye lasers (Hartig, 1978; Ziegler and Hudson, 1980). A recent and now widely-used solid state laser material is sapphire (Ah03) doped with Ti3+ (Moulton, 1986). This medium is used extensively for femtosecond pulse generation due to its enormous gain curve covering the wavelength region between 660 and 986 nm or beyond. The coherent near infrared output of a Ti:sapphire laser can be doubled in BBO to generate near UV-visible radiation between 350 and 490 nm. Its UV third harmonic can be tuned from 233 - 327 nm, while fourth harmonic generation covers a range of wavelengths between 210 and 245 run. Other inorganic crystals designed, manufactured and tested (French et al., 1991) for frequency doubling include LiB30S (LBO) (Chen et al., 1989; Borsutsky et al., 1991; Seifert et al., 1994), CSB30S (CBO) (Wu et al., 1993), CsLiB 60 lO (CLBO) (Mori et al., 1995; Kiriyama et al., 2000), KBe2B03F2 (KBBF) (Chen et al., 1996) and LhB 40 7 (LB4) (Komatsu et al., 1997; Petrov et al., 1998). These materials are transparent down to - 160 nm, and have been used to generate second harmonic VUV radiation as short as 170 nm. However, at the shortest wavelengths these crystals tend to

ROBERT H. LIPSON et al.

264

suffer from unsuitable dispersion of birefringence. While it may appear that crystal transparency must ultimately limit their usefulness for generating wavelengths shorter than 160 nm, strategies have been considered which overcome this restriction. Chen and co-workers (1995) have shown theoretically that under specific experimental conditions nonlinear crystals with strong absorptions could be used to generate hannonics in the VUV and soft-x-ray spectral regions. A typical nonlinear optical crystal has an energy gap that corresponds to the energy difference nmg between the top of the valence band and the bottom of the conduction band. Typically the second hannonic frequency at 2liJI is < COg. However, consider the leading term in the quantum mechanical expression for i 2); (9)

where ek is the unit vector identifying the polarization of wave vector k, jl is the electric dipole moment operator, t"1jg are the complex frequencies for the j +- g transitions, and the summation is over all states. The resonance denominator proportional to (nag - liJI) in i 2) is similar to that found for the linear susceptibility ill. Therefore, if two light fields are used, one with frequency liJI > mg and the other with a frequency w.z such that 2 w.z < mg, the second hannonic power generation at 2liJI can be increased despite the linear absorption increase because both denominators in Eq. 9 would be minimized by approximately the same order of magnitude. Proof of principle for many of these concepts has been demonstrated experimentally for the single resonance case (Krol et al., 1993). Double resonance experiments, if carried out in transmission, will require very thin crystals, on the order of 100 A. While these small thicknesses would allow intense hannonic light to be generated with minimal phasematching restrictions, the large heat loads generated in this way could also be damaging. This problem could be rectified by working in reflection with bulk crystals. The dipole hannonic radiation generated in the backwards direction in a conventional frequency-doubling experiment is absent due to phase-matching. However, reflected second harmomc light generated by the double-resonance scheme above is expected to originate primarily from the surface of the crystal, and not from the bulk. The advantage, however, of using bulk crystals in this configuration is that they would assist in heat dissipation.

2.2

Organic Crystals

Organic materials represent a relatively new and exciting development

9. Tunable Short Wavelength Generation and Applications

265

for optical applications because their nonlinearities and laser damage thresholds can often exceed those of inorganic crystals, and because they can be easily designed and modified (Marder et al., 1991; Messier et al., 1991; Nalwa and Miyata, 1997; Kuzyk and Dirk, 1998). A two-tiered approach is required for synthesis of nonlinear organic materials (Marder et al., 1991; Shui, 1994). The first involves the design of individual molecules, while the second is concerned with ways of producing bulk samples having the requisite symmetry properties. As noted above, molecules are sought which are very polarizable. In this regard, the organic molecules of choice tend to contain delocalized 7t-electron systems. It has been shown that the magnitude of the second-order hyperpolarizability, Pijk. increases with each double bond involved in conjugation, but levels off when the number of these double bonds becomes large (Morley et al., 1987). However, the magnitude of these effects and the degree of conjugation necessary before the nonlinearity saturates depends on the specific chemical nature of the molecules. Centro symmetric molecules like benzene and napthalene are not suitable for frequency doubling as their P values are zero. Chemically, however, their inversion centres can be removed by introducing substituents. Commonly, highly asymmetric 7t-systems are made which include an electron donor (D, such as alkyl, alkoxyl, amines) and an electron acceptor (A, halogen, cyano, and nitro) placed para to each other on the aromatic ring, or at the two ends of a polyene unit. The resultant P values of these so-called D-A or 'push-pull' molecules are determined by the strength of the substituents, related to the ionization potentials of the filled donor and empty acceptor orbitals, the mediating units between D and A, and the planarity of the overall 7t-system. With regard to this last point, intramolecular hydrogen bonding is often used to force co-planarity between the aromatic ring and donor groups with a large degree of rotational freedom, such as amines. Typical Pvalues range between 10-30 - 10-27 esu. The strongest spectroscopic feature for a D-A molecule is an electronic charge transfer (CT) transition where a D-A 'benzene' structure is converted to a 'quinone' zwitterionic form (Figure I). Large P values have been obtained for polyene systems by picking A substituents which can gain aromaticity in their charge-separated resonance form (Marder et al., 1994). However, a conflict exists between a large hyperpolarizability and optical transparency at the doubled frequency, 2m. Specifically, the optical absorption edge shifts to longer wavelengths with increasing strength of D and A. Molecules for which these absorption maxima lie in the near UV are technically useless for short wavelength generation. Still, this is a burgeoning field because of potential uses for the frequency doubling of diode lasers (It - 820 nm).

266

ROBERT H. LIPSON et al.

"Benzene" structure

"Quinone" structure

Figure 1: Schematic diagram showing the charge transfer transition structural change in a benzene-type O-A molecule

For simpler nonlinear optical applications such as the Pockels (electrooptic) effect, polar molecules must be assembled into a bulk fonn with their dipoles parallel to each other to enhance a particular component of 2). In practice, polymeric organic material can be aligned or poled by heating them above their glass transition temperature and then applying a DC electric field to line them up (Samyn et al., 2000). The molecules are then cooled in the presence of the electric field to freeze them into positions to yield a non-zero i 2). The degree of alignment induced by the electric field which decays over time, however, is usually small and high applied fields can induce material electric breakdown. Other approaches include self-assembly (Li et al., 1990; Katz et al., 1991) and Langmuir-Blodgett film assembly (Kajikawa et al., 1992). Zyss and co-workers (Joffre et al. 1992) have also begun synthesizing molecules with zero pennanent dipole moments but nonzero multipoles. This reduces the tendency of the polar molecules to align themselves in an anti-parallel fashion that fonns unit cells with a centre of symmetry. Second harmonic generation requires crystal engineering methods that prevent simple head-to-tail alignment of the polar molecules which can cancel out the asymmetry properties of the bulk. Strategies include the use of hydrogen bonds (Seto and Whitesides, 1993), chiral groups (Rieckhoff and Peticolas, 1964; Halbout et al., 1979; Halbout and Tang., 1982; Rosker and Tang, 1984), and organic salts (Marder et al., 1994). Hydrogen bonds can offset the electro-static interactions between asymmetric molecules which favour the fonnation of centrosymmetric structures and help direct molecular arrangements which favour macroscopic nonlinear optical effects. Chiral substituents, on the other hand, ensure a non-centrosymmetric crystal structure but do not guarantee a favourable alignment of the molecules and therefore an optimized 2). While chiral compounds such as amino acids or saccharides typically have small p values they are transparent at wavelengths shorter than 300 nm, making them potentially useful for UV generation. The

t

t

9. Tunable Short Wavelength Generation and Applications

267

Coulombic interactions associated with salts appear to dwarf the dipoledipole interactions that result in the formation of centro symmetric crystals (Marder et al., 1994). These materials are thought to form a sheet structure where the anionic layers provide the driving force favouring a net polar alignment of the cation sheets over an anti-parallel configuration.

2.3

Continuous-Wave UV Laser Systems

Several groups are using continuous wave (cw) lasers to obtain much higher resolution spectra. Pratt and co-workers have described an ultrahigh resolution UV laser which they have used to record fully rotationally resolved LIF excitation spectra of large polyatomic molecules (Majewski et al., 1995). In their experiments, a single mode ring dye laser capable of stabilized scans of 90-120 GHz in the visible, and operating on Rhodamine 6G (595-630 nm), Kiton Red (620-640 nm) and/or DCM (630-700 nm) dye solutions, is pumped by - 1 W of the 514.5 nm line of an argon-ion laser. UV light is generated by placing a Lil0 3 doubling crystal in the auxiliary waist of the ring laser to produce - 1 m W of output power in the UV. Phasematching the SHG crystals doubles the scan range to 180-240 GHz, or 6-8 cm- I . The spectrometer has a frequency bandwidth resolution of 2.5 MHz at an excitation wavelength of 300 nm, corresponding to an enviable resolving power ｶＯｾ＠ v = 4 x 108 • A second intermediate resolution system reported by Bitto and Willmott (1992) allows both precise high resolution frequency-resolved and timeresolved experiments to be carried out. It uses the visible (532 nm) pulsed amplified cw output of a ring laser to generate Fourier transform-limited pulses with up to 100 mJ of energy. UV pulses with approximately 150 MHz frequency bandwidth and energies up to - 300 ｾ＠ are subsequently generated by SHG in nonlinear crystals.

2.4

Optical Parametric Oscillators

Optical parametric oscillation (Giordmaine and Miller, 1965, 1966) can be viewed as the inverse of SHG, or more precisely, two-colour three-wave frequency mixing (Massey and Johnson, 1976). Here, interaction with a 2) nonlinear crystal converts a strong pump beam at frequency COp into two new waves at frequencies ｾ＠ ('idler') and lOs ('signal'). Due to energy and momentum conservation, the pump, idler and signal photon frequencies and wave vectors are related by;

i

OJ p

kp (OJ p)

= OJ + OJ s

=k

j

j

(OJ) + ks (OJs )

(lOa) (lOb)

268

ROBERT H. LIPSON et al.

and the strengths of the signal and idler waves are proportional to the intensity of the pump beam and the square of the crystal nonlinear susceptibility (Harris, 1969; Baumgartner and Byer, 1979). Of the infinite number of frequency combinations suggested by Eq. lOa the pair emerging from the crystal is that which satisfies the phase-matching condition implied by the wave vector relationship Eq. lOb. This will be determined by the orientation of the crystal with respect to kp (wp) , and if the nonlinear crystal is placed inside a resonator, gain can achieved for either the signal or idler waves, or both. Frequency tuning can then be achieved by either crystal rotation or temperature adjustment. The recent interest in OPOs and their commercialization as a viable all solid-state replacement for dye lasers can be attributed to the availability of injection seeded single-mode pump laser sources and damage-resistant nonlinear materials such as KTP, LBO, and especially BBO (Tang et al., 1990; Haub et al., 1991). However, it has also been shown that the idler and signal waves must be actively controlled for single-mode operation, for example by injecting seed radiation of the appropriate frequency from another single-mode laser source (Fan et al., 1988). The output of a BBO OPO pumped by the 355 nm output of an injection seeded Nd:YAG laser can be angled-tuned from wavelengths < 0.422 IJlll to > 1.68 f..lm (Cheng et al., 1988), and also doubled into the UV by SHG. Tunability between 0.33 f..lm and 1.37 IJlll is possible from a similar device pumped by the 266 nm output of aNd: YAG laser (Bosenberg et al., 1989).

3.

VUV GENERATION

Isotropic gases are most often used for coherent VUV generation because, as noted above, nonlinear solid state materials that are widely transparent in this spectral region do not exist. In the absence of DC electric fields, and in isotropic media where 2) is identically zero, the term in Eq. 4 leading to nonlinear effects involves the nonlinear susceptibility, ;t). Three third-order optical schemes are considered below: non-resonant third harmonic generation (THG) , two-photon resonance-enhanced four-wave sum- and difference-mixing, and coherent anti-Stokes Raman shifting.

i

3.1

Non-resonant Third Harmonic Generation

Figure 2a shows an energy level diagram for non-resonant THG (Hilbig et al., 1986), where lUoutput = 3w, and w is the input laser frequency.

269

9. Tunable Short Wavelength Generation and Applications

---,---.-- Ie> Ｍｾｉ･＾＠

'-'-'-'-'- I. P.

'-'-1. P.

cq

--+--t--Ib> ｾｵｶ＠

=

3cq

--+--+--Ib> -+--+--+---Ie>

--4--4--

la>

----L...--'---Ig> (a)

--..1.---lg> (b)

Figure 2. (a) Energy level scheme for one color non-resonance third harmonic generation at the fundamental frequency WI. (b) Energy level scheme for four-wave sum- and differencemixing at 2w\ + iV2 and 2w\ - iV2 respectively. I.P - ionization potential. [Redrawn from Hilbig et aI., 1986.]

When a Gaussian beam of frequency co, and input power P0J7 is focussed into an atomic medium, the resultant third harmonic (frequency-tripled) output power P3fJ) is given by; (Bjorklund, 1975) (11) The geometrical phase-matching function F, describing the dispersive phase velocity relationship between the input and output waves, depends on the product bM, where b is the confocal beam parameter of the focussed radiation, and M is the wave-vector mismatch between the input and output light beams;

(12)

The indices of refraction, n(A3fJ) and n(AfJ), for the medium correspond to those at the third harmonic and input laser beam wavelengths, A3fJ) and A0J7 respectively. Usually the product NI..t3)(3co)1 is small for most media, when the fundamental laser frequency does not match a resonance condition for an allowed transition. However, in general P 3fJ) cannot be increased simply by using a larger nonlinear gas pressure because M is also a function of the number density N. Instead, it is the product G = NF(bM) [or (bMiF(bM)]

270

ROBERT H. LIPSON et al.

which must be optimized. THG is most often generated in a tight focussing configuration, where the confocal beam parameter of a fundamental laser output, b is much less than the length of the medium, L. Typically, pulsed lasers are used to maximize the P {J)3 power dependence in Eq. 11. It should be noted, however, that there have been some notable successes using continuous-wave fundamentals when resonant enhancement techniques as described below are employed (Timmermann and Wallenstein, 1983; Nolting et al., 1990; Nolting and Wallenstein, 1990). More importantly, tight focussing corresponds to the following phase-matching function, (Bjorklund, 1975) F(bllk);

F(b&) = 1[2(b&ye Mk =0

L1k 0). In a multi-component mixture with constant N, the generated third harmonic power will follow F(bllk), and be a maximum when bilk = -2. In a single negatively dispersive medium, phase-matching may be optimized by varying N. In that case, the generated third harmonic power will be determined by the function G, which peaks at bilk = -4. THG can be also optimized by changing the beam focus to vary b. Even though the refractive index is a strong function of A. near a resonance line, coherent VlN generation at any allowed wavelength can be maximized by adding different amounts of a positively dispersive buffer gas to a constant number density of the negatively dispersive nonlinear medium. The discussion above is only valid for a pure fundamental TEMo,o mode. Yiu et al. (1979) have considered the effect of third harmonic generation in a tight focussing regime using a fundamental electric field having a cylindrical spatial mode structure. They calculated that a pure, high order angular mode, TEMo,1 will generate a TEMo,31 mode at the third harmonic frequency with a single maximum but lower efficiency than a pure TEMo,o Gaussian. Conversely, the phase-matching functions F and G, for an incident pure higher-order radial mode, TEMp,o would be multi-peaked. The function F exhibits maxima at bilk = -11.5 and -21.5 for TEM\,o and TEM 2,o modes, respectively, while G peaks at bilk = -13 and -22.7. Saturation of THG is

9. Tunable Short Wavelength Generation and Applications

271

attributed to light intensity-dependent changes in the index of refraction of the medium (Kerr effect) which destroy the phase-matching condition (Puell et al., 1980).

a

Ne

(72.0S • 73.S8 nm ; 74.3 • 74 36 mn)

III

Ar

(8S.7· 86.68 nm; 86.8 ·86.98 mn: 97.4 ·104.7S nm)

Kr

I III

(1\ \.6 ·1\6.S run; 120.2 ·123.6 nm)

I I III

Xe

(liB ·117.0 nm; 117.6 ·119.2 mn: 126.7· 129.S nm; 140.1 ·146.9)

Hg (132·140mn; 141·I84mn) I.,,!

