advances in photochemistry...advances in photochemistry volume 18 editors david h. volman department...

ADVANCES IN PHOTOCHEMISTRY

Volume 18

Editors

DAVID H. VOLMAN Department of Chemistry, University of California, Davis, California

GEORGE S. HAMMOND Center for Photochemical Sciences, Bowling Green State University,

Bowling Green, Ohio

DOUGLAS C. NECKERS Center for Photochemical Sciences, Bowling Green State University,

Bowling Green, Ohio

A WILEY-INTERSCIENCE PUBLICATION

JOHN WILEY & SONS, INC.

New York 0 Chichester 0 Brisbane 0 Toronto Singapore


Volume 18


Volume 18

Editors

DAVID H. VOLMAN Department of Chemistry, University of California, Davis, California

GEORGE S. HAMMOND Center for Photochemical Sciences, Bowling Green State University,

Bowling Green, Ohio

DOUGLAS C. NECKERS Center for Photochemical Sciences, Bowling Green State University,

Bowling Green, Ohio

A WILEY-INTERSCIENCE PUBLICATION

JOHN WILEY & SONS, INC.

New York 0 Chichester 0 Brisbane 0 Toronto Singapore

This text is printed on acid-free paper.

Copyright 0 1993 by John Wiley & Sons, Inc.

All rights reserved. Published simultaneously in Canada.

Reproduction or translation of any part of this work beyond that permitted by Section 107 or 108 of the 1976 United States Copyright Act without the permission of the copyright owner is unlawful. Requests for permission or further information should be addressed to the Permissions Department, John Wiley & Sons, Inc., 605 Third Avenue, New York, NY 10158-0012.

Libmry of Congress Cataloging in Publication Data: Library of Congress Catalog Card Number: 63-13592 ISBN 0-471-59133-5

1 0 9 8 7 6 5 4 3 2 1

CONTRIBUTORS

Andre M. Braun Lehrstuhl fur Umweltmesstechnik Engler-Bunte-Institut Universitat Karlsruhe 7500 Karlsruhe, Germany

George S. Hammond Center for Photochemical Sciences Bowling Green State University Bowling Green, OH 43403

Graham Hancock Physical Chemistry Laboratory Oxford University South Parks Road Oxford OX1 342, United

Kingdom

Dwayne E. Heard School of Chemistry Macquarie University Sydney, NSW 2109 Australia

Laurent Jakob Lehrstuhl fur Umweltmesstechnik Engler-Bunte-Institut Universitat Karlsruhe 7500 Karlsruhe, Germany

Claudio A. Oller do Nascimento Escola Politechnica da

Univesidade de S5o Paulo 01OOO S5o Paulo, SP, Brasil

Douglas C. Neckers Center for Photochemical Sciences Bowling Green State University Bowling Green, OH 43403

Esther Oliveros Lehrstuhl fur Umweltmesstechnik Engler-Bunte-Institut Universitat Karlsruhe 7500 Karlsruhe, Germany

V. Ramamurthy Central Research and

Development Experimental Station The Du Pont Company Wilmington, DE 19880-0328

Oscar M. Valdes-Aguilera Center for Photochemical Sciences Bowling Green State University Bowling Green, OH 43403

Richard G. Weiss Department of Chemistry Georgetown University Washington, DC 20057

PREFACE

Volume 1 of Advances in Photochemistry appeared in 1963. The stated purpose of the series was to explore the frontiers of photochemistry through the medium of chapters written by pioneers who are experts. As editors we have solicited articles from scientists who have strong personal points of view, while encouraging critical discussions and evaluations of existing data. In no sehse have the articles been simply literature surveys, although in some cases they may have also fulfilled that purpose.

In the introduction to Volume 1 of the series, the editors noted developments in a brief span of prior years which were important for progress in photochemistry: flash photolysis, nuclear magnetic resonance, and electron spin resonance. Since then two developments have been of prime significance: the emergence of the laser from an esoteric possibility to an important light source; the evolution of computers to microcomputers in common laboratory use for data acquisition. These developments have strongly influenced research on the dynamic behavior of excited state and other transients.

With an increased sophistication in experiment and interpretation, photo- chemists have made substantial progress in achieving the fundamental objective of photochemistry: Elucidation of the detailed history of a molecule which absorbs radiation. The scope of this objective is so broad and the systems to be studied are so many that there is little danger of exhausting the subject. We hope that the series will reflect the frontiers of photochemistry as they develop in the future.