II

I,

•

' .. ,,1 11 ,,1 11 ,

I,

"It

"I.

,

.I.

,

.!

It.ll,

'''tt'

50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210

Generated VlN / XUV Wavelength / nm. Figure 3. VUV and XUV spectra tuning regions for non-resonant third hannonic generation

in Ne, Ar, Kr, Xe or Hg nonlinear media. [Redrawn from Hilbig et al., 1986.]

The nonlinear media most commonly used for THG are Xe (Hilbig and Wallenstein, 1981; Zapka et al., 1981), Kr (Cotter, 1979a, 1979b; Langer et al., 1980; Mahon et al., 1978), Ar (Hilbig and Wallenstein, 1983; Marinero et al., 1983), and Ne (Hilbig et al., 1984a; 1984b), rare gases, or metal vapours such as Hg (Hilbig et al., 1986) (Figure 3). While metal vapours must be produced in high temperature heat pipe ovens, rare gases can be housed in either a room temperature gas cell with a MgF2 or LiF output window for VUV generation, or expanded in a spatially localized supersonic jet. The latter arrangement can provide a near ideal 'windowless' environment for:XUV generation (Rettner et al., 1984). Typical conversion efficiencies of - 10-6-10.3%, leading to intense coherent VUV outputs, are possible using commercial pulsed dye laser peak powers of 1-10 MW (-10 ns pulse duration). It is also straightforward to triple the third harmonic output of a Nd:YAG laser at -355 nm to produce VUV light at -118 nm (Kung et al., 1973; Zych and Young, 1978). Its virtue, despite the fixed wavelength, is its technical simplicity. As described below, one area where such a source has found wide application is in the soft-ionization of organic molecules (Lockyer and Vickerman, 1997; Butcher, 1999).

272

3.2

ROBERT H LIPSON et al.

Two-Photon Resonance-Enhanced Four-Wave Mixing

Two-photon resonance-enhanced four-wave mixing has been thoroughly reviewed elsewhere (Jamroz and Stoicheff, 1983; Vidal, 1987; Hepburn, 1995; Lipson et aI., 2000) and so only the salient features are presented here. As shown in Figure 2b, in two-photon resonance-enhanced four-wave mixing, three input fundamental photons from two lasers, two at angular frequency COt, and one at angular frequency ｾＬ＠ are used to produce a fourth output photon at either the sum (2COt + ｾＩ＠ and/or the difference (2COt - ｾＩ＠ frequency, both of which can lie in the VUV or XUV (Hodgson et ai., 1973; Herman et ai., 1985). The expression 'four-wave mixing' is used because three input waves generate a fourth output signal; FWSM denotes sumfrequency generation, and FWDM, generation of the difference frequency. In a expression similar to Eq. 11, the intensity of the generated VUV light, PVlN , is governed by a P l 2P2 incident laser power dependence for beams at frequencies Wt and ｾＬ＠ respectively, and by a phase-matching function F(bM) that is identical to Eq. 13 when a tight focussing geometry is used. However, the wave-vector mismatch here is now defined as M = IkvlN - 2kl - k 21,where k\, k2' and kVlN are the wave vectors for the input and output light beams, respectively. At first glance resonant four-wave mixing, like non-resonant tripling, appears only possible in spectral regions where the nonlinear medium is negatively dispersive. However, most atomic elements used for this purpose are, to a good approximation, inherently negatively dispersive for virtually every VUV and XUV wavelength in their tuning ranges, since their first resonance lines, which carry most of the oscillator strength, come to the blue of the majority of the input laser frequencies used in the nonlinear optical process. Still, broad tunability is often only achieved at the expense of optimal phase-matching. The dispersion criterion for FWDM is much less stringent in a tight focussing situation because M can be either positive or negative. The phase matching function; (14)

is a maximum when flk = 0 (Bjorklund, 1975), and ordinarily, the FWDM conversion efficiency is about an order of magnitude better than that for FWSM. One significant difference between resonance enhanced four-wave mixing and non-resonant THG is that an appreciable VUV output conversion efficiency, (on the order of 1 part in 107 to 10\ for the former can be achieved using moderate incident pulsed laser powers by judiciously tuning

9. Tunable Short Wavelength Generation and Applications

273

frequencies CO) and OJ}, to be either one- or two-photon resonant with allowed transitions (flag, flbg) in the medium. This approach minimizes one or more of the resonance denominators in the expression for the third-order nonlinear susceptibility (Ann strong et al., 1962; Orr and Ward, 1971);

(15)

Here, the defmitions are similar to those for Eq. 9. Experimentally, two-photon resonance enhancement is most commonly used since it can substantially increase i 3 ) with minimal two-photon absorption in the medium, provided the pulse durations of the fundamental laser beams are reasonably short (:s 10 ns). The one-photon resonance condition, on the other hand, can lead to substantial reduction in the coherent output due to strong linear absorption of the fundamental. Additional resonance enhancement is also possible if the VUV output frequencies correspond to allowed transitions in the medium, neg. This effect is usually small when the unstructured continuum of an atomic medium is used, due to the small magnitude of the bound-continuum (cle3 jL Ig) matrix element (Condon and Shortley, 1964). A better conversion efficiency however is possible if the generated VUV wavelength is resonant with a transition to an auto ionized level embedded in the continuum. Likewise, VUV light generated at wavelengths longer than the ionization potential of the nonlinear medium can often be in resonance with strongly allowed (Rydberg state ｾ＠ ground state) transitions. While the conversion efficiency can then be very high (up to 5%) (Muller et al., 1988) the possibility of linear reabsorption is also large, and the generated light intensity can change dramatically with wavelength. While this characteristic is the opposite of what is desired in a tunable spectroscopic light source, any resultant intensity fluctuations can readily be handled by appropriate nonnalization techniques. At all times, input laser frequencies must be chosen for atomic transitions giving non-zero electric dipole matrix elements in the numerator of Eq. 15. With Russell-Saunders (LS) coupling the two-photon selection rules are; LV.

= 0, ± 2; M = 0; LV = 0, ± 2

,

(16)

where L, S, and J are the multi-electron atom quantum numbers for the orbital, spin, and total angular momentum, respectively. The LV = 0, ±2 rule is the only important one for heavier elements where spin-orbit effects (j-j coupling) dominates. An examination of the polarization selection rules for the magnetic quantum number;

ROBERT H UPSON et al.

274 Il.m/ (Il.m

j) =0, ± 2 (linearly polarized light);

(17a)

Il.m/(Il.m

j) = ±2 (circularly polarized light),

(17b)

shows that parasitic and non-tunable third hannonic light generated at 3lVt during the four-wave mixing experiment can, for certain two-photon resonances, be eliminated using circularly polarized light. Many possible phenomena can limit the two-photon resonant four-wave mixing conversion efficiency (Georges et al., 1977). These include medium ionization, laser-induced Stark shifts, the effects of population redistribution on the phase-matching conditions, possibly the effects due to stimulated emission from the two-photon excited state, and the use of multimode laser inputs. Of all these, the dominant saturation mechanism is two-photon absorption leading to changes in level populations, and hence the refractive index and phase-matching of the medium (Bjorklund, 1975; Vidal, 1980). Qur laboratory routinely generates VlN light between -174 nm and 70 nm by four-wave mixing in Mg and Hg vapour, as well as in Xe and Kr gas (Lipson, 1999; Lipson et al., 2000). Energy levels diagrams and tuning curves for these elements are shown in Figure 4. It is quite common to obtain VlN photon numbers exceeding 1010 per pulse by generating the fundamental beams using commercial dye lasers. The monochromaticity of the VlN is then determined essentially by these sources. Current commercial pulsed dye lasers have narrow output line widths, often < 0.07 cm- I , which yield VlN line widths of< 0.12 cm-I . The corresponding frequency resolving power, vlll. v, is better than 106 at the shortest wavelengths. This, combined with the resultant high spectral brightness due to the low divergence of the VlN output, represents a considerable improvement over the best monochromators available, and current synchrotron sources, as shown in Table 4 (Herman et al.• 1997). Experimentally, gas-phase spectral resolution will be a convolution of the source linewidth, Doppler broadening, and other broadening mechanisms such as predissociation and/or autoionization. Typically, individual rotational lines having a full-width at half maximum of less than about 0.5 cm- I in the VUV can easily be resolved. Four-wave mixing in molecular gases has also been demonstrated (Bischel et al., 1979; Tsukiyama et al., 1990). Of particular note has been the use of narrow bandwidth ArF excimer laser light at 193 nm for twophoton resonance enhanced four-wave sum-mixing and third-harmonic generation in H2 to generate extreme ultraviolet radiation as short as 64 nm (Srinivasan et al., 1983; Hirakawa et al., 1993; Hirakawa et al., 1997).

275

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Figure 4. Energy level diagrams and tuning ranges for four-wave sum- and difference-mixing in (a) Kr; (b) Xe; (c) Hg and (d) Mg. The wavelengths (nm) of the fundamental laser beams indicated are those which provide two-photon resonance enhancement of the third-order nonlinear susceptibility of the medium. I. P. - ionization potential of the nonlinear medium.

Lastly, with the development of high-power solid state and excimer laser systems capable of producing band-width limited picosecond (ps) to femtosecond (fs) pulses, tunable short pulse VUV sources generated by fourwave mixing have also been realized for ultrafast time-resolved spectroscopic studies (Kung, 1974; Glownia et al., 1994). While the nonlinear process is essentially identical with that described above for nanosecond pulses, there are also some marked differences. Firstly, the twophoton resonance condition that must be strictly obeyed with nanosecond pulses is relaxed with fs pulses because of the large bandwidth of the fundamental and the high intensities of the fundamentals (> 1013 W cm·2). Secondly, these short pulses result in AC Stark shifts that tend to shift the atomic levels into multiphoton resonance. Since ground state AC Stark energy shifts are negative while excited states move in the opposite sense, a given transition frequency vfi will increase in an intense laser field. Therefore, a two-photon resonance can be achieved if 2 VI > vfi. Resonant four-wave difference mixing in Xe has been used to generate short VUV pulses> 130 nm using fundamental pulses at A.I = 248.5 nm obtained using both a fs KrF excimer laser output (Momma et al., 1993) and continuum radiation produced in high-pressure Ar, seeded in a KrF-excimer gain

276

ROBERT H. LIPSON et al.

module (Bischel et al., 1979). Difference mixing capitalizes on the relaxed phase-matching criterion. Shorter wavelength femtosecond pulses tunable between 102 nm and 124 nm can be generated in Kr and Ar by four-wave difference mixing using fs pulses at Al = 193 nm obtained either directly from an ArF excimer laser (Nazarkin et al., 1997) or by amplifying the fourth hannonic of a Ti:sapphire laser operating at 774 nm, in an ArF excimer gain module (Kittlemann et al., 1996; Wittmann et al., 2000). The intensities of the short pump pulses alone are also sometimes sufficiently high to initiate parametric processes leading to intense ps to fs VUV emission (Pummer et al., 1983; Shahidi et al., 1988; Tiinnermann et al., 1992; Tiinnermann et al., 1993; Le Blanc et al., 1995). Table 4. Comparison of some characteristics of second and third generation synchrotrons and VUV lasers sources generated by four-wave mixing in nonlinear media Properties

2nd generation synchrotrona )

3M generation synchrotronb)

Four-wave mixing sources

E-range

10- 1 - 3 X 104 eV

optimized for 10 eV to 1 keY

6 eV-19 eV

intensity

< 109 photonslpulse' 0.1 % band pass

up to 1013 photonsisec/O.Ol % bandpass

10 12 photons I pulse

resolution

10-3 eV (0.03 A) at 540 eV; 10-2 eV (0.02 A) at 40-100 eV

< 10 meV at 100 eV;

50.5 cm- I

ElM as high as lOS

repetition rate

-I to 500MHz

< 500 MHz; > 1.5 MHz typically < 50 Hz

pulse duration

> lOOps

< 70ps.

510 nsec

a) I. Munro and A. P. Sabersky, in Synchrotron Radiation Research, H. Winick and S. Doniach (editors), Plenum Press, New York (1980). b) Characteristics of the Advanced Light Source: A Brief Summary, http://www-als.lbl.gov/alslworkshopslalscharacter.html

3.3

VUV and XUV Laser Setups at the Forefront

Several groups have successfully pushed the envelope in terms of:XUV source frequency resolution (Cromwell et al., 1989; Trickl et al., 1989; Ubachs et al., 1997; Hollenstein et al., 2000; Merkt, 2000). Each system combines pulsed amplification of a single-mode ring dye laser, frequencydoubling, and either non-resonant tripling or two-photon resonant four-wave mixing in a pulsed jet. Coherent outputs with > 108 photons/pulse in frequency bandwidths of > 210 MHz (or 0.007 cm- I ) were achieved, corresponding to a remarkable best resolving power of vI!l.v > 107 at

9. Tunable Short Wavelength Generation and Applications

277

wavelengths < 95 run. Experiments carried out using these instruments have contributed significantly to our current understanding of atomic and molecular Rydberg states. Two other experiments are worth noting here. In the first, Merkt and coworkers (1998) measured ultra-high resolution Rydberg state spectra using an XUV -millimeter wave double-resonance scheme. Here, high principal quantum number n* Rydberg states in atoms were prepared by excitation with 0.1 cm- I bandwidth XUV laser light generated by two-photon resonance enhanced four-wave sum-mixing in Kr gas. Millimeter waves, generated by a backward wave oscillator operating in the 118 - 180 GHz range, were then used to induce transitions from the prepared Rydberg states to higher n* levels in a skimmed supersonic jet. The resultant states were subsequently pulsed field-ionized and the zero energy kinetic energy (ZEKE) electrons detected in a time-of-flight (TOF) spectrometer. Transition linewidths under these experimental conditions are determined either by excited state lifetimes or transit time effects, since the Doppler broadening is negligible. Atomic transitions in Ar with linewidths as narrow as 250 kHz could be measured in this way in the vicinity of n* = 200! In the second experiment Eikema et al. (1997) were able to significantly improve on the measurement of the He ground state Lamb shift using 250 MHz bandwidth XUV (near 58 run) radiation generated by a fifth harmonic process in a N2 jet.

3.4

VUV Radiation by Stimulated anti-Stokes Raman Scattering

Raman scattering occurs when a pump photon at frequency llJp is inelastically scattered in a medium to generate the first Stokes component at frequency COs I = llJp -.1.. Here.1. is the frequency difference between two levels in the medium, for example, the first vibrational interval of the ground state, and a given vibrational transition will be Raman active if there is a polarizability change with change in a particular normal mode coordinate, Q; that is, 8aJ8Q O. Stimulated Raman scattering produces an exponential increase in the intensity of the first Stokes component in the direction of the pump beam (Hellwarth, 1962; Eckhardt et al., 1962). When that component becomes sufficiently intense it too can act as a pump for the second Stokes component at frequency COs2 = COs I - .1., and so on, until there is a series of output frequencies at llJp, COsh COs2,"" COsN. The first anti-Stokes component at frequency £iJASI is generated by a fourwave mixing process involving the beams at llJp and COsh such that £iJASI = 2% - COs I (Figure 5a). The dominant process which generates the JIb antiStokes component is £iJASN = % + £iJAS(N-I) - COsI. As such, the intensity of the JIb anti-Stokes component depends on the intensities of the pump beam, the

*

278

ROBERT H. LIPSON et al.

fIrst Stokes component and the preceding anti-Stokes order. EffIcient phasematching dictates that the generated components will emerge from the sample at different angles (Figure 5b). Furthermore, since a given antiStokes order depends on the previous one, high order generation is usually ineffIcient. Still, in many ways anti-Stokes Raman scattering is technically simpler than four-wave mixing because only a pump laser and gas cell are required, and if the pump laser is tunable then each anti-Stokes order will be as well. H2 is often used as the Raman medium since !:1 = E( v' = 1, J = 1) - E(v" = 0, J =1) is large at - 4160 cm- I (Wilke and Schmidt, 1979), although many liquids and other gases have also been investigated (Eckhardt, 1966; Fenner et ai, 1973). The Raman output light intensity can be increased by cooling the molecules into the lower energy state. More importantly, cooling reduces the Raman linewidth, which in turns leads to an enhanced Raman gain. Under these conditions the optimum gas density, dispersion, and resultant phase mismatch can be reduced, resulting in more effIcient, higher order anti-Stokes generation.

a)

cql

b)

ｾＱ＠ ｾ＠

ｾ＠

Figure 5. (a) Energy level scheme for the generation of the first anti-Stokes line at WASI by the stimulated Raman effect; liJp and tiJsl are the frequencies of pump and Stokes beams, respectively, and 11 denotes the energy difference between two (typically vibrational) levels in the medium. (b) Wave-vector geometry of the pump, Stokes and anti-Stokes beams to satisfy the phase-matching criterion.

KrF (wavelength 248 nm) and ArF (wavelength 193 nm) excimer lasers, whose outputs already lie close to the VUV, have been use to generate high order short wavelength anti-Stokes Raman lines (Dobele et ai., 1987). For example, Huo and co-workers (1992) reported a pulse energy of 0.28 Jll for

9. Tunable Short Wavelength Generation and Applications

279

the 11 th anti-Stokes order at 116 run, when 50 mJ of KrF radiation (pulse duration 20 ns) was focused with an/= 100 cm lens into 1.8 atmospheres of H2 cooled to liquid N2 temperature The conversion efficiency was 5.5 x 10-6 • The Raman cell was 110 cm long and 40 mm in diameter. Two lasers can also be used to generate intense high order stimulated Raman anti-Stokes radiation (Mennicke et al., 1976). For example, Wallmeirer and Zacharias (1998), by using two lasers operating in the visible and the UV, were able to generate 9 anti-Stokes orders between 250.8 run and 136.8 run, with peak powers of - 5kW in the VUV. This biharmonic pumping technique is similar to coherent anti-Stokes Raman scattering (CARS), except that in CARS the Stokes component at lUsl is provided by a second laser instead of generated within the Raman medium from noise photons. High order CARS is capable of generating coherent VUV light with increased conversion efficiency and better reproducibility than that achieved by pure stimulated Raman scattering (Schultz von der Gathen et al., 1990).

3.5

Multiphoton Techniques

An alternative method that can be used to explore VUV and XUV states of atoms and molecules involves the simultaneous absorption of two or more photons. There have been numerous reviews on this subject (Johnson and Otis, 1981; Anderson, 1992; Ashfold and Howe, 1994). One of the distinct advantages that multiphoton spectroscopy enjoys, compared to sequential one-photon techniques like double resonance spectroscopy, is the technical simplicity of being able to use commercially available, tunable, longer-wavelength lasers to excite higher energy transitions. In addition, multiphoton transitions can access high [-states, where [ is the quantum number of the electronic angular momentum, and for molecules possessing a centre of symmetry, g or u parity states which are electric-dipole forbidden by single-photon excitation. Finally, unlike the cross-section of a single-photon transition, multiphoton cross-sections are polarization dependent, which, in cases of extreme rotational congestion, can provide a possible means of establishing the symmetry of the excited state (Bray and Hochstrasser, 1979; Wirth et al., 1981; Nascimento, 1983). Relatively high laser intensities are required experimentally since multiphoton cross-sections are small (on the order of 10-51 cm4 s and 10-82 cm6 S2 for coherent two-photon and three-photon excitations, respectively, compared to - 10- 17 cm2 for a single-photon transition). Suitable conditions can, however, easily be achieved by focussing the outputs of a tunable ns pulse duration Nd:YAG or excimer-pumped dye laser. While strong focussing makes direct absorption detection impractical, if not nearly impossible, multiphoton resonances can be readily detected either in fluorescence excitation or, more commonly, by monitoring the formation of

280

ROBERT H LIPSON et al.

ions. The latter is inherently very sensitive as most Rydberg states probed by multiphoton absorption lie close to the first ionization potential, and therefore usually require the absorption of only one additional photon to produce an ion. In general, the scheme where multiphoton absorption is combined with resultant ion formation and detection, is called (m + n) resonance enhanced multiphoton ionization, (REMPI). The integers m and n refer to the number of photons required to populate and subsequently ionize the state of interest, respectively. When the resultant ions are mass dispersed in a time-of-flight (TOF) mass spectrometer, excitation spectra can be recorded for individual species in chemical or isotopic mixtures.

3.6

The Push to Shorter Frequencies

Supersonic gas jet expansions are widely used by the spectroscopic community to produce gas phase molecules with internal energy distributions characterized by only a few degrees Kelvin (Smalley et al., 1977). The resultant spectra are relatively simple and therefore amenable to analyses, often for the first time. Non-resonant tripling (and four-wave mixing) in jets, however, does not capitalize on the cooling properties of the gas samples, but instead, on their spatially localized nature which make them a near-ideal 'windowless' environment for XUV generation (Marinero et aI., 1983; Rettneretal., 1984;Pageetal., 1987). The extension of third-order nonlinear optics to even shorter wavelengths is limited by the need for fundamentals which already lie in the VUV. On the other hand, higher order processes can be used to reach this spectral region and still allow the use of longer wavelength lasers. The theory of such an approach is well established (Harris, 1973; Tomov and Richardson, 1976; Reintjes et al., 1978). Although it may be expected that the shorter wavelength output would be relatively weak, the following polarization ratio;

(18)

shows that if the incident field is sufficiently large, (for an input intensity J, E/(V cm- I ) = 27.5[J/(W cm-2)t2), the higher order (q + 2) harmonic intensity may be comparable or even exceed that for the lower order (q). This condition can be achieved using ultra-short fundamentals. Just like third harmonic generation, higher order harmonic generation must be done in a negatively dispersive single component medium when using focused beams, but there is an additional benefit in that resonance enhancement is now possible at the second-, third- and/or fourth-photon energies. Fifth harmonic

9. Tunable Short Wavelength Generation and Applications

281

generation in Ne and He using the quadrupled output of a Nd:YAG laser at 266.1 run has been used to generate XUV light at 53.2 run (Reintjes et al., 1976). Similarly seventh harmonic light at 38 run was generated in He with the same fundamental input (Reintjes et al., 1977). It is straightforward to envisage using tunable lasers operating in the visible and near-UV, and fifthorder frequency mixing to generate tunable coherent XUV radiation between 40 and 100 nm. The area of physics dealing with the properties of electrons, atoms and molecules in intense laser fields is now so extensive that an exhaustive review is beyond the scope of this chapter (Gibbon and Forster, 1996). However, there are several aspects of this discipline which merit discussion here due to their impact on the development of coherent short wavelength radiation sources. The type of interactions that atoms and molecules experience in an intense laser field are usually differentiated by the magnitude of the Keldysh parameter y(Keldysh, 1965);

ｲ］ｾＲ｣ｄ＠

(lP

'

(19)

where IP is the ionization potential of the atom or molecule and is the ponderomotive potential of the laser. The ponderomotive potential is defined as the average oscillation (wiggle) kinetic energy of an electron in the electric field of the laser with strength E; (20) where m is the electron mass, (j) is the frequency of the laser, the intensity I is given in W cm-2 and A is in J.lm (Augst et al., 1989). The condition y> 1 is termed the multiphoton ionization regime (McPherson et al., 1987), while y < 1 is referred to the tunneling ionization regime (Augst et al., 1991). However, there are experimental measurements of threshold ionization intensities for the rare gases that agree with the relatively simple model involving tunneling ionization even under conditions where y> 1 (Gibson et al., 1990). In addition to producing highly charged ions, intense short-pulse interactions (- lOIS W cm·2) with rare gases can also generate high-order harmonic light. This process arises through a sequence of field ionization and ponderomotive free-space electron oscillation, followed by radiative recombination of the active electron with the ion, all within one half-wave of

282

ROBERT H. LIPSON et al.

the laser-light (Sander, 2000). One of the most significant observations has been that the intensities of the generated harmonics do not decrease with the harmonic order, but instead exhibit a plateau region where they are approximately constant. On the other hand, this plateau is found to have a fairly well-defmed upper energy limit above which the harmonic intensity decreased rapidly (L'Huillier and Balcou, 1993; FOldes et al., 1996; Schniirer et al., 1999). As noted above, the extent of harmonic response is determined primarily by the ponderomotive energy, and simple theory shows that the intensity drop off can be predicted as - IP + 3 (Becker et al., 1990; Krause et at., 1992). The harmonic conversion efficiency increases with increasing gas density and laser confocal beam parameter. Furthermore the generated light is coherent, which indicates that in the strong-field limit, phase matching is almost independent of the process order. However, this condition can be destroyed if the input laser liberates a high density of free electrons. Still, at this point, contributions to the harmonic spectrum from ions can compete with that from the neutrals. Furthermore, since ions normally have larger IPs than the neutrals, this can translate into a shorter wavelehgth output. Generation down to 7.2 nm (the 109th harmonic of 806 nm light) (Macklin et at., 1993) and 9.9 nm (the 25 th harmonic of 248 nm light) has been reported (Sarukura et at., 1991). Several other innovations present themselves when considering the interaction of relativistic free electrons with strongly coherent terawatt femtosecond pulses. In a collinear geometry with a laser pulse each electron will experience the incident electric and magnetic fields and a ponderomotive force which will tend to push them off-axis. However, those electrons directly on-axis will not feel the ponderomotive force during the entire optical pulse. Similarly, those electrons far off-axis will only experience a weak electric field and, therefore, a weak ponderomotive force. There will however be an optimal offset where the ponderomotive interaction is a maximum and the electrons will be expelled from the laser focus. Since those electrons at that position will only be affected by a few optical cycles of the fundamental beam, the higher order VUV harmonic output could be on the order of attoseconds (10- 18 sec) in duration! (Christov et aI., 1998) Another very interesting application for femtosecond laser pulses is the creation a plasma wave in a gaseous medium. Each intense pulse exerts pressure to create regions of negative and positive charge. Although the resultant electric field affects both electrons and ions with equal force, the ions do not move significantly while the electrons, which are light, are induced to oscillate back and forth due to an electrostatic restoring force. Such an array of electron pendulums will create an electric wake field that will accelerate electrons injected into the plasma (possibly by a second orthogonally directed laser pulse altering the local trajectories of some of the plasma wave electrons). Under proper conditions the injected electrons will

9. Tunable Short Wavelength Generation and Applications

283

be accelerated by the plasma wave in the direction of the propagating laser, and gain on the order of 109 V energy from the electric field over distances of - I cm. A comparable electron energy would require tens of meters in a conventional linear accelerator (Dawson, 1989; Ulmstadter et al., 1996a; 1996b). It is not hard to imagine such a table-top device which can produce femtosecond x-ray pulses, competing effectively with synchrotrons. One solution to coherent short wavelength generation that builds on synchrotron technology is the free electron laser (Brau, 1998). Relativistic electrons propagating through a series of alternating poled magnets (called a wiggler) will, in the rest frame of the electrons, undergo a series of forced oscillations perpendicular to the wiggler axis, causing them to radiate at the oscillation frequency. Within the stationary frame of the laboratory the electrons are travelling close to the speed of light, and the emitted light appears to be moving collinearly forward with the electron beam. The emitted wavelength AL is Doppler-shifted to a shorter wavelength than the period of the wiggler, Aw such that;

(21)

Here, e is the electron charge, Bw is the average magnetic induction, mo is the electron rest mass, c is the speed of light, and r is the energy of the electron in units of its rest energy, which is 0.511 MeV. When a laser with wavelength AL co-propagates with the electron beam, the electrons will form bunches spaced at the laser wavelength and radiate in phase with laser, resulting in light amplification. The laser light can either be from an external source or from the free electron laser itself. Free electron lasers offer several advantages including broad tunability, and high output powers due to their ability to remove large amounts of waste heat via the electrons extremely quickly. Their main disadvantage is cost. Successful operation in the infrared and visible has been reported (Hogan et al., 1998; Babzien et al., 1998), and several groups have also designed free electron lasers that are expected to operate in the VUV and soft-X-ray spectral regions (Rossbach, 1996; Tatchyn et al., 1996). The problems associated with X-ray lasing based on population inversions are well understood (Matthews and Rosen, 1988). Soft X-ray photons are at least 102 times more energetic than optical photons, and therefore, at least 102 times more energy must be used to pump the electrons in the atomic gain medium. Of course, there must be an appropriate set of energy levels in a given atom on which to create a population inversion.

284

ROBERT H UPSON et al.