DAVID H. VOLMAN GEORGE S. HAMMOND DOUGLAS C . NECKERS

Davis, California Bowling Green, Ohio Bowling Green, Ohio

vii

CONTENTS

Time-Resolved FTIR Emission Studies of Photochemical Reactions 1 GRAHAM HANCOCK AND DWAYNE E. HEARD

A Model for the Influence of Organized Media on Photochemical Reactions 67

V. RAMAMURTHY, RICHARD G. WEISS, AND GEORGE s. HAMMOND

Up-Scaling Photochemical Reactions 235 AND& M. BRAUN, LAURENT JAKOB, ESTHER OLIVEROS, AND

CLAUDIO A. OLLER DO NASCIMENTO

Photochemistry of the Xanthine Dyes 315 DOUGLAS c. NECKERS AND OSCAR M. VALDES-AGUILERA

Index 395

Cumulative Index, Volumes 1-18 402

ix

TIME-RESOLVED FTIR EMISSION STUDIES OF PHOTOCHEMICAL

REACTIONS

Graham Hancock and Dwayne E. Heard

Physical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, United Kingdom

CONTENTS

I. Introduction 11. Fundamentals of Fourier transform spectroscopy

111. A practical guide to time-resolved FTIR emission studies A. Stop-scan (SS) time-resolved Fourier transform

spectrometers 1. Historical development 2. The Oxford stop-scan time-resolved FTIR emission

spectrometer Continuous-scan (CS) time-resolved Fourier transform spectrometers Comparison of S S and CS time-resolved FTIR spectrometers

Internal state distributions of the fragments of molecular photodissociation Product state distributions from laser-initiated bimolecular reactions

B.

C.

IV. Applications A.

B.

2 5

10

10 10

12

22

28 31

31

37

Advances in Photochemistry, Volume 18, Edited by David Volman, George S . Hammond, and Douglas C. Neckers ISBN 0-471-59133-5 Copyright 1993 by John Wiley & Sons, Inc.

1

2 G . HANCOCK AND D. E. HEARD

C . Kinetic studies of chemical reactions and energy transfer processes 48

V. Conclusions 57 Acknowledgments 58 References 58

INTROD ICTIO i

Fourier transform infrared (FTIR) emission spectroscopy is now the method of choice for observation of weak sources of infrared radiation in a wide variety of applications. The advantages of Fourier transform techniques [ 11, namely, that all light frequencies are observed simultaneously at the detector (the multiplex or Fellgett advantage [2]), and that the optical throughput is greater than for a dispersive instrument with light-confining slits (Jacquinot advantage [3]), have enabled high-resolution spectra to be obtained routine- ly in short periods of time with excellent sensitivity.

In the field of chemical reaction dynamics, the nascent product state distributions of many bimolecular chemical reactions have been measured by FTIR analysis of the infrared chemiluminescence from vibrationally excited products [4-91. In most of these studies reaction has been initiated by simple mixing of the reagents. The nascent product state distributions are obtained free of the effects of collisional relaxation by extrapolation of the distributions observed at various positions in a flow-tube (measured relaxation [lo, 111) or by carrying out the reactions at low total pressure such that the excited species are completely quenched at the walls before any relaxation occurs (arrested relaxation [lo, 123). Many of the fundamental concepts on which molecular dynamics is based, for example, understanding the partitioning of the available energy into the internal degrees of freedom of the reaction products, or the efficacy of reagent vibrational versus translational energy in overcoming barriers in the potential energy surface, have emerged from such experiments [13,14].

Conventional FTIR instruments, in which the interferometer mirror is translated at a constant velocity, are ideally suited to the analysis of steady state infrared emission. However, time resolution of the infrared emission is required in many applications, such as the measurement of absolute rate constants for the formation or subsequent relaxation of a vibrationally excited species. It is then necessary to follow the intensity of the emission (at a particular wavenumber if state-specific rate constants are required) as a function of time. For continuous-wave experiments, crude time resolution

TIME-RESOLVED FTIR EMISSION STUDIES 3

can be obtained by varying either the distance between the point of mixing of the reagents and the FTIR observation point [lo, 11,151 or the gas flow rate for a fixed mixing-spectrometer separation [ 161. Analysis of the transformed spectra at different reaction times allows the populations to be monitored with time and, if desired, extrapolated to obtain the initial distribution [lo].