Heavier atoms having a large number of protons could inherently bind their inner electrons more effectively than lighter atoms due to an enhanced Coulomb attraction. However, the medium must be ionized to eliminate the strong electron screening effects which reduce the effective nuclear charge experienced by the electrons to near-zero. Ultimately, removing - Z electrons to obtain an effective nuclear charge of Z (- ten times larger than for those elements used in optical lasers) requires an amount of energy that scales as Z2 per electron. Overall then, the energy required to pump and ionize an X-ray gain medium must be at least 103 times larger than that for an optical laser. Furthermore, since the excited state lifetime scales as T, energy must be supplied to an X-ray laser at least 104 times faster. Towards these ends, a tremendous amount of time, effort and money has been put into building very powerful laser systems, such as Novette at the Lawrence Livermore National Laboratories capable of delivering 10 14 W in a pulse of less than a nanosecond. Gain has been observed primarily on the 3p-3s (J= 2 -1) transitions of Ne-like ions of elements such as Se (Matthews et al., 1985), Ti (Boehly et al., 1990), Ag (Fields et al., 1992), Y (Da Silva et al., 1993) and others (MacGowan et al., 1992). The upper lasing levels were pumped by collisions between the electrons (stripped from the neutral atoms produced after the metal foil was vaporized by a laser pulse) and the resultant ions. Since highly reflective broadband mirrors for the X-ray region are not available, the input laser beam was line-focussed instead onto the metal target to create a cylindrically shaped plasma. In this way, amplification could take place on a single pass. Other transitions which are particularly promising for isoelectronic scaling into the X-ray region include the 4d - 4p (J= 0 - 1) lines of Ni-like ions (Daido et al., 1995, 1996). Wavelengths as short as 3.56 nm are possible by isoelectronic extrapolation of the series to Ni-like AuSI +. One problem associated with the collisional excitation scheme concerns the resultant axial inhomogeneities in the plasma, which distort the plasma column and limit the amplification. One extremely promising solution developed by Rocca and co-workers (1999) is the use of fast discharge excitation of capillary channels to pump plasma recombination and collisionally excited soft X-ray lasers. This type of discharge can deposit large amounts of electrical energy into a plasma column with excellent axial homogeneity, and length-to-diameter ratios approaching 1000: 1. Most significantly, the experimental setup is a very compact table-top device. For example, gain of 0.6 cm- I was observed on 3p - 3s (J= 0 -1) line ofNe-like Ar at 46.9 nm (Rocca et al., 1994). An enhanced average laser output of - 1 mW for this laser, at a repetition rate of 7 Hz, was subsequently achieved by using ceramic Ah03 capillaries which are resistant to ablation and assist in heat dissipation (Benware et al., 1998). Another promising table-top device involves a two-laser driver. One

9. Tunable Short Wavelength Generation and Applications

285

nanosecond infrared laser pulse is used to create the ionic gain medium, while a second a sub-picosecond pulse is used to excite a transient population inversion. High gain can be realized because the rise time of the excitation pulse is comparable to the excited state relaxation times. For example, a compact high gain (19 em-I) laser operating at 32.6 nm was achieved in this way on the 3p- 3s (J= 0 -1) line ofNe-like Ti (Nickles et al., 1997). While none of the X-ray laser systems described above is available commercially, the push down to wavelengths of - 4 nm is driven by the maximum contrast expected between water and biological materials. To this end, it has been suggested that harmonic generation by the interaction of a femtosecond pulse and a solid surface could provide another possible mechanism to realize this possibility (Murnane et aI., 1991; Gibbon, 1996). However achieved, the impact of soft X-ray lasers in the fields of medical imaging, microscopy and lithography is expected to be nothing short of profound (Suckewer and Skinner, 1990).

4.

APPLICATIONS

The number of potential applications using coherent short wavelength radiation is so vast that the emphasis will here be limited to only some of the work that has been carried out in the 6 - 19 eV energy range using coherent sources generated by nonlinear optical means.

4.1

Rare Gas Dimer Spectroscopy

Stoicheff and co-workers at the University of Toronto (Herman et al., 1985) were the first to use four-waving sources to determine the properties of the electronic states of diatomic molecules (Eden, 2000). One of this groups most impressive accomplishments was quantitative characterization of the lowest lying ungerade electronic states of Xe2 (Lipson et al., 1984, 1985), Kr2 (LaRoque et al., 1986), and Ar2 (Herman et al., 1988) homonuclear rare gas van der Waals excimers dissociating to RgeSo) + Rg* (ns, ns'); n = 6, 5, 4 for Xe, Kr, and Ar, respectively. Specifically, they recorded fluorescence excitation spectra of the Al u, BOu+, and CO u+ +- XOu+ transitions of jet-cooled samples at vibrational (all dimers) and rotational resolution (Kr2 and Ar2), and reported precise spectroscopic constants and potential energy curves, often for the first time. They were able to establish the importance of Hund's case (b) coupling for the low v'-levels of the Ar2 A-excited state versus a Hund's case (c) scenario for levels higher in the potential (Herman et al., 1987), and to deduce, from lifetime measurements, the dependence of the A1u +- XOg+ transition moment on internuclear separation (Madej et al., 1986). The magnitude of their success can only be

286

ROBERT H LIPSON et al.

appreciated when one realizes how little quantitative infonnation about the dimer electronic structures was available despite decades of prior experimental work, including the observation of VUV lasing. Our group at the University of Western Ontario, and others, have used coherent four-wave mixing VUV sources to extend the breakthrough Toronto single-photon studies on Xe2 to higher energy Rydberg states (Tsukiyama et al., 1988; Tsukiyama and Kasuya, 1992; Tsukiyama, 1993; Pibel et al., 1994; Mao et al., 1997a, 2000a, 2000b). We (Dimov et al., 1994; Hu et al., 1995; Lipson et al., 1995a; Dimov et al., 1995; Hu et al., 1996, 1997a, 1997b, 1998) and others (Green and Wallace, 1994) have also recorded (2 + 1) REMPI spectra of jet-cooled Xe2 and Kr2, which were mass-selected after ionization in a TOF mass spectrometer. The analyses of these spectra provided the first precise vibrational constants for many of the gerade excited states that dissociate to products Xe*(6p, 5d, 4f) + Xe and Kr*(5p) +Kr. TOF mass detection is particularly important for the study of the heteronuclear RgXe dimers (Rg = Ne, AI, Kr) because comparable concentrations of the homonuclear Rg2 and Xe2 species will be readily formed under jet conditions, with spectra that overlap the heteronuclear dimer transitions of interest when detection methods are used that exclude mass selectivity. As shown in Figure 6 the single isotopomer (2 + 1) REMPIITOF spectra ofKrXe (Lipson et al., 1995b; Mao et al., 1996), AIXe (Dimov et al., 1996), and NeXe (Mao et al., 1997b) in the vicinity of the Xe*(6p) atomic lines, recorded with linearly polarized fundamentals, are relatively free of signals from other species, and show rich vibronic structure which has been fully analyzed. Rotational branch structure was not resolved for these heavy van der Waals molecules, due in part to the relatively large VUV laser linewidths used. However, changes in bond lengths upon excitation could be deduced from Franck-Condon simulations of the vibrational band intensities. Although the first lines of each rotational branch of a two-photon transition could not be resolved, electronic excited state symmetries could be inferred from our (2 + 1) REMPIITOF spectra using circularly polarized fundamentals. The polarization ratio Ro, which is defined as; (22)

287

9. Tunable Short Wavelength Generation and Applications

a)

, ＮｾｉＧ＠

80000

I

. ＺＮｉｍｾＬ＠

79500

1

t

ｾ＠ ." . . .I ,

79000

I

78500

Jl

84

Kr 132Xe

II

78000

b) 40

JIJk 80000

.

I

I

,11,(,.I.!. :II,' .

79500

79000

I

Ar 132Xe

! .,II! . "

78500

I .

78000

" 80000

79500

79000

Wavenumber / cm-

78500

78000

1

ｆｩｾｲ･＠ 6. Overview (2 + 1) REMPI / TOF mass spectra for: a) 84Kr132Xe, b) 4OAr132Xe, and c)

'

:

':

"

,

..

0

4

2

6

8

10

14

12

Bond length / A = Ne, Ar, and Kr. The unusual shapes of the potential energy curves are detennined primarily by the R-dependent repulsion which arises when the ground state Rg atom penetrates and polarizes the Xe*(6p) Rydberg orbital.

Figure 8; Calculated potential energy curves for RgXe*(6p), Rg

a) 84050

84050

ＬｾＭＺＮ［＠

,....-y=-:= 84000

83950

84000

83950

83900 40

c) 84050

Ar 132Xe .;:>.;:>

84000

83950

Wavenumber / em

83900 -1

Figure 9. REMPIfTOF mass spectra involving the excited n = I Rydberg state of 40Ar\32Xe dissociating to Ar(ISo) + Xe* 5d[3/2]lo: a) Single-photon (I + I') REMPI VUV laser spectrum; b) (2 + I) REMPI spectrum obtained using linearly polarized fundamentals; c) (2 + 1) REMPI spectrum obtained using circularly polarized fundamentals.

291

9. Tunable Short Wavelength Generation and Applications

RgXe*( 6p) states are built on either the RgXe+ ground electronic state or first excited state, the dissociation energies of the neutral RgXe* (5d) states are considerably less than those for RgXe*(6p). This is attributed directly to the destabilizing repulsion energy predicted from the model above. The pcharacter of these states due to i-mixing yields weaker yet measurable twophoton spectra (Figures 9b,c and lOb,c). The polarization measurements and known excited state dissociation products immediately allow the ArXe and KrXe Rydberg states in Figures 9 and 10 to be assigned to Q= 1 symmetry.

a)

84

__v__v

Kr 131 Xe

_A'-------'A______

83900

83850

83800

83850

83800

b)

83900

c) ,.

--""'-

83900

J

84

"

"-

83800

83850

Wavenumber / em

Kr 131 Xe ;>;>

-1

n

Figure 10. REMPIITOF mass spectra involving the excited = I Rydberg state of 84Kr132Xe dissociating to KreSo) + Xe* 5d[3/21 1°: a) Single-photon (I + I') REMPI VUV laser spectrum; b) (2 + I) REMPI spectrum obtained using linearly polarized fundamentals; c) (2 + I) REMPI spectrum obtained using circularly polarized fundamentals.

5.

ZEKE SPECTROSCOPY

One of the most exciting new developments in the study of molecular ionization has been the introduction of high resolution zero kinetic energy (ZEKE) photoelectron spectroscopy (Muller-Dethlefs et al., 1984; Reiser et al., 1988). The method has been reviewed elsewhere (MUller-Dethlefs and Schlag, 1991; Muller-Dethlefs et al., 1995) and therefore only the salient features are mentioned here. ZEKE spectroscopy involves the detection and

292

ROBERT H. UPSON et al.

discrimination of very slow (ZEKE) photoelectrons from fast electrons produced by REMPI or direct ionization. Experimentally, ZEKE electrons will be released from a molecule when the wavelength of a tunable light source is scanned across a threshold for ionization which corresponds to the energy separation between the initial state of the molecule and the resultant ion state. For this reason, this technique is also often called threshold photoelectron spectroscopy. Since zero kinetic energy electrons detection is technically very demanding, near ZEKE electrons are usually generated instead by pulsed field extraction of long-lived molecular Rydberg states, and separated from the fast photoelectrons in an electron time-of-flight spectrometer. High n*-Rydberg state stability is attributed to Stark l, m( mixing due to weak DC fields in the excitation volume, and inhomogeneous electric fields created by nearby ions (Merkt and Zare, 1994; Martin et al., 1997). Overall, the technique has come to be known as pulsed-field ionization zero kinetic energy (PFI-ZEKE) spectroscopy. In comparison, conventional photoelectron spectroscopy uses a fixed high energy photon source to ionize a molecule, and the kinetic energies of the resultant photoelectrons are measured. While this approach has been used extensively to determine ion structure, it suffers from several disadvantages, the most important being a limited resolution of - 5 - lOmeV. ZEKE spectroscopy, on the other hand, offers a spectral resolution on the order of 0.2 cm- I , which is usually more than adequate to resolve rotational fine structure of small to medium molecules, and the vibrational structure of most heavier species. While it is common to associate the ultimate resolving power ofPFI-ZEKE spectroscopy with the laser bandwidth, it is actually the magnitude of the pulsed ionizing field, E, which is important. ｔｨｾｵｬｳ･､＠ electric field lowers the ionization limit by a factor of (4 - 6).J E , and therefore even a modest amplitude will result in a sampling of a fairly wide This problem can be range of Rydberg states at each threshold. circumvented by using an alternating polarity pulse sequence for fractional Stark state selection and electric field ionization (Dietrich et al., 1996; Dessent et al., 1999). The use of coherent VUV and XUV sources for single-photon PFI-ZEKE was pioneered by White and co-workers (Tonkyn and White, 1989), who recorded ground state ion spectra of diatomics (Tonkyn et al., 1989, 1992), polyatomics (Wiedmann el al., 1991; Tonkyn et al., 1991; Lee et al., 1992), and van der Waals molecules (Tonkyn and White, 1991). Unlike a resonance-enhanced multiphoton scheme which usually requires a known, bound intermediate state, single-photon PFI-ZEKE using coherent VUV sources can be viewed as a more universal approach (Hepburn, 1996). In this context Hepburn and co-workers have made many important contributions to the study of ionic excited states using PFI-ZEKE spectroscopy and VUV and XUV lasers generated by four-wave mixing

9. Tunable Short Wavelength Generation and Applications

293

(Kong et al., 1993, 1994; Mank et al., 1995; Kong and Hepburn, 1995, 1996; Hepburn, 1997). Not only could they determine accurate ionization potentials but they were also able to deduce dynamical information about the Rydberg states involved in pulsed field ionization through rotational branch intensity simulations (Buckingham et al., 1970). In one example, the intensity anomalies in the PFI-ZEKE spectra of the a 31;+ state ofNO+ could be attributed to auto ionization caused by a 'complex resonance' perturbation between the high-n * Rydberg series near the ionization limit and an interloper Rydberg state with a low principal quantum number and a large oscillator strength (Giusti-Suzor and Lefebvre-Brion, 1984). However, the mechanism for the intensity anomalies in the PFI-ZEKE spectrum of CO+ A2nl/2 (v+ = 0), which is perturbed by X 21;+(v+ = 10), is different. Here, core relaxation induced by collisions between the Rydberg electron and the ion-core of the neutral dominates. PFI-ZEKE spectroscopy can also be used to rotationally resolve vibrational states of the ground state ion in a region of its potential where the Franck-Condon factors' between the initial neutral state and the final ion states are small to negligible. For example, spectra of ground state O2+ levels between v+ = 6 and 24 levels were reported that resulted in 70% of the potential energy curve to become well characterized (Kong and Hepburn, 1994). In this case, the v+-Ievels were thought to be pumped directly because they are weakly coupled to one or more steeply repulsive neutral continuum states. This work also illustrates that PFI-ZEKE spectroscopy has the potential to prepare state-selective ions that can be used to study ionmolecule reactions. Recently, Zhu and Johnston (1991) devised a variant of PFI-ZEKE they called mass-analyzed threshold ionization (MATI) spectroscopy, where the ZEKE ions produced by pulsed field ionization are detected instead of the ZEKE electrons. This is a more challenging experiment since the kinetic energy differences between the slow and fast ions are very small. Still, MATI can provide the same information as PFI-ZEKE using electrons with the additional benefit of possible mass discrimination in a TOF mass spectrometer (Guilhaus, 1995), thereby allowing high resolution photoelectron spectra to be obtained for more demanding samples such as cluster jets. Hepburn and his group realized that the design of a MATI spectrometer would allow threshold ion-pair production spectroscopy (TIPPS) to be done, where highly vibrationally excited ion-pair states are dissociated by a pulsed electric field (Martin and Hepburn, 1997). Neutral ion-pair molecular states are ones that yield a pair of oppositely charge ions at their diabatic dissociation limits (Lawley and Donovan, 1993). Since the ion-pair dissociation limit is independent of rotation, the rotational line spacings in a TIPPS spectrum are determined by the neutral

294

ROBERT H LIPSON et al.

molecule. Furthennore, there are no centrifugal barriers to dissociation since the Coulombic interaction (ex: R- 1) binding the ion-pair state is much stronger than the effective centrifugal contribution (ex: R-2) to the potential. Thus, knowing two of three quantities: the IP of the A atom of an AB molecule, the electron affinity of the B atom, and the first dissociation energy of the AB neutral molecule, allows the third to be established from a simple thennodynamic cycle (Martin and Hepburn, 1998). However, a short extrapolation is necessary to find field-free quantities, since the pulsed electric field will deplete the highest energy ion-pair states. Still, this procedure introduces considerably less error than Birge-Sponer or thennochemical techniques. More importantly TIPPS can, in principle, be applied to all molecules. Recently, Shiell et al. (2000) reported the first threshold ion-pair spectrum of a triatomic molecule, H2 S corresponding to

c

.2 '0 :;

E

iii

,.... C ::I

.d ...

o

Figure JJ. The top trace is a simulation of the experimental H2S TIPP spectrum shown below, obtained by monitoring the H+ ion-pair yield. The highly structured spectrum is due to the excitation and pulsed-field dissociation of weakly bound ion-pair states just below each fieldfree threshold. The simulation was carried out assuming a rotational temperature oL105 K. [Reproduced with the permission of the authors.]

the H2S -+ H+ + SH- ion-pair channel near 15.175 eV. Their result, obtained by monitoring the H+ ion-yield, is shown in Figure 11. Since H2S is a bent asymmetric rotor, the spectrum could be simulated by calculating the

9. Tunable Short Wavelength Generation and Applications

coupling of its

lAI

295

ground state distribution of J"K K levels to each H+ + .'

h

SH-(J') threshold. The calculated result also shown in Figure 11 is in good agreement with experiment and is consistent with that expected from a direct dissociation of the ground state. An accurate value for Do(H-SH) could also be derived which is in good agreement with the results of photofragment measurements, but is more precise by a factor of 5.

6.

ANALYTICAL SPECTROSCOPY

In the last few years there has been a growing appreciation that the specific combination of VUV lasers and mass spectrometry provides a nearideal tool for organic molecule soft-ionization and detection, when compared to other experimental methods based on either longer wavelength coherent sources or non-optical strategies. It is well known that unambiguous molecular identification by conventional methods such as electron ionization (El) (Futrell, 1985), chemical ionization (CI) (Fales et al., 1971; Baldwin and McKafferty, 1973), and fast atom bombardment (FAB) (Barber et al., 1982; Caprioli, 1990) often requires an understanding of the complex fragmentation patterns that depend on the thermal energy in the parent species, as well as the excitation and ionization methods used. Similarly, ion yields from surface adsorbed molecules using secondary ion mass spectrometry, (SIMS) (Benninghoven and Sichtermann, 1978) and matrix assisted laser desorption and ionization, (MALDI) (Karas and Bahr, 1996) are strongly influenced by solid matrix effects. Since the number of neutral molecules and fragments produced by SIMS and MALDI are estimated to be orders of magnitude larger than the direct ion yields, a significant enhancement in detection sensitivity could be achieved if these neutral species were post-ionized following desorption. This approach, termed surface analysis by laser ionization, (SALI) (Becker and Gillen, 1984, 1985), decouples surface desorption from post-ionization, and therefore allows the two steps to be independently optimized. It is next worthwhile to compare REMPI post-ionization to a nonresonant single-photon SALI process. REMPI is particularly effective for elemental analysis, and different schemes have now been devised for almost every atom in the periodic table (Hurst et al., 1979). This approach is highly sensitive, selective, and uniformly efficient since a focussed laser can ionize almost every species in its focal volume with relative ease. The results of organic molecule detection using REMPI are, however, considerably less satisfying. First, there is the challenging task of determining the different electronic structures and oscillator strengths for each individual molecule. More importantly, REMPI tends to fragment the organic molecules in unpredictable ways (Schlag and Neusser, 1983),

296

ROBERT H. LIPSON et al.

resulting in the opposite of a structurally ideal mass spectrum which has a single peak at the parent ion mass. Non-resonant ionization with VUV lasers can minimize many of the problems above because detailed prior spectroscopic knowledge of the molecule is not required. Furthermore, quantitative measurements are also possible since single-photon ionization cross-sections tend to be more uniform from molecule to molecule than those for REMPI. Most significantly, the first ionization potential of most organic molecules lie in the 7 - 13 eV range, and therefore VUV lasers sources can be viewed as a 'universal detector' (Schuhle et ai., 1988). This burgeoning area of analytical chemistry has recently been reviewed (Butcher, 1999, 2000; Lipson and Shi, 2001). Our group has studied a series of O,S-dialkyl dithiocarbonates (xanthate esters) to explore the potential ofVUV lasers for organic soft-ionization (Shi et al., 1998). Our choice of molecules was driven by issues in the separation and concentration of valuable trace metals from crude ore by flotation (Persson, 1994; Shergold, 1984; Klassen and Mpokrousov, 1963; Gaudin, 1957). Mineral grains (size ｾ＠ 150 mm), produced by crushing and grinding, are rendered hydrophobic in aqueous solution by the addition of organic molecules called collectors. The coated grains attach themselves to air bubbles which float to the solution surface where they are removed and analysed usually by mass spectrometry. Due to the extensive fragmentation associated with conventional mass spectrometric techniques it is nearly impossible to correlate a particular fragment with a given parent molecule in mixtures of like compounds. One of the most important classes of organic collectors are the alkali salts of O-alkyl dithiocarbonates, ROC(S)S', commonly known as xanthates (Ackerman et ai., 1987) where, typically, the organic R-group is a short alkyl chain of 2-5 carbon atoms. Xanthate anions will selectively adsorb to the metal sulphide because either there is no adsorption at the non-sulphide mineral/water interface or, if there is adsorption, the organic groups are too short to impart a hydrophobic character to the grain. Some selectivity between different xanthate collectors is possible by the addition of separate chemicals called depressants or activation modifiers (Kim and Chryssoulis, 1996) and/or by changes in solution conditions (pH, for example). Usually, several collectors are added to the flotation circuit, and the conditions changed empirically to achieve the best results. Since xanthate salts have exceedingly low vapour pressures, volatile xanthate ester derivatives (Degani and Fochi, 1978), ROC(S)SR', were ionized as proof-of-principle using VUV wavelengths near 129.7 nm generated by four-wave difference mixing in Kr. Here Rand R' are either identical or different alkyl chains. Typical results for CH3SC(S)OC2Hs by focussed UV two-photon (A -216

297

9. Tunable Short Wavelength Generation and Applications

run; power density - 108 W cm-2) and unfocussed non-resonant VUV excitation are shown in Figures 12a and b, respectively. Since the UV excitation wavelength chosen is one-photon resonant with the sulfur n -+ 11:* molecular transition (Rosengren, 1962), the parent mass peak intensity in Figure 12 a is markedly reduced due to extensive fragmentation. In contrast, the dominant feature in the VUVITOF mass spectrum shown in Figure 12b is the parent ion, and signals due to low molecular mass ions are effectively absent.

a)

20

40

60

80

100

120

140

160

180

200

120

140

160

180

200

rrVz

b)

20

40

60

80

100

rrVz Figure 12. TOF mass spectra of O-ethyl, S-methyl dithiocarbonate by: a) UV laser twophoton ionization and b) VUV laser single photon ionization.

The VUVITOF mass spectra of CH3SC(S)OC2Hs, C2HsSC(S)OC2HS, i-C3H7SC(S)OC2H5' and n-C 3H7SC(S)OC 3H7-i are shown in Figure 13. A similar spectrum for a mixture of these esters can be compared in Figure 14, where the composition of the liquid mixture was established empirically to yield similar gas phase intensities for each parent ion. It can immediately be concluded that VUV single-photon ionization, coupled with TOF mass spectrometry, can readily distinguish components of gas-phase xanthate mixtures by parent mass. Furthermore, initial calculations suggest that the detection sensitivity of the experimental apparatus is in the femtomole regime. Preliminary studies in our lab have shown that a wide range of organic

ROBERT H. LIPSON et al.

298

molecules with different functional groups can be soft-ionized using VUV lasers. For technical simplicity most of these results were obtained with 118 nm radiation generated by non-resonant third harmonic conversion of the 355 nm output of aNd: YAG laser in phase-matched Xe/Ar gas mixtures.

ｾ＠

a)

CH,SC0C,H,

I "

,

I

,

,

I

20

｟ｾ＠

I,

,

,

I

,

,

,

I

,

,

,

I

S oo

4O

.A.______

_ ,

,

I

100

I

,

,

I

120

,

,

,

,

140

ｾ＠

1

__C_2H_5_sc_llo_c_2H_5_______• ___ "

,

I

20

•

,

,

I

•

,

40

•

I

••

,

I

00

80

S 00

80

,

I ,

I

,

,

,

100

I

!

,

•

,

•

120

140

ＬＶｾ＠

140

,

,

,

100

,

,

!

I

,

!

I

200 [mlz]

180

_______ ,

•

,

e

,

100

,

,

,

,

I,

•

,

200 [mlz]

180

S

c) , I,

II

Ｚｾ［ＲＧＢ＠

II I I I

20

ｾ＠ •

I

ＬｾＭ［ＳＷＱ＠

40

100

120

,.,A:"e ", 100

II I

20

40

00

80

I

I

I

100

I

,i;

120

I

•

i

I

140

I

I

i

I

100

Ｌｾ＠

I, ,

200 [mlz]

180

1 I

180

I

;

;

200 [mlz]

Figure 13. Individual VUV laserffOF mass spectra for: a) CH 3SC(S)OC2Hs; b) C2HsSC(S)OC2HS; c) i-C3H7SC(S)OC2Hs. and d) n-C3 H7SC(S)OC3Hri.

Many of the features in the early mass spectra of jet-cooled samples capable of hydrogen bonding such as alcohols, ROH; R = CH3-, CH3CHr , ionized by monochromatized vacuum ultraviolet synchrotron radiation, were due to cluster ions belonging to the series (ROH)nH+, n ｾ＠ 5 (Cook et al" 1980). The production of these hydrated species were attributed to the two following fragmentation pathways:

(ROH)n +hv ｾ＠

(ROH)n_1H+ +RO'+e-

(ROH)n +hv ｾ＠

(ROH)n_1H+ +RO-

(23)

299

9. Tunable Short Wavelength Generation and Applications 2

3

4

= 1.0: 4.4: 9.0: 24.8

,

,

,

I

,

,

,

,

20

,

,

t

40

,

,

60

,

,

,

,

80

,

,

,

I

•

100

I

,

,

120

,

,

,

,

140

,

,

It,

160

,

I

,

180

,

,

,

,

,

200

m/z Figure 14. VUV laserffOF mass spectrum for a mixture of I) CH)SC(S)OC2Hs, 2) C2HsSC(S)OC2HS' 3) i-C)H7SC(S)OC2Hs. and 4) n-C)H7SC(S)OC)H7-i in a volume ratio of VI: V2: V): V4 = 1.0: 4.4 : 9.0: 24.8.

m/z = 97

m/z m/z

I " , I"

o

20

= 33

I

m/z

= 65

=

129

ｈｍ･ｾ＠

-

m/z

I " , I " , I " , I."

40

Pure methanol vapor

60

= 161

I", I",'",',"

It"

It"

,. ,

80 100 120 140 160 180 200 220 240

mI z Figure 15. VUV laserffOF mass spectra of methanol (Me) and methanol clusters formed in the vapor phase above a room temperature liquid sample, and entrained in He to form a supersonic jet. The mass peaks correspond to protonated HMe" (n S 5) species produced by the dissociation mechanism discussed in the text.

300

ROBERT H. LIPSON et al.

A similar series was also observed in our VUV laser / TOF mass spectra for jet-cooled methanol, Figure 15 (Shi et ai, 2001), and for ethanol and ethanol/methanol mixtures. In all cases, the hydrated trimer ion was found to be the strongest signal over a wide range of stagnation pressures used to create the jets (Figure 16), and therefore we believe that the observed cluster distribution reflects the nascent distribution of alcohol clusters formed above the liquid surface. If the reactions in Eq. 23 are valid, then the most stable neutral cluster is the tetramer. This conclusion is in agreement with other experiments (Weltner and Pitzer, 1951), and calculations (Curtis, 1977; Hagemeister et ai, 1998; Buck et ai., 1998) which predict that this cluster has a cyclic structure which minimizes ring strain but maximizes hydrogenbonding cooperativity effects. Tunable VUV laser radiation has been used to initiate ion-molecule reactions where the probability of deuteron transfers from the methyl group ---H(Et)2 - -.- - H(Et)3 ...•.... H(Et)

0.11 0.10

...

e 0.09 ｾ＠

ｾＰＮＸ＠ ｾ＠

e 0.07

-

e; 0.06 0.05 0.04

,.,

, ,

, Ｎｾ＠

...

'

Ｍｾ＠

ｾ＠ ........... .•...•.

..

IS

20

.. . .

.. . ..

"

'

.

0.03 ...................o....L.......................-L..................o....J'-'--'..........ｾ＠ 10

'.

ｾ＠

25

.

...........................................................

30

35

40

PHe (psia) Figure 16. Plots of the intensity ratio of the hydrated ethanol HEt" cluster TOF mass spectral signal, I(HEt,,) to monomer signal, I(Et) as a function of the helium stagnation pressure, PHe, used to create the supersonic jet. The graph shows no marked changes in cluster size distribution with stagnation pressure, which implies that the observed distributions reflect the nascent distribution of alcohol clusters above the liquid sample.

and proton transfers from the hydroxyl group in methanol dimer were measured as a function ofVUV photon energy between 10.91 and 10.49 eV (Tsai, 1999). These results strongly indicate that VUV lasers are an ideal technology for probing these important reactions, and could supplement and refine fascinating experiments currently being done by electron impact (Lyktey et ai., 1996; Furlani and Garvey, 1997; Lyktey et ai., 2000;

9. Tunable Short Wavelength Generation and Applications

301

Charlebois et al., 2000).

7.

SUMMARY AND CONCLUSIONS

This chapter has dealt with various schemes that have successfully been used to generate coherent short wavelength light, and some of the recent applications in molecular and analytical spectroscopy. One of the purposes of this work was to show that, because of the fundamental physical constraints governing laser action, many of the successes in coherent short wavelength generation correlate with exciting new directions in longer wavelength laser development, nonlinear optics, materials science, and high field, high intensity electron and photon based physics. The discussion was not intended to provide a comprehensive review of such a broad field, but instead, a flavor of its exciting evolution. It is not unreasonable, however, to believe that the future of such sources is bright, and that their impact will begin to cut broadly across many academic, medical and industrial pursuits.

ACKNOWLEDGMENTS This work was supported by the Natural Sciences and Engineering Research Council of Canada, The University of Western Ontario, and the Center for Chemical Physics of the University of Western Ontario. The contributions of former group members Drs. S. S. Dimov, X. K. Hu, and D. M. Mao are gratefully acknowledged. The authors thank Professor John Hepburn for Figure 11. Y. J. S. thanks the Ontario Government for an Ontario Graduate Scholarship.

REFERENCES Ackerman, P.K., Harris, G.H., Klimpel, R.R., and Aplan F.F., 1987, Int. J. Min. Process. 21:105. Anderson, S., 2001, The laser marketplace 2001 - Part I: Nondiode lasers. Laser Focus World. Anderson, S.L., 1992, Adv. Chern. Phys. 82:177. Armstrong, JA, Bloembergen, N., Ducuing, J., and Pershan, P.S., 1962, Phys. Rev. 127:1918. Ashfold, M.N.R., and Howe, J.D., 1994, Ann. Rev. Phys. Chern. 45:57. Augst, S., Meyerhofer, D.D., Strickland, D., and Chin, S.L., 1991, J. Opt. Soc. Am. B 8:858. Augst, S., Strickland, D., Meyerhofer, D.D., Chin, S. L., and Eberly, J. H., 1989, Phys. Rev.

Lett. 63:2212. Babzien, M., Ben-Zvi, I., Catravas, P., Fang, J.-M., Marshall, T.C., Wang, X.J., Wurtele, J.S., Yakimenko, V., and Yu, L.H., 1998. Phys. Rev. E 57:6093.

302

ROBERT H LIPSON et al.