A more desirable approach is to take advantage of the short pulse duration, high energy, and high repetition rates of pulsed laser sources to cause photolytic production of one of the reagents, allowing exact specifica- tion of zero time for the reaction. This method is standard for studies of molecular dynamics in which products are observed by laser-induced fluorescence [17], but has been little used for monitoring products by IR emission because it is difficult to obtain good signal-to-noise ratios for spectrally resolved emission from specific vibration-rotation states as a function of time. Interference filters are able to isolate spectrally the emission from one particular molecule (or one vibrational mode within a polyatomic molecule), and in several experiments the temporally resolved filtered emission has been studied under varying conditions of pressure and temper- ature in order to measure the rates of bimolecular reactions or energy transfer processes [ 18-22]. Early time-resolved experiments employed circular variable filters with slightly improved but still poor spectral resolution (30- 60 cm- l) to measure the vibrational distribution and relaxation of molecular fragments excited by UV photodissociation [23] or translational-to- vibrational energy transfer [24]. However, the time dependences at any given wavenumber under these conditions correspond to emission from a wide range of excited levels. Time-resolved experiments using dispersive grating spectrometers to analyse pulsed emission from laser-initiated reactions [25] again only allowed low (11-22 cm-') spectral resolution owing to optical throughput problems. Individual rovibronic spectral features could not be resolved, and vibrational distributions were deconvoluted from the data by spectral fitting routines.

The use of FTIR techniques in studies of time-resolved IR emission has been a relatively recent development, and two of the major practitioners in the field have provided excellent reviews of progress up to 1989 [26,27]. This chapter does not attempt a historical survey of the method, but instead describes progress since 1989, suggests possible further areas of study, and most importantly tries to provide the experimentalist with a practical guide to the use of the technique for studying a wide range of photochemical systems.

Commercial Fourier transform spectrometers operating at moderate resolution (1 cm-l) require fractions of seconds to complete a scan of the interferometric mirror (scans may only take tens of milliseconds if only low spectral resolution is required). A new strategy must now be used to study the

4 G. HANCOCK AND D. E. HEARD

transient IR emission signals originating from the products of laser-initiated reactions or photofragmentations which typically decay in tens or hundreds of microseconds. A number of early experiments which employed time- resolved FTIR methods are documented [28-361, but during the last six years we have seen extensive development of fast time-resolved FTIR spectrometers to tackle problems in gas-phase reaction dynamics and molecular photodissociation. Two fundamentally different approaches have been employed. The first, referred to as the stop-scan (SS) method, records the entire temporal evolution of the IR emission while the interferometric mirror is held stationary at each of its sampling positions. The key feature is that the complete temporal evolution of every wavenumber in the product emission spectrum is obtained from only one scan of the interferometric mirror [37]. A SS instrument was developed in the authors’ laboratory [37-381 to study the IR vibrational fluorescence of the products of atom-radical reactions and molecular photodissociations, and the modification of a low-cost teaching interferometer for use as a fast time-resolved instrument is described in detail in later sections. Free radicals with low internal energy were generated by IR multiple photon dissociation (IRMPD) [39], and their reactions with atomic partners were studied in a discharge flow system. For example, nascent vibrational distributions in CO(u’) and HF(o’), generated in the exothermic reaction between oxygen (3P) atoms and the monofluorocarbene radical CHF, were measured together with kinetic parameters from the temporal evolution of the emission spectrum [40]. The second approach is referred to as the continuous-scan (CS) method, in which the interferometric mirror is never stationary throughout the duration of the IR transient. The photolysis laser is triggered when the continuously moving mirror reaches each interferogram sampling point, and the subsequent pulse of IR radiation is digitized at some user-defined delay following the laser pulse. It is assumed that the IR signal is constant during digitization (a very short sample-and- hold gate width is used) and hence following one sweep of the mirror an interferogram (and therefore a FT spectrum) corresponding to one time point in the transient is obtained. The application of the CS method to reaction dynamics was introduced by Sloan and co-workers in 1985 [41]. Electroni- cally excited O(’D,) was generated from the 248-nm photolysis of ozone, and nascent OH(u’, N ’ ) distributions were measured after reaction with a variety of hydrogen-containing molecules [7,41-451. The same strategy has been successfully applied to time-resolved observations of nascent fragments from the UV photolysis of a variety of molecules by Leone and co-workers [46- 501. The high resolution of the CS method has allowed emission from polyatomic fragments to be analyzed to obtain vibrational distributions in electronically excited states formed by UV photolysis of suitable precursors [49, SO]. Recent modifications to the technique allow the observation of


transient signals which do not conveniently decay before the next sampling position is reached [Sl].

The chapter is set out in the following way. Section I1 contains elements of the theory of Fourier transformations which, rather than being exhaustive (and exhausting), aims to cover the details and limitations of the technique which are of importance for the experimentalist to understand. Section I11 contains descriptions and comparisons of the S S and CS methods and outlines the advantages and pitfalls of each, together with recommendations for their suitability for specific applications. Section IV presents recent results from time-resolved FTIR emission experiments, emphasizing photochemical applications.

11. FUNDAMENTALS OF FOURIER TRANSFORM SPECTROSCOPY

Fourier transform methods have revolutionized many fields in physics and chemistry, and applications of the technique are to be found in such diverse areas as radio astronomy [52], nuclear magnetic resonance spectroscopy [53], mass spectroscopy [54], and optical absorption/emission spectroscopy from the far-infrared to the ultraviolet [55-571. These applications are reviewed in several excellent sources [l, 54,581, and this section simply aims to describe the fundamental principles of FTIR spectroscopy. A more theoretical development of Fourier transform techniques is given in several texts [59-611, and the interested reader is referred to these for details.

For a conventional dispersive spectrometer operating between 400 and 4000cm-' at a resolution of 1 cm-', only 0.03% of the radiation that enters the instrument reaches the detector at any one time. Such losses can be disastrous if weak emissions are to be observed in the infrared, where detectors are limited by background noise from the black-body surroundings and their own thermal energy. The time taken to record a high signal-to-noise ratio (SNR) spectrum at high spectral resolution becomes prohibitively long. In Fourier transform spectrometers the incident beam of collimated IR radiation is passed into a Michelson interferometer [62,63]. A beamsplitter divides the beam into two equal parts which are reflected by two mutually perpendicular plane mirrors, one of which can be translated along the optical axis. Following recombination at the beansplitter, the two spatially coherent beams interfere to give a stationary pattern of interference fringes. If the radiation is monochromatic this will occur for all values of the translating mirror, but if it is broad band a pattern is only seen in the vicinity of zero- path difference between the two mirrors. If a detector is placed at the center of


the interference pattern (which consists of circular rings for a monochromatic source) and the path difference between the two beams is precisely varied by translating one mirror, the resulting interferogram has encoded information about the source of radiation. The multiplex or Fellgett advantage resulting from this approach [2] gives a superior SNR of M”’ over a dispersive instrument, where M is the number of resolution elements in a spectrum (defined as the spectral bandwidth divided by the resolution). The same SNR can thus be achieved in a considerably shorter measurement time. The Fellgett advantage is only realized in spectral regions where detectors are background-noise limited, and is lost for detectors which are shot-noise limited (e.g., photomultipliers). The throughput or Jaquinot advantage [3] of an interferometer is typically a factor of 100 or more [64], and is realized for all spectral regions.

The signal seen at the detector for a given value of the optical path difference (OPD), given by the symbol 6, is dependent upon the wavelengths, amplitudes, and phases of the components of the radiation. Constructive interference for all components occurs only at 6 = 0, where the maximum signal is observed (often referred to as the centerburst or central maximum). The signal that is seen at the detector as a function of 6, I ( 6 ) for an ideal interferometer, is given by

Z(6) = - B(V){l + cos 27cV6)dV : j: where B(S)dS is the intensity of the spectral component in the wavenumber range V to 5 + dS. The first term of the expression is constant and is subtracted from the interferogram before computation of the spectrum. The spectrum B(V) is then calculated from an inverse Fourier transformation of the modulated part of Eq. (1):

B(V) = 1(6) cos(2nV6)d6 s_a The cosine Fourier transform given by Eq. (2) is only applicable if the interferogram is perfectly symmetrical about 6 = 0. In practice additional wavenumber-dependent phase shifts are present, owing to beamsplitter characteristics or refraction effects, and cause the interferogram to appear partially asymmetric. The modulated part of Eq. (1) then becomes

I ( 6 ) = - B(V) COS(27cV6 + O(ii))dV : J-: (3)


where O(V) is a wavenumber-dependent phase error. The interferogram now contains some sinusoidal character, and a complex Fourier transformation is required:

Z(6) exp{ -2ni56)dd (4)

yielding a spectrum consisting of real and imaginary parts, representing contributions from the cosine and sine components of the interferogram respectively. The integral expressions (3) and (4) cannot be realized in practice for two reasons:

1. The optical path difference over which the interferogram can be

2. Digitization of Z(6) can only be performed at finite intervals of 6. digitized is limited by the dimensions of the interferometer.

The integrals are thus replaced by summations:

m = + M B(V) = A6 I(mA6) exp( -2niVmA6)

m = - M

The interferogram is digitized at a total of 2M + 1 points with a sample interval of A6 and computation now involves evaluation of a sum over the 2M + 1 values of 6 for each value of V in the spectrum. A Fourier transform instrument is able to record an interferogram corresponding to a spectrum with comparable SNR and spectral resolution to a dispersive instrument in a fraction of the time. However the time advantage for very high resolution studies was not realized until the advent of fast digital computers, which, using the “Fast Fourier Transform” algorithm developed by Cooley and Tukey [65] , could calculate the Fourier transforms of interferograms with very large numbers of M very quickly.