Baldwin, M.A., and McLafferty, F.W., 1973, Org. Mass Spectrom. 7: 1353. Barber, M., Bordoli, RS., Elliot, 0.1, Sedgwick, R.D., and Taylor, A.N., 1982, Anal. Chem. 54:645A. Basov, N.G., Danilychev, V.A., and Popov, Yu. M., 1971, Sov. J. Quantum Electron. 1:29. Baumgartner, RA., and Byer, RL., 1979, IEEE J. Quantum Electron. QE-15:432. Becker, C.H., and Gillen, K.T., 1984, Anal. Chem. 56:1671. Becker, C.H., and Gillen, K.T., 1985, J. Vac. Sci. Technol. A 3:1347. Becker, W.o Long, S., and McIver, 1 K., 1990, Phys. Rev. A 41:4112. Benerofe, S.1., Yin, G.-Y., Barty, C.P.1., Young, IF., and Harris, S. E., 1991, Phys. Rev. Lett. 66:3136. Benninghoven, A., and Sichtermann, W.K., 1978, Anal. Chem. 50: 1180. Benware, 8.R., Macchietto, C.D., Moreno, C.H., and Rocca, J.1., 1998, Phys. Rev. Lett. 81:5804. Bilt, S., Weaver, E.G., Dunning, F.B., and Tittel, F.K, 1977, Opt. Lett. 1:58. Bischel, W.K., Bokor, 1, Kligler, D.l, and Rhodes, C.K., 1979, IEEE 1. Quantum Electron. QE-15:380. Bitto, H., and Willmott, P.R., 1992, Chem. Phys. 165:113. Bjorklund, G.c., 1975, IEEE 1. Quantum Electron. QE-ll:287. Boehly, T., Russotto, M., Craxton, R.S., Epstein, R., Yaakobi, B., Da Silva, L.B., Nilsen, 1., Chandler, E.A., Fields, D.1., MacGowan, B.1., Matthews, D.L., Scofield, lH., and Shimkaveg, G., 1990, Phys. Rev. A 42:6962. Bolt, RF., Gashurov, G., and Liu, Y.S., 1985, Laser Focus 21 (October): 110. Bordui, P.F., and Fejer, M.M., 1993, Annu. Rev. Mater. Sci. 23:321. Borsutsky, A., Brunger, R, Huang, C., and Wallenstein, R, 1991, Appl. Phys. B 52:55. Bos,F., 1981, Appl. Opt. 20:3553. Bosenberg, W.R., Cheng, L.K., and Tang, c.L., 1989, Appl. Phys. Lett. 54: 13. Brau, C.A., 1984, Rare Gas Halogen Excimers in Excimer Lasers (2 nd edition) (C.K. Rhodes, ed.), Topics in Applied Physics 30:87-137, Springer-Verlag, Berlin. Brau, C.A., 1988, Science 239: 1115. Bray, RG., and Hochstrasser, R.M., 1979, Mol. Phys. 31: 1199. Buck, U., Siebers, l-G., and Wheatley, R.1., 1998, J. Chem. Phys. 108:20. Buckingham, A.D., Orr, B.1., and Sichel, 1M., 1970, Phil. Trans. Roy. Soc. London A 268:147. Butcher, D.1., 1999, Microchemical Journal 62:354. Butcher, D.1., 2000, Microchemical Journal 66:55. Caprioli, R.M., 1990, Anal. Chem. 62:477A. Charlebois, lP., DeLeon, R.L., and Garvey, J.F., 2000,1. Phys. Chem. A 104:6799. Chen, C., Wu, Y., Jiang, A., Wu, B., You, G., Li, R., and Lin, S., 1989 J. Opt. Soc. Am B 6:616. Chen, C., Xu, Z., Deng, D., Zhang, 1, Wong, G.K.L., Wu, 8., Ye, N., and Tang, D., 1996, Appl. Phys. Lett. 68:2930. Chen, T.-l, Zitter, R N., and Tao, R., 1995, Phys. Rev. A 51:706. Cheng, L.K., Bosenberg, W.R., and Tang, C.L., 1988, Appl. Phys. Lett. 53: 175. Christov, I.P., Murnane, M.M., and Kapteyn, H.C., 1998, Opt. Comm. 148:75. Condon, E.U., and Shortley, G.H., 1964, The Theory of Atomic Spectra. Cambridge, University Press. Cook, K.D., Jones, G.G., and Taylor, lW., 1980, Int. J. Mass Spectrom. Ion Phys. 35:273. Cotter, D., 1979a, Opt. Lett. 4: 134. Cotter, D., 1979b, Opt. Comm.31:397. Cromwell, E., Trickl, T., Lee, Y.T., and Kung, A.H., 1989, Rev. Sci. Instrum. 60:2888. Curtis, L.A., 1977,1. Chem. Phys. 67:1144. Da Silva, L.B., MacGowan, B.1., Mrowka, S., Koch, lA., London, R.A., Matthews, D.L., and Underwood, J.H., 1993, Opt. Lett. 18: 1174.

9. Tunable Short Wavelength Generation and Applications

303

Daido, H., Kato, Y., Murai, K., Ninomiya, S., Kodama, R., Yuan, G., Oshikane, Y., Takagi, M., Takabe, H., and Koike, F., 1995, Phys. Rev. Lett. 75:1074. Daido, H., Ninomiya, S., Imani, T., Kodama, R, Takagi, M., Kato, Y., Murai, K., Zhang, J., You, Y., and Gu, Y., 1996, Opt. Lett. 11:958. Dawson, J.M., 1989, Sci. Am. 260:54. Degani, I., and Fochi, R, 1978, Synthesis 5:365. Demtroder, W., 1996, Laser Spectroscopy Basic Concepts and Instrumentation (2nd edition). Springer-Verlag, Berlin, p.341. Dessent, C.E.H., Haines, S.R, and Muller-Dethlefs, K., 1999, Chem. Phys. Lett. 315:103. Dietrich, J.-J., Muller-Dethlefs, K., and Ya. Baranov, L., 1996, Phys. Rev. Lett. 76:3530. Dimov, S.S., Cai, J.Y., and Lipson, RH., 1994, J. Chern. Phys. 101:10313. Dimov, S.S., Hu, X.K., Mao, D.M., and Lipson, RH., 1995, Chern. Phys. Lett. 239:332. Dimov, S.S., Hu, x.K., Mao, D.M., Cai, J.Y., and Lipson, RH., 1996, J. Chem. Phys. 104:1213. Dobele, H.F., Hor, M., and Rowekamp, M., 1987, Appl. Phys. B 42:67. Dreyfus, R W., and Hodgson, R T., 1972, Appl. Phys. Lett. 20: 195. Dreyfus, R W., and Hodgson, R T., 1974, Phys. Rev. A 9:2635. Dubinskii, M.A., Cefalas, A.C., Sarantopoulou, E., Spyrou, S.M., Nicolaides, C.A., Abdulsabirov, R Y., Korableva, S.L., and Semashko, V.V., 1992, J. Opt. Soc. Am. B 9:1148. Eckhardt, G., 1966, IEEE J. Quantum Electron. QE-2:1. Eckhardt, G., Hellwarth, R W., McClung, F.J., Schatz, S.E., Weiner, D., and Woodbury, E.J., 1962, Phys. Rev. Lett 9:455. Eden, J.G., 2000, CanJ. Phys. 78:397. Eikema, K.S., Ubachs, W., Vassen, W., and Hogervorst, W., 1997, Phys. Rev. A 55:1866. Endert, H., Scaggs, M., Basting, D., and Stamm, U., 1999, J. Laser Appl. 11: 1. Ewing, J.J., and Brau, C.A,1975, Appl. Phys. Lett. 27:350. Fales, H.M., Nagai, Y., Milne, G.W.A., Brewer, Jr., H.B., Bronzert, T.1., and Pisano, L.J., 1971, Anal. Biochem. 43:288. Fan, Y.x., Eckardt, RC., Byer, RL., Nolting, J., and Wallenstein, R, 1988, Appl. Phys. Lett. 53:2014. Fenner, W.R, Hyatt, H.A., Kellam, J.M., and Porto, S.P.S., 1973, J. Opt. Soc. Am. 63:73. Fields, D.J., Walling, R.S., Shimkaveg, G.M., MacGowan, B.1., Da Silva, L.B., Scofield, J.H., Osterhead, A.L., Philips, T.W., Rosen, M.D., Matthews, D.L., Goldstein, W.H., and Stewart, RE., 1992, Phys. Rev. A 46: 1606. Foldes, LB., Bakos, J.S., Veres, G., Bakonyi, Z., Nagy, T., and Szatmari, S., 1996, IEEE J. Quantum Electron. QE-2:776. Franken, P.A., Hill, A.E., Peters, C.W., and Weinreich, G., 1961, Phys. Rev. Lett. 7:118. French, R.H., Ling, J.W., Ohuchi, F.S., and Chen, C.T., 1991,Phys. Rev. B 44:8496. Furlani, T.R, and Garvey, J.F., 1997, Mol. Phys. 92:449. Futrell, J.H., 1985, In Electron Impact Ionization (T.D. Mark and G.H. Dunn, eds.), WienNew York, Springer-Verlag, p.364. Gaudin, A.M., 1957, Flotation. McGraw-Hili Book Co., New York. Georges, A.T., Lambropoulos, P., and Marburger, J.H., 1977, Phys. Rev. A 15:300. Gerardo, J.B.,and Johnson, A.W., 1973, IEEE J. Quantum Electron. QE-9:748. Gibbon, P., 1996, Phys. Rev. Lett. 76:50. Gibbon, P., and Forster, E., 1996, Plasma Phys. Control. Fusion. 38:769. Gibson, G., Luk, T.S., and Rhodes, C.K., 1990, Phys. Rev. A 41:5049. Giordmaine, J.A., and Miller, RC., 1965, Phys. Rev. Lett. 14:973. Giordmaine, J.A., and Miller, R.C., 1966, Appl. Phys. Lett. 9:298. Giusti-Suzor, A., and Lefebvre-Brion, H., 1984 Phys. Rev. A 30:3057. Glownia, J.H., Gnass, D.R, and Sorokin, P.P., 1994, J. Opt. Soc. Am. B 11:2427. Green, D.S., and Wallace, S.C., 1994, J. Chem. Phys. 100:6129.

304

ROBERT H LIPSON et al.

Guilhaus, M., 1995, J Mass Spectrom. 30:1519. Hagemeister, F.G., Gruenloh, C.1., and Zwier, T.S., 1998, J. Phys. Chern. A 102:82. Halbout, J.M., Blit, S., Donaldson, W., and Tang, c.L., 1979, IEEE J Quantum Electron. QE-15: 1176. Halbout, J.-M., and Tang, C.L., 1982, IEEE J. Quantum Electron. QE-18:410. Harris, S.E., 1969, Proc. IEEE 57:2096. Harris, S.E., 1973, Phys. Rev. Lett. 31 :341. Hartig, W., 1978, Opt. Commun. 27 :447. Haub, J.G., Johnson, M.J., Orr, 8.J., and Wallenstein, R, 1991, Appl. Phys. Lett. 58:1718. Hellwarth, R W., 1962, Phys. Rev. 130: 1850. Hepburn, J.W., 1995, In Laser Techniques o/Chemistry Vol. XXIII (A8. Meyers and T.R Rizzo, eds.), John-Wiley & Sons Inc., p.149. Hepburn, J. W., 1996, Chern. Soc. Rev. 25:281. Hepburn, J.W., 1997,J. Chern. Phys. 107:7106. Herman, P.A., Koike, M., Hsu, C.W., Blank, D., Yang, X.M., Suits, AG., Lee, Y.T., Evans, M., Ng, C.Y, Flaim, C., and Padmore, H. A., 1997, Rev. Sci.lnstrum. 68:1945. Herman, P.R., LaRocque, P.E., and. Stoicheff, B.P., 1988, J. Chern. Phys. 89:4535. Herman, P.R., LaRocque, P.E., Lipson, RH., Jamroz, W., and. Stoicheff, 8.P., 1985, Can. J. Phys. 63: 1581. Herman, P.R., Madej, A.A., and Stoicheff, 8.P., 1987, Chern. Phys. Lett. 134:209. Hilbig, R., Hilber, G., Lago, A, Wolff, B., and Wallenstein, R., 1986, Comments At. Mol. Phys.18:157. Hilbig, R, Hilber, G., Timmermann, A., and Wallenstein R., 1984, In Laser Techniques in the Extreme Ultraviolet (S.E. Harris and T.B. Lucatorto, eds.), AlP Conference Proceedings, 119 New York, American Institute of Physics, p.l. Hilbig, R, Hilber, G., and Wallenstein, R., 1986, Appl. Phys. B 41:225. Hilbig, R, Lago, A, and Wallenstein, R., 1984, Opt. Comm. 49:297. Hilbig, R., and Wallenstein, R., 1981, IEEE J. Quantum Electron. QE-17: 1566. Hilbig, R., and Wallenstein, R., 1983, Opt. Commun. 44:283. Hirakawa, Y., Nagai, A., Muraoka, K., Okado, T., and Maeda, M., 1993, Opt. Lett. 18:. Hirakawa, Y., Okado, T., Maeda, M., and Muraoka, K., 1997, J. Opt. Soc. Am. B 14: 1029. Hodgson, R.T., 1971,J Chern. Phys. 55:5378. Hodgson R T., and Dreyfus R.W., 1972, Phys. Rev. Lett. 28:536. Hodgson, R.T., Sorokin, P.P., and Wynne, J.1., 1973, Phys. Rev. Lett. 32:343. Hoff, P.W., Swingle, J.C., and Rhodes, C.K., 1973, Appl. Phys. Lett. 23:245. Hoffman, J.M., Hays, A.K., and Tisone, G.C., 1976, Appl. Phys. Lett. 28:538. Hogan, M.1., Pellegrini, C., Rosenzweig, J., Anderson, S., Frigola, P., Tremaine, A., Fortgang, c., Nguyen, D.C., Sheffield, R.L., Kinross-Wright, J., Varfolomeev, A, Varfolomeev, AA., and Tolmachev, S., 1998, Phys. Rev. Lett. 81:4867. Holland, F., Huber, K.P., Hoy A.R, and Lipson, R.H., 1991, J Mol. Spectrosc. 145:164. Hollenstein, U., Palm, H., and Merkt, F., 2000, Rev. Sci. Instrum. 71:4023. Hooker, S.M., and Webb, C.E., 1994, Prog. Quant. Electr. 18:227. Hu, X.K., Mao, D.M., Dimov, S.S., and Lipson, R.H., 1996, Phys. Rev. A. 54:2814. Hu, X.K., Mao, D.M., Dimov, S.S., and Lipson, R.H., 1997a, J. Chern. Phys. 106:9411. Hu, x.K., Mao, D.M., Dimov, S.S., and Lipson, R.H., 1997b, J Chern. Phys. 106:9419. Hu., X.K., Mao, D.M., Dimov, S.S., and Lipson R.H., 1995, Chern. Phys. 201:557. Hu, X.K., Mao, D.M., Shi, Y.1., Dimov, S.S., and Lipson, R.H., 1998, J. Chern. Phys. 109:3944. Hughes, W.M., Shannon, J., and Hunter, R., 1974, Appl. Phys. Lett. 24:488. Huo, Y., Shimizu, K., and Yagi, T., 1992, J. Appl. Phys. 72:3258. Hurst, G.S., Payne, M.G., Kramer, S.D., and Young, J.P., 1979, Rev. Mod. Phys. 51:767. Jamroz, W., LaRocque, P.E., and Stoicheff, 8.P., 1982 Opt. Lett. 7:148-150 Jamroz, W., and Stoicheff, B.P., 1983, Progress in Optics XX:326.