Replacement of Fourier integrals by summations has two important ramifications of practical importance:

1. The resolution of the spectrum is reduced, as no data points are taken in the interferogram beyond a,,,,, = i MA6; higher resolution requires greater mirror travel. At the extrema of the mirror travel there is a discontinuity in the value of the interferogram (it suddenly becomes zero), and this is equivalent to multiplying it by a boxcar function D(6) given by


One of the fundamental theorems of Fourier transforms states that multiplying two functions in one Fourier domain is equivalent to convoluting the two functions in the other domain [60]. The FT spectrum thus has a lineshape corresponding to the Fourier transformation of D(d), which is the sinc function

sin(2~5d,,,) B(5) = 2d,,, = 26,,, sinc(2~5d,,,)

27158,,, (7)

where d,,, is the maximum mirror travel. The complex nature of the spectral lineshape [given by Eq. (7)] resulting from Fourier transformation with boxcar truncation has some interesting properties. The full width at half maximum (FWHM, often used as a definition of resolution) is given by 0.6034/6,,,, and is hence inversely proportional to the maximum mirror travel. Lines in the transformed spectrum appear with subsidiary side lobes or “feet,” an inconvenient distortion which may be mistaken for other spectral features or even swamp a genuine weak feature. This problem is circumvented by “apodization” (Greek for removal of the feet) of the interferogram. New improved lineshapes with suppressed feet are obtained by multiplying the interferogram by an apodization function prior to Fourier transformation. The “weighted” interferogram now reaches zero at f d,,, rather more gradually but, as information at the extreme ends of the interferogram is being thrown away, the spectral resolution is degraded in the form of a broader FWHM. Triangular apodization is popular (reduced sidelobes, FWHM = 0.88/6,,,), and gives the same sinc’ lineshape as encountered in a diffraction-limited grating spectrometer. Commercial instruments employ sophisticated apodization functions, optimizing the FWHM and suppression of sidelobes, and are specific to the SNR available [55 ,59 ] .

2. For a finite sampling interval Ad, more than one superposition of cosine/sine waves can give rise to the recorded interferogram. For the transformed spectrum to be unique, the sampling interval Ad must be sufficiently small to detect modulations in the interferogram due to the shortest wavelength present in the spectrum, the so-called Nyquist criterion [66] :

Here V,,, is the maximum wavenumber present (Nyquist wavenumber). Radiation 5 above V,,, must be removed by suitable optical filtering or additional features and noise will be folded back onto the spectrum. Aliasing, as this phenomenon is called, places constraints on the operation of real


interferometers. Experimentally precise sampling at Ad intervals is performed using the sinusoidal interferogram of a monochromatic source. Helium-neon lasers are used for this purpose to provide an accurate reference of the optical path difference, and values of A6 [and hence V,,, from Eq. (S)] may be chosen to be 0.5nAH,,,, where n is an integer. Sampling in S S and CS interferometers is discussed in Section 111. If lines in an aliased spectrum do not overlap those in the normal spectrum, more efficient “undersampling” may be used, giving a greater mirror travel and hence enhanced resolution for the same number of data points [67] .

A consequence of using a discrete FT is a spectrum with data points equally placed in wavenumber, with a spectral spacing determined by the Nyquist wavenumber and number of interferogram points. It is stressed that the spectral spacing is not equal to the resolution of the instrument, which depends not only upon d,,, but also upon the apodization used. As the interferograms contain some asymmetry, data should be recorded on both sides of the centerburst. Collection of a complete double-sided interferogram (whose N data points are real numbers), gives the complex Fourier transform spectrum:

forming a Hermitian sequence, that is, B ( N / 2 + K ) is the complex conjugate of B(K). The imaginary part would be zero for a symmetric interferogram. The power spectrum is then given by

A disadvantage of calculating the power spectrum is that the spectral information and noise are computed to have positive values (noise is generally randomly positive and negative) due to the squared terms in Eq. (lo), and the height of the baseline may increase above its true value for a noisy spectrum. Recording a single-sided interferogram and performing a cosine Fourier transformation would obviate the need to calculate a power spectrum, and would immediately improve the spectral resolution by a factor of 2 for the same number of data points. However, distortions in the spectrum will result unless the phase error e(5) in Eq. (3) is known for each 5 [SS]. ‘The term e ( i ) can be found by recording a very short double-sided interferogram and calculating

e(v) = tan- lCT,in(v)/T,os(S)I (1 1)

Phase correction algorithms can then use O(V) to correct the asymmetry in a


much longer single-sided interferogram [68,69]. The discrete nature of interferogram sampling ensures that no single data point will exactly correspond to 6 = 0, generating an additional phase error. Commercial instruments employ a white light reference beam, whose sharp centerburst indicates 6 = 0 precisely, which, together with phase correction packages, allow single-sided data to be taken.

In practice, a number of experimental factors degrade the resolution from that of an ideal interferometer. Emission sources are finite in size, and hence the beam entering the interferometer is slightly divergent. A critical angle of divergence for a given resolution and Nyquist wavenumber can be calculated [ S S ] , and for large area sources, an aperture (or Jacquinot stop) may be required to increase the quality of collimation, reducing the throughput advantage. Typically emission occurs only from the region of a laser focus in photochemical applications, and a Jacquinot stop may only be required at very high resolution. Mirror misalignment during a scan can also degrade resolution [ S S ] , and for high resolution work, commercial instruments employ dynamic alignment of the moving mirror (the reference laser forms a two-dimensional image) during a scan. Unapodized resolutions of <0.002 cm-' are obtained in this way. Sampling errors, such as missed or extra points, or variable sample interval Ad, degrade the resolution and increase the noise level, the effect being most severe for errors near to the centerburst.

111. A PRACTICAL GUIDE TO TIME-RESOLVED FTIR EMISSION STUDIES

In this section it is hoped to provide useful experimental details which demonstrate the simplicity of both the time-resolved FTIR technique and its incorporation into an experiment. Two major implementations of the technique have emerged, namely stopped and continuous-scan; they are dealt with separately below in Sections 1II.A and 1II.B. A detailed comparison of the two methods is then presented in Section 1II.C.

A. Stop-Scan (SS) Time-Resolved Fourier Transform Spectrometers

1. Historical Development, In this category of FT spectrometer the complete time-evolution of the IR transient is digitized whilst the interferometric mirror is held stationary at each sampling point. The transient can be initiated repeatedly and signal averaged to achieve an adequate SNR. The


stop-scan (SS) idea is by no means a new one, and its potential for the study of short-lived phenomena was first proposed over 20 years ago [70]. Early implementations of the technique are reviewed by Sloan [27] and are only briefly considered here. The first demonstrations by Murphy and co-workers used pulsed electron bombardment to investigate the production and relaxation of vibrationally excited molecules [30,35,70]. Pulses of energetic electrons lasting several milliseconds and repeated at 80 Hz impinged on slowly flowing mixtures of gases at reduced pressures. A pulse of IR emission was generated, and decayed completely between electron pulses. Many pulses were averaged and digitized with about 50-ps temporal resolution before the moving mirror was translated to the next sampling position using a reference laser interferogram and a servo feedback mechanism. After rearrangement of the data, interferograms were constructed as a function of time, which, after Fourier transformation, gave a series of time-resolved spectra. Modulated discharges have also been employed to excite atomic and molecular emissions in the infrared, generating time-resolved spectra at 4 cm- ' resolution [29]. Some quasi stop-scan experiments have employed a very slowly moving mirror, as opposed to a stationary one, with the entire emission pulse digitized and signal averaged many times between two adjacent reference laser sampling points [36,71-731 again using electron bombardment as the excitation source. An assumption was then made that all electron beam pulses occurring between sampling points have the same mirror position, that is, it was stationary. However, results from quasi SS spectrometers, some of which have appeared very recently [72,73], may be subject to errors as discussed in reference 27. The location of the moving mirror is only precisely known at the HeNe zero crossings, and also the temporal behavior of the IR transient will change as the mirror is translated between these.

Palmer et al. have very recently modified a commercial FTIR spectrometer (IBM 44 FT-IR) for stop-scan operation [74,75]. The moving mirror position is controlled with a feedback loop using path difference modulation of the reference laser intensity, together with lock-in amplifiers to detect the mirror position. The interferogram can be sampled at intervals as small as %AHeNe, and hence V,,, is 31,600 cm-'. The software developed only allows data collection at approximately 1.6Hz, but with a mirror settling time of < 20 ms much more efficient data collection should be possible. The stability of the moving mirror imposes an upper limit on the SNR of the measured spectra and is f15nm. A number of applications have been reported in photoacoustic and photothermal spectroscopy [75-771 and two- dimensional FTIR correlation spectroscopy [78], although so far the time- resolved examples show only low resolution emission spectra (100 cm-', 2 ms) [79,80]. In another example, Siebert et al. recently described the modification of a Bruker IFS 88 FTIR spectrometer for use as a stop-scan


time-resolved instrument [8 13. Absorption spectra at moderate resolution (4 cm-') were taken following laser initiation of a biological system, with a good temporal resolution of l o p . Signal averaging was performed at each interferogram sampling point.