9. Tunable Short Wavelength Generation and Applications

305

Joffre, M., Yaron, D., Silbey, RJ., and Zyss, J., 1992,). Chern. Phys. 97:5607. Johnson, P.M., and Otis, C.E., 1981, Ann. Rev. Phys. Chern. 32: 139. Kajikawa, K., Anzai, T., Takezoe, H., Fukuda, A., Okada, S., Matsuda, H., Nakanishi, H., Abe, T., and Ito, H., 1992, Chern. Phys. Lett. 192: 113. Karas, M., and Bahr, U., 1996, In Mass Spectrometry in Biornolecular Sciences (R. M. Caprioli, A. Malorni and G. Sindona, eds.), Nato ASI Series Vol. 475 Kluwer Academic Publishers, The Netherlands, p.33. Kato, K., 1986, IEEE J. Quantum Electron. QE-22:1013. Katz, H.E., Scheller, G., Putvinski, T.M., Schilling, M.I., Wilson, W.L., and Chidsey, C.E.D., 1991, Science 254: 1485. Keldysh, L.V., 1965, Sov. Phys. JETP 20:1307. Kim, lY., and Chryssoulis, S.L., 1996, Min. Metall. Process. 13:69. Kiriyama, H., Matsuoka, S., Nakano, F., and Yamakawa, K., 2000, Opt. Rev. 7:281. Kitamoto, A., Kondo, T., Shoji, I., and Ito, R., 1995, Opt. Rev. 2:280. Kittlemann, 0., Ringling, J., Korn, G., Nazarkin, A., and Hertel, LV., 1996, Opt. Lett. 21:1159. Klassen, V.I., and Mpokrousov, V.A., 1963, An Introduction to the Theory of Flotation. Butterworth, London. Koehler, H.A., Ferderber, L.l, Redhead, D. L., and Ebert, P. J., 1972, Appl. Phys. Lett. 21:198. Komatsu, R., Sugawara, T., Sassa, K., Sarukura, N., Liu, Z., Izumida, S., Segawa, Y., Uda, S., Fukuda, T., and Yamanouchi, K., 1997, App. Phys. Lett. 70:3492. Kong, W., and Hepburn, J.W., 1994, CanJ. Phys. 72:1284. Kong, W., and Hepburn, J.W., 1995, J. Phys. Chern. 99:1637. Kong, W., and Hepburn, J.W., 1996, Int. J. Mass Spectrorn.lon Process. 159:27. Kong, W., Rodgers, D., and Hepburn, lW., 1993,J. Chern. Phys. 99:8571. Kong, W., Rodgers, D., and Hepburn, lW., 1994, Chern. Phys. Lett. 221:301. Krause, lL., Schafer, K. 1, and Kulander, K. C., 1992, Phys. Rev. Lett. 68:3535. Krol, D.M, DiGiovanni, DJ, Pleibel, W., and Stolen, R.H., 1993, Opt Lett. 18:1220. Kung, A.H, 1974, Appl. Phys. Lett. 25:653. Kung, A.H., Young, IF., and Harris, S.E., 1973, Appl. Phys. Lett. 22:301. Kuzyk, M.G., and Dirk, C.W., 1998, Characterization Techniques and Tabulations for Organic Nonlinear Optical Materials. Marcel Dekker Inc., New York. Langer, H., Puell, H., and Rohr, H., 1980, Opt. Cornrnun.34:137. LaRocque, P.E., Lipson, R.H., Herman, P.R., and Stoicheff, B.P., 1986, J. Chern. Phys. 84:6627. Lawley, K.P., and Donovan, R.J., 1993, J. Chern. Soc. Faraday Trans. 89: 1885. Le Blanc, S.P., Qi, Z., and Sauerbrey, R., 1995, Appl. Phys. B 61:439. Lee, M.-T., Wang, K., McKoy, V., Tonkyn, R.G., Wiedmann, R.T., Grant, E.R., and White, M.G., 1992, J. Chern. Phys. 96:7848. L'Huillier A., and Balcou, P., 1993, Phys. Rev. Lett. 70:774. Li, D., Ratner, M.A., Marks, TJ., Zhang, C., Yang, J., and Wong, G.K., 1990, J. Am. Chern. Soc. 112:7389. Lin, J.T., 1990, Opt. Quantum Electron. 22:S283. Lipson, R.H., 1999, Physics in Canada 55:223. Lipson, R.H., Dimov, S.S., Cai, lY., Wang, P., and Bascal, H.A., 1995a, J. Chern. Phys. 102:5881. Lipson, R.H., Dimov, S.S., Hu, X.K., Mao, D.M., and Cai, lY., 1995b, J. Chern. Phys. 103:6313. Lipson, R.H., Dimov, S.S., Wang, P., Shi, YJ., Mao, D.M., Hu, X.K., and Vanstone, J., 2000, Instrurn. Sci. Tech. 28:85. Lipson, R.H., and Field, R.W., 1999, J. Chern. Phys. 110: 10653. Lipson, R.H., LaRocque, P.E., and Stoicheff, B.P., 1984, Opt. Lett. 9:402.

306

ROBERT H UPSON et al.

Lipson, RH., LaRocque, P.E., and Stoicheff, B.P., 19851. Chern. Phys. 82:4470. Lipson, RH., and Shi, Y.I., 2002, In Ultraviolet Spectroscopy and UV Lasers (P. Misra and M. Dubinskii, eds.), Marcel Dekker. Lipson, RH., LaRocque, P.E., and Stoicheff, B.P., 1988, J. Chern. Phys. 89:4535. Lockyer, N.P., and Vickennan, J.C., 1997, Laser Chern. 17:139. Lyktey, M.M.Y., Deleon, RL., Shores, K.S., Furlani, T.R, and Garbey, J.F., 2000,1. Phys. Chern. A 104:5197. Lyktey, M.Y.M., Rycroft, T., and Garvey, J.F., 1996,1. Phys. Chern. 100:6427. MacGowan, B.J., Da Silva, L.B., Fields, OJ. Keane, C.J., Koch, J.A, London, R.A., Matthews, D.L., Maxon, S., Mrowka, S., Osterheld, AL., Scofield, J.H., Shimkaveg, G., Trebes, J.E., and Walling, R.S., 1992, Phys. Fluids B 4:2326. Macklin, J.J., Kmetic, J.D., and Gordon III, C. L., 1993, Phys. Rev. Lett. 70:766. Madej, A.A., Hennan, P.R, and Stoicheff, B.P., 1986, Phys. Rev. Lett. 57:1574. Mahon, R, McIlrath, TJ., and Koopman, D.W., 1978, Appl. Phys. Lett. 33:305. Majewski, W.A, Pfanstiel, J.F., Plusquellic, D.F., and Pratt, D.W., 1995, In Laser Techniques of Chemistry Vol. XXIII (A.B. Meyers and T.R Rizzo, eds.), Wiley, New York, p.IOI. Mank, A., Nguyen, T., Martin, J.D.D., and Hepburn, J.W., 1995, Phys. Rev. A 51:RI. Mao, D.M., Hu, x.K., Bascal, H.A, Dimov, S.S., and Lipson, RH., I 997b, 1. Chem. Phys. 107:4817. Mao, D.M., Hu, X.K., Dimov, S.S., and Lipson, RH., 1996,1. Phys. B. 29:L89. Mao, D.M., Hu, X.K., Dimov, S.S., and Lipson, RH., I 997a, J. Mol. Spectrosc. 181:435. Mao, D.M., Hu, XX, Shi, YJ., and Lipson, RH., 1999, J. Chern. Phys. 111:2985. Mao, D.M., Hu, X.K., Shi, Y.J., and Lipson, RH., 2000a, Chern. Phys. 257:253. Mao, D.M., Hu, X.K., Shi, Y.J., Ma, J., and Lipson, RH., 2000b, Can 1. Phys. 78:433. Marder, S.R, Beratan, D.N., and Cheng, L.-T., 1991, Science 252:103. Marder, S.R, Cheng, L.-T., Tiemann, B. G., Friedli, A C., Balnchard-Desce, M., Perry, J.W., and Skindlwj, J., 1994, Science 263:511. Marder, S.R, Perry, J.W., and Yakymyshyn, C.P., I 994a, Chern. Mater. 6:1137. Marder, S.R., Sohn, J.E., and Stucky, G.D., 1991, Materials for Nonlinear Optics. ACS Symposium Series 455. Marinero, E.E., Rettner, C.T., Zare, R.N., and Kung, AH, 1983, Chern. Phys. Lett. 95:486. Martin, J.D.D., and Hepburn, J.W., 1997, Phys. Rev. Lett. 79:3154. Martin, J.D.D., and Hepburn, J.W., 1998, J. Chem. Phys. 109:8139. Martin, J.D.D., Hepburn, J.W., and Alcaraz, C., 1997,1. Phys. Chern. A. 101:6728. Massey, G.A., and Johnson, J.C., 1976, IEEE 1. Quantum Electron. QE-12:721. Matthews, D.L., and Rosen, M. D., 1988, Sci. Am. (Decernber):86. Matthews, D.L., Hagelstein, P.L., Rosen, M.D., Eckart, M.J., Ceglio, N.M., Hazi, A.U., Medecki, H., MacGowan, B.J., Trebes, J.E., Whitten, B.L., Campbell, E.M., Hatcher, C.W., Hawryluk, AM., Kauffman, RL., Pleasance, L.D., Rambach, G., Scofield, J.H., Stone, G., and Weaver, T.A., 1985, Phys. Rev. Lett. 54:110. McPherson, A., Gibson, G., Jara, H., Johann, U., Luk, T.S., McIntyre, LA, Boyer, K., and Rhodes, C.K., 1987,1. Opt. Soc. Am. B 4:595. Mennicke, H., Meyer, J., and Sinnott, T:, 1976, Phys. Lett. 57A:477. Merkt, F., and Schmutz, H., 1998,1. Chern. Phys. 108:10033. Merkt, F., and Zare, RN., 1994, J. Chem. Phys. 101:3495. Merkt, F., 2000, Chimia 54:89. Messier, J., Kajzar, F., and Prasad, P., 1991, Organic Molecules for Nonlinear Optics and Photonics. Nato ASI Series E: Applied Science, 194. Miyazaki, K., Sakai, H., and Sato, T., 1986, Opt. Lett. 11:797. Momma, C., Eichmann, H., Tiinnennann, A., Simon, P., Marowsky, G., and Wellegehausen, B., 1993, Opt. Lett. 18:1180. . Moore, C.E., 1971, Natl. Bur. Stand. Cir. N 467, U.S. GPO, Washington, D.C., Vol. 3. Mori, Y., Kuroda, I., Nakajima, S., Sasaki, T., and Nakai, S., 1995, Jpn. 1. Appl. Phys.

9. Tunable Short Wavelength Generation and Applications

307

34:L296. Morley, J.O., Docherty, V.J., and Pugh, D., 1987, J. Chem. Soc. Perkin Trans. I/p.1351. Moulton, P.F., 1986, J. Opt. Soc. Am. B 3:125. Mukenheim, W., Lokai, P., Burghardt, 8., and Basting, 0.,1988, Appl. Phys. B 45:259. Muller III, C.H., Lowenthal, D.O., DeFaccio, M.A., and Smith, A.V., 1988, Opt. Lett. 13:651. Muller-Dethlefs, K., Sander, M., and Schlag, E.W., 1984, Chem. Phys. Lett. 112:291. Muller-Dethlefs, K., and Schlag, E.W., 1991, Annu. Rev. Phys. Chem. 42:109. Muller-Dethlefs, K., Schlag, E.W., Grant, E.R., Wang, K., and McKoy, B.V., 1995, Adv. Chem. Phys. XC: 1. Mullin, C.S., Kim, D., Feller, M.B., and Shen, Y.R., 1995, Phys. Rev. Lett. 74:2678. Murnane, M.M., Kapteyn, H.C., Rosen, M.D., and Falcone, R. W., 1991, Science 251:531. Muschenborn, H.J, TheiS, W., and DemtrOder, W., 1990,Appl. Phys. B. 50:365. Nalwa, H.S., and Miyata, S., 1997, Nonlinear Optics of Organic Molecules and Polymers. CRC Press, Boca Raton. Nascimento, M.A.C., 1983, Chem. Phys. 74:51. Nath, G., and Haussiihl, S., 1969, Appl. Phys. Lett. 14: 154. Nazarkin, A., Korn, G., Kittelmann, 0., Ringling, J., and Hertel, LV., 1997, Phys. Rev. A 56:671. Nickles, P.V., Shlyaptsev, V.N., Kalchnikov, M., Schnurer, M., Will, I., and Sander, W., 1997, Phys. Rev. Lett. 78:2748.

Nolting, J., and Wallenstein, R., 1990, Opt. Commun. 79:437. Nolting, J., Kunze, H., Schutz, I., and Wallenstein, R., 1990, Appl. Phys. B 50:331. Ohwa, M., and Obara, M., 1987 Appl. Phys. Lett. 51:958. Orr, 8.J., and Ward, J.F., 1971, Mol. Phys. 20:513. Page, R.H., Larkin, R.J., Kung, A.H., Shen, Y.R., and Lee, Y. T., 1987, Rev. Sci. Instrum. 58:1616. Patterson, E.L., Gerardo, J.B., and Johnson A.W., 1972, Appl. Phys. Lett.ll:293. Persson, I., 1994, J. Coord. Chem. 32:261. Petrov, V., Rotermund, F., Noack, F., Komatsu, R., Sugawara, T., and Uda, S., 1998, J. Appl. Phys.84:5887.

Pibel, C.D., Yamanouchi, K., and Tsuchiya, S., 1994, J. Chem. Phys. 100:6153. Puell, H., Scheingraber, H., and Vidal, C.R., 1980, Phys. Rev. A 22:1165. Pummer, H., Egger, H., Luk, T.S., Srinivasan, T., and Rhodes, C.K., 1983, Phys. Rev. A. 28:795. Reintjes, J., Eckardt, R.C., She, C.Y., Karangelen, N.E., Elton, R.C., and Andrews, R.A., 1976, Phys. Rev. Lett. 37: 1540. Reintjes, J., She, C.-Y., and Eckardt, R. C., 1978, IEEE J. Quantum Electron. QE-14:581. Reintjes, J., She, C.Y., Eckardt, R.C., Karangelen, N.E., Andrews, R.A., and Elton, R.C., 1977, Appl. Phys. Lett. 30:480.

Reiser, G., Habenicht, W., Muller-Dethlefs, K., and Schlag E.W., 1988, Chem. Phys. Lett. 152:119. Rettner, C.T., Marinero, E.E., Zare, R.N., and Kung, A.H., 1984, J. Phys. Chem. 88:4459. Rieckhoff, K.E., and Peticolas, W.L., 1964, Science 147:610. Rocca, J.J., 1999, Rev. Sci. Instrum. 70:3799. Rocca, U., Shlyaptsev, V., Tomasel, F.G., Cortazar, 0.0., Hartshorn, D., and Chilla, J.L.A., 1994,Phys. Rev. Lett. 73:2192.

Rosengren, K.J., 1962, Acta Chern. Scand. 16:2284. Rosker, M.J., and Tang, C.L., 1984, IEEE J. Quantum Electron. QE-20:334. Rossbach, J., 1996, Nuc. Instrum. Methods Phys. Res. A 375:269. Samyn, C., Verbiest, T., and Persoons, A., 2000, Macromol. Rapid Commun.ll:1. Sander, W., 2000, Nuc. Instrum. Methods Phys. Res. A 445:296. Sarukura, N., Hata, K., Adachi, T., Nodomi, R., Watanabe, M., and Watanabe, S., 1991, Phys. Rev. A. 43:1669.