In our laboratory a low-cost teaching interferometer has been modified for use as a S S time-resolved instrument to study the infrared vibrational chemiluminescence arising from a variety of photochemical processes [37,38, 40, 82, 831. The entire temporal evolution of the emission spectrum is obtained from a single interferometric scan, with spectral resolution of 2 cm-' and temporal resolution down to 10 ns. One of the major difficulties encountered with S S interferometers is that of keeping the mirror stationary during the signal averaging of the IR transient. Small position inaccuracies will, after Fourier transformation, manifest themselves as decreased SNR [27]. Elaborate stabilization electronics and position-sensing techniques have been used [29], but as described in the next section, spectra of excellent SNR have been straightforwardly obtained at this resolution without the need for such expensive instrumentation.

2. The Oxford StopScan Time-Resolved FTIR Emission Spectro- meter. Earlier studies in this laboratory [84] showed that infrared radiation was emitted following the IRMPD of a number of halocarbons in the presence of oxygen atoms in a low-pressure discharge flow system. Attempts to resolve spectrally the emission with a dispersive scanning monochromator were unsuccessful because of throughput losses. Kinetic studies of the total unresolved emission revealed in some cases more than one emitting species, but their identities remained speculative. A time-resolved interferometer was designed and constructed to resolve the pulse of weak IR chemiluminescence, enabling identification, energy partitioning, and kinetic measurements to be made. The instrument is described in detail in references 37 and 38. The aims of this section are to highlight the insights into time-resolved interferometry that are gained from practical use of the technique, and to demonstrate that an inexpensive instrument is capable of solving important questions in chemical physics.

The interferometer is a modified Michelson teaching instrument (Ealing Optics) and is shown schematically in Figure 1, and follows the basic design previously used to study CW emissions in the visible and near IR [67]. The beamsplitter and compensator plates were constructed of CaF, ; the latter compensates the phase differences between the radiation transmitted and reflected by the beamsplitter. One face of the beamsplitter was coated with a 4-nm thick layer of gold, deposited by ion sputtering, to give approximately equal reflection and transmission in the midinfrared. The NaCl collimation and collecting lenses were used with a variety of detectors (InSb, HgCdTe,


Pulsed photolysis

I I I

i Fixed i mirror I I

ctor

laser

detector

Figure 1. Schematic diagram of the Michelson stop-scan interferometer used for time- resolved FTIR emission studies. Reproduced with permission from Ref. 38.

Ge, Si depending on the spectral region studied), suitably filtered to limit the spectral bandwidth in order to prevent aliasing errors. The entire interferometer was housed in an air-tight aluminum box, mounted on a steel baseplate to ensure mechanical stability, and was flushed with N, to prevent atmospheric absorption of IR radiation. The box is portable, and has been readily interfaced to a number of experimental configurations (including a reactive ion plasma etching chamber) with a minimum of alignment. A helium-neon laser with an interference filter to cut out all emission lines except 1 = 632.8 nm provides the reference interferogram, and passes through the center of the main optics and is detected by a photodiode. The scanning mirror is moved via a micrometer drive allowing approximately 8 nm mirror travel per step. The micrometer has a tungsten-carbide tip and is connected to the stepper motor via a rubber belt to ensure smooth mirror travel.

A Zenith 2-158 PC computer (8 MHz 20 MB, 640 kB RAM 8087 coproc- essor) is interfaced to the experiment and controls the interferometer sampling, data acquisition, and subsequent sorting/Fourier transformation of the data to produce time-resolved spectra. Two digitizers were employed to record the temporal evolution of the IR emission and were fully dedicated


to the computer. The first was home-built, and incorporated a 10-bit 2.2-ps conversion speed A-D chip, with 8192 channels. The second was a Biomation 8100 transient recorder, with only 8 bit digitization but a faster temporal resolution of 10 ns per channel. Also interfaced to the computer is a standard stepping motor controller, a 60-ps A-D converter for recording the energy of the photolysis laser, and an average-crossing detector which enables the computer to identify the correct position for sampling of the IR interferogram. On stepping the mirror, a sinusoidal voltage is produced at the helium-neon laser detector. The detector output is fed to a low-pass filter which, after a few He-Ne cycles, produces a DC voltage which is an average of the oscillating laser interferogram. Once the average is established, the subsequent incoming sinusoidal He-Ne laser signal at the photodiode is compared with this average, and standard TTL high or low pulses are sent to the computer depending on whether the reference signal is above or below the average value. Each time the mirror is stepped by 158.2 nm (corresponding to a change in the optical path difference of 316.4nm or one half of a laser wavelength), the square-wave output undergoes a TTL high-low logic change, thus allowing the computer to sample the IR interferogram ac- curately. Figure 2 shows a block diagram of the hardware, illustrating the control structure of the instrument. In a recent development the He-Ne laser signal is digitized by a slow A-D converter, and the computer can directly decide when the average value is reached. The average is updated throughout a scan.