308

ROBERT H. LIPSON et al.

Sauerbrey, R., and Langhoff, H., 1985,IEEE J. Quantum Electron. QE-l1: 179. Sauerbrey, R. 2000, In Electro-Optics Handbook (2 nd edition) (R.W. Wayant and M.N. Ediger, eds.), McGraw-Hill Handbooks. New York. Schlag, E.W., and Neusser, H.J., 1983, Acc. Chem. Res. 16:355. Schnurer, M., Cheng, Z., Hentschel, M., Tempea, G., Kalman, P., Brabec, T., and Krausz, F., 1999, Phys. Rev. Lett. 83:722. Schuhle, U., Pallix, lB., and Becker, C.H., 1988, J. Am. Chem Soc. 110:2323. Schulz-von der Gathen, V., Bornemann, T., Komas, V., and Dobe1e, H.F., 1990, IEEE J. Quantum Electron. 26:739. Seifert, F., Ringling, J., Noack. F., Petrov, V., and Kittelmann, 0., 1994, Opt. Lett. 19: 1538. Seto C.T., and Whitesides, G.M., 1993, J. Am. Chem. Soc. 115: 1330. Shahidi, M., Luk, T.S., and Rhodes, C.K., 1988, J. Opt. Soc. Am. B 5:2386. Shergold, H.L., 1984, In The Scientific Basis of Flotation (J. I. Ives, ed.), Martinus Nijhoff Publishers, The Hague, p.229. Shi, Y.J., Hu, X.K., Mao, D.M., Dimov, S.S., and Lipson, R.H., 1998, Anal. Chem.70:4534. Shi, Y.J., Mallik, 8., Lacey, D., Constas, S.C., and Lipson, R.H., 2001, in preparation. Shiell, R.C., Hu, XX, Hu, Q.J., and Hepburn, J.W., 2000, J. Phys. Chem. A 104:4339. Shui, S., 1994, Contemp. Phys. 35:21. Silfvast, W.T., 1996, Laser Fundamentals. University Press, Cambridge, p.l92. Smalley, R.E, Wharton, L., and Levy, D.H., 1977, Acct. Chem. Res. 10: 139. Srinivasan, T., Egger, H., Pummer, H., and Rhodes, C.K., 1983, IEEE J. Quantum Electron. QE-19:1270. Suckewer, S., and Skinner, C.H., 1990, Science 247:1553. Tang, C.L., Bosenberg, W.R., Ukachi, T., Lane, R.J., and Cheng, L.K., 1990, Laser Focus World 26 (October): 107. Tatchyn, R., Arthur, J., Baltay, M., Bane, K., Boyce, R., Cornacchia, M., Cremer, T., Fisher, A, Hahn, S.-J., Hemandez, M., Loew, G., Miller, R., Nelson, W.R., Nuhn, H.-D., Palmer, D., Patterson, J., Raubenheimer, T., Weaver, J., Wiedemann, H., Winick, H., Pellegrini, C., Travish, G., Scharlemann, E.T., Caspi, S., Fawley, W., Halbach, K., Kim, K.-J., Schlueter, R., Xie, M., Meyerhofer, D., Bonifacio, R., and De Salvo, L., 1996, Nuc. Instrum. Methods Phys. Res. A 375:274. Telle, H., Huffer, W., and Basting, D., 1981, Opt. Commun.38:402. Timmermann, A, and Wallenstein, R., 1983, Opt. Lett. 8:517. Tomin, V.I., Alcock, AJ., Sarjeant, W.J., and Leopold, K.E., 1978 Opt. Commun. 26:396. Tomov, I.V., and Richardson, M. C., 1976,IEEE J. Quantum Electron. QE-12:521. Tonkyn, R.G., and White, M.G., 1989, Rev. Sci.lnstrum. 60, 1245. Tonkyn, R.G., and White, M.G., 1991, J. Chem. Phys. 95:5582. Tonkyn, R.G., Wiedmann, R., Grant, E.R., and White, M.G., 1991,J. Chern. Phys. 95:7033. Tonkyn, R.G., Wiedmann, R. T., and White, M.G., 1992, J. Chem. Phys. 96:3696. Tonkyn, R.G., Winniczek, J.W., and White, M.G., 1989, Chem. Phys. Lett. 142:137. Trickl, T., Vrakking, M.J.J., Cromwell, E., Lee, Y.T., and Kung, A.H., 1989, Phys. Rev. A 39:2948. Tsai, S.-T., Jiang, J.-C., Lee, Y.T., Kung, A.H., Lin, S.H., and Ni, C.-K., 1999, J. Chem. Phys. 111:3434. Tsukiyama, K., 1993, J. Mol. Spectrosc. 159:572. Tsukiyama, K., and Kasuya, T., 1992, J. Mol. Spectrosc. 151:312. Tsukiyama, K., Momose, M., Tsukakoshi, M., and Kasuya, T., 1990, Opt. Commun. 79:88. Tsukiyama, K., Tsukakoshi, M., and Kasuya, T., 1988, Chem. Phys. 127:393. Tunnermann, A, Morna, C., Mossavi, K., Windolph, C., and Wellegehausen, B., 1993,IEEE J. Quantum Electron. QE-29:1233. Tunnermann, A., Mossavi, K., and Wellegehausen, B., 1992, Phys. Rev. A 46:2707. Ubachs, W., Eikema, K.S.E., Hogervorst, W., and Cacciani, P.C., 1997, J. Opt. Soc. Am. B 14:2469.

9. Tunable Short Wavelength Generation and Applications

309

Uchino, 0., Mizunami, T., Maeda, M., and Miyazoe, Y., 1979, Appl. Phys. 19:35. Uehara, Y.,Sasaki, W., Kasai, S., Saito, S., Fujiwara, E., Kato, Y., Yamanaka, c., Yamanaka, M., Tsuchida, K., and Fujita. J., 1985, Opt. Lett. 10:487. U1mstadter, D., Chen, S.-Y., Maksimchuk, A., Mourou, G., and Wagner, R., 1996, Science 273:472. Umstadter, D., Kim, J.K., and Dodd, E., 1996, Phys. Rev. Lett. 76:2073. Vidal, C.R., 1980, Appl. Opt. 19:3897. Vidal, C.R., 1987, In Tunable Lasers, Topics in Applied Physics (L.F. Mollenauer and J.C. White, eds.), Berlin, Springer-Verlag, 59:57. Wallmeier, H., and Zacharias, H., 1998, Appl. Phys. B 45:263. Wayant, R.W., 1972, Phys. Rev. Lett. 28:533. Weltner Jr., W., and Pitzer, K.S., 1951, J. Am. Chem. Soc. 73:2606. Wiedmann, R.T., Grant, E.R., Tonkyn, R.G., and White, M.G., 1991, J. Chem. Phys. 95:746. Wilke, V., and Schmidt, W., 1979, Appl. Phys. 18: 177. Wirth, M.J., Koskelo, A., and Sanders, MJ .1981, Appl. Spectrosc. 35: 14. Wittmann, M., Wick, M.T., Steinkellner, 0., Farmanara, P., Stert, V., Radloff, W., Kom, G., and Hertel, LV., 2000, Opt. Commun. 173:323. Wu, Y., Sasaki, T., Nakai, S., Yokotani, A., Tang, H., and Chen, c., 1993, Appl. Phys. Lett. 62:2614. Yamada, K., Miyazaki, K., Hasama, T., and Sato, T., 1989, Appl. Phys. Lett. 54:597. Yang, K. H., and DeLuca, J.A., 1976, Appl. Phys. Lett. 29:499. Yui, Y.M., McIlrath, TJ.and Mahon, R., 1979, Phys. Rev. A 20:2470. Zapka, W., Cotter, D., and Brackmann., U., 1981, Opt. Commun. 36:79. Zhu, L., and Johnson, P., 1991, J. Chem. Phys. 94:5769. Ziegler, L.D and Hudson, B.S., 1980, Opt. Commun. 32:119. Zych, LJ., and Young, J.F., 1978, IEEE J. Quantum Electron. QE-14:147.

Chapter 10

Femtosecond Laser Ionisation Mass Spectrometry RAVI P. SINGHAL Department ofPhysics and Astronomy. University of Glasgow. Glasgow G12 8QQ. Scotland

1.

INTRODUCTION

Observation of a process requires a probe whose duration is of the order of or less than the characteristic timescale of the event. This paradigm has guided the progress in the study of dynamic processes and has been the catalyst in attempts to produce ever-shorter probes. In many instances, the timescale associated with transition phenomena can be of the order of the period of optical radiation - for 500 nm, the period is 1.7 fs (1 fs = 1O- 15S). Molecular vibrations typically involve energies of 0.1 eV and have a timescale of tens of fs. Molecular rotations are 100 times slower still and may be studied with ps (1 ps = 1O- 12 S) pulses. The situation holds generally in disparate fields: for example, phase transitions in highly excited semiconductors, impulsive phonon generation in solids, biological changes at molecular level etc. proceed on the sub-ps timescale. It appears that the production of short laser pulses has progressed in discrete jumps of 103, each instigated by a technological breakthrough. The first ruby laser had a pulse duration of 1 millisecond and was quickly followed by other lasers of JlS duration. Q-switching reduced the pulse duration to ns (1 ns = 1O-9s) while mode-locking made possible the production of laser pulses lasting a few ps. Technological hurdles put a halt to the production of shorter pulses of significant intensities, and it was only a decade later that Shank and collaborators were able to compress and amplify optical pulses to about 10 fs duration. Each jump towards shorter pulse length has been marked by much new science becoming accessible. Femtosecond pulses not only allow time resolutions comparable with the speed of changes at the molecular level; they also deliver unique An Introduction to Laser Spectroscopy. Second Edition. Edited by Andrews and Demidov, Kluwer AcademiclPlenum Publishers. New York, 2002

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experimental capabilities. For example, the collision interval between constituents of a medium is typically 100 fs or longer. Femtosecond studies in condensed matter could, therefore, be conducted in an environment relatively free from collisions. Molecular vibrations with a typical period of a few hundred fs are essentially frozen when 10 fs pulses are used as probes. Controlled delay of the probe pulse allows one to map out with high resolution the vibration dynamics of molecules. The studies of transition states of chemical reactions using fs spectroscopy earned Zewail the 1999 Chemistry Nobel Prize. Another unique feature of femtosecond laser pulses is their ability to deliver very high intensities. Because the energy of a focused laser pulse is delivered in a short time, the intensities can be very large. The associated electric fields far exceed those responsible for binding atoms and molecules. Not surprisingly, new phenomena like tunnelling, coulomb explosion etc. are observed. At even higher intensities exotic effects appear - for example in the past two years there has been a series of papers which describe the observation of laser-induced fission of uranium and other nuclear reactions with numerous projected applications in medicine etc. Because the pulse duration is of the order of, or shorter than, the time taken by chemical bonds to form or break, it is, in principle, possible to control the rate of a chemical reaction. In fact, by controlling the shape and intensity of the laser pulse, it is possible to favour a particular reaction channel. Coherent control of chemical reactions holds tremendous promise for industrial and other applications. This chapter attempts to address some of the unique features of femtosecond lasers in ionisation of atoms and molecules. Due to the high intensities involved, the usual multiphoton description of events is no longer valid. In the large electric fields of a femtosecond pulse, multiple ionised states of molecules may be created which explode to give characteristic signatures in the mass spectra. The speed of energy absorption by the molecule is such that dissociative channels are bypassed, sometimes completely, and by controlling the intensity it is possible to produce a high yield of the parent molecular ion with limited accompanying fragmentation. Near-complete ionisation of molecules happens at intensities of about 2 x 10 15 W cm-2 • This observation opens the possibility of a simpler method for analytical detection of molecules without the need for frequent calibrations with standards. Another advantage which accrues from the ability to bypass dissociation pathways is that labile molecules like the explosives with one or more nitro-groups may be identified from the production of unfragmented parent molecular ions. With the recent success in reducing the physical size of femtosecond lasers, the fs laser ionisation mass spectrometry (FLMS) promises to be a viable analytical technique of the future.

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Femtosecond lasers bring their own peculiar challenges. Measurement of pulse duration, pulse intensity, pulse shape all require a lot of thought. A ten femtosecond pulse contains only a few cycles of radiation and therefore has a large wavelength spread. This makes fs lasers relatively broad bandwidth sources. Because light of different wavelengths travels in optical elements like lenses etc. at different speeds, dispersion may cause serious temporal stretching of the pulse. The use of reflective optics is one way to avoid some of the problems associated with dispersion.

2.

PRODUCTION OF FEMTOSECOND PULSES

The early history of the realisation of sub-picosecond laser pulses is described in articles by Shank (1983). The chirped-pulse amplification (CPA) technology for the amplification of short pulse lasers enables the production of high power femtosecond laser pulses which have found applications in ultrafast spectroscopy, medicine, high harmonic generation etc. The concept of chirped-pulse amplification is illustrated in Figure 1. A chirped pulse has a time-dependent frequency order. A positively chirped pulse is one in which the lower frequencies are at the front of the pulse with the higher frequencies arriving in the later part of the pulse. This complicated procedure is necessary in order to be able to amplify laser pulses to high power levels without nonlinear distortions and also preventing damage to the amplifier medium. These effects arise because the refractive index of the solid-state amplifier medium becomes intensity-dependent, causing wavefront distortion and self-focusing - which can result in power densities that exceed the damage threshold of the amplifier medium. As a typical example, the CPA arrangement, Figure 2, used at the Lasers for Science Facility, Rutherford Appleton Laboratory (RAL) is described in the following (Taday et ai., 1998) Short 5 - 50 fs pulses of wavelength 790 nm at a repetition rate of 82 MHz are generated at a few nJ level in a mode-locked Ti-sapphire oscillator. In an all-reflective grating arrangement, operated in a Littrow geometry to minimise aberration, the pulses are stretched to about 200 ps. A Faraday rotator placed before the stretcher isolates the oscillator from down-beam reflections and allows the input and output beams in the stretcher to be collinear. The stretched pulses are sent into a preamplifier/amplifier arrangement that increases the pulse energy from 5 nJ to 2 mJ - a gain of 4 x 105 . The Pockels cell between the preamplifier and the amplifier stages reduces the pulse repetition rate from 82 MHz to 10Hz. The 180 ps long pulses are then compressed to 50 fs with the help of an all-reflective grating arrangement.

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Amplified fs pulse

Low energy fs pulse

Low energy stretched pulse

Amplified stretched pulse .. ... ｾ＠

Stretcher

Amplifier

Compress

, i

Figure 1. The principle of the chirped pulse amplification technique.

.-5• ｾ＠

8

Phase II 1 TW 30 m130 fs

Figure 2. The table-top terawatt system at Rutherford Appleton Laboratory (Taday et al., 1998). [Reproduced with the pennission ofCLRC Rutherford Appleton Laboratory.]

The RAL system has been recently upgraded by the addition of another amplifier stage. The layout of the new facility, ASTRA, is shown in Figure 3 and is capable of delivering intensities up to 10 19 W cm·2 in the high intensity target area (Langley et ai., 1999).

10. Femtosecond Laser Ionisation and Mass Spectrometry

. ._ _. 3rd

315

_ ..._ - . Ultra-high intensity target area

. multipeu TIS

0.5 J, 50 fs, 800 om

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