Figure 1 shows the interferometer sampling IR radiation from a flow tube in which atoms (e.g., 0, N) react with free radicals (e.g., CF,, CHF, NCO) following pulsed initiation of the process. Atoms were formed in a microwave discharge of a diatomic precursor diluted in argon and then mixed with the radical precursor in a conventional flow system. Radicals were formed by IRMPD of the precursor, using a pulsed line-tunable CO, laser (Lumonics K 103,0.5 Hz, 6 J per pulse, or Laser Applications Ltd., 10 Hz, 6 J per pulse). The IRMPD production of free radicals ensures relatively low internal excitation [39] compared with ultraviolet photolysis methods. Following laser photolysis, the radicals react with the atoms to produce a pulse of product IR chemiluminescence at the laser focus, a region approximately 2mm in diameter at the center of the flow tube, which is imaged into the interferometer.

Time-resolved chemiluminescence spectra are obtained as follows. For a given optical path difference, the entire temporal profile of the pulse of product IR chemiluminescence is recorded at the detector, amplified, digitized (up to 1011s resolution) and stored directly on hard disc for a preset number of photolysis laser shots, the number depending on the SNR of the system. The CO, laser energy for each shot is recorded by a pyroelectric


ZERO CROSSING

REFERENCE LASER

ZENITH 1 1 INTERFEROMETER^ I A°C z-158 PC MIRROR DRIVE U I I I I

COOLED TO 77K TRANSIENT

PULSE

Figure 2. Block diagram illustrating the control structure of the stop-scan instrument hardware.

joulemeter and stored for subsequent normalization-this also allows rejection of a laser shot if its energy is below a predetermined value. The internal reference of the He-Ne laser is then used to step the moving mirror on to the next sampling position (the path difference is incremented by n/2 He-Ne wavelengths, where n is an integer). The pulse of chemiluminescence following C 0 2 photolysis is again recorded, digitized, and stored for this optical path difference. The temporal shape of the emission changes as a function of path difference, as shown in Figure 3, because the component spectral frequencies in the emission, originating from a variety of transitions in the excited molecules, have different time dependences and, as the optical path difference is changed, will interfere differently when recombined at the


0

Figure 3. The overall temporal profile of the infrared emission as seen by the detector at four positions of optical path difference 6, in the vicinity of the position of zero optical difference 6 = 0. The data shown are for emission from highly vibrationally excited CO,, taken with a temporal resolution of 3 ps and with one shot of the CO, laser per mirror position, and illustrate how both the intensities and the time profiles of the emission, arising from the production and decay of many vibrational levels, change as a function of 6. Reproduced with permission from Ref. 37.

beamsplitter. The degree of change between temporal profiles at different positions of the mirror is most noticeable for a system containing more than one emitting species which are formed and removed at different rates. Intuitively it is obvious that temporal profiles at different values of 6 must be different in order for the interferograms and hence spectra to change with time. The sequence described above is repeated for all the required sampling points in the interferogram. The experimental spectral resolution depends only on the maximum path difference, 6,,,, travelled from the position of the zero path difference. Double-sided interferograms are taken to avoid phase errors, and as the mirror travel is limited in this instrument to kO.25 cm-' from the centerburst, this results in d,,, = 0.5cm and thus an unapodized resolution limit of 1.22 cm-'. The number of points required in an interferogram is 4V,,,6,,, . Thus, for example, if sampling is performed every second zero-crossing of the laser reference, giving V,,, = 7901 cm-l, and an unapodized spectral resolution of 3 cm- ' is required (a,,, = 0.2 cm), then the


number of interferogram points is 6320. The laser must be fired 6320n times to achieve a SNR improvement of nli2 from that of a single shot scan. Typically in these experiments n varied from 1 to 5 when using the 0.5-Hz laser, but considerably more averaging was performed on weaker emissions with the 10-Hz laser. A large computer disc space is needed to store such a time-resolved scan; for example, if 150 digitized time points are used for each temporal decay, and 6000 sampling points are used in the interferogram, the required memory space is 1.72 MBytes.

After the scan is finished the raw data are in the form shown in Figure 4,

1 Time/psec

Pho to1 y si s Laser

Figure 4. Three-dimensional representation of the time-resolved data near S = 0 as they appear following one interferometric scan. The sampling interval employed was 1.2656pm, corresponding to a Nyquist wavenumber of 3950.7 cm-'. Selection of an interferogram at any time delay following the photolysis laser pulse is possible, and is shown here for t = 150 p s . Reproduced with permission from Ref. 37.


0 -.

that is, as a two-dimensional file of sequential temporal decays as a function of optical path difference. The data are then sorted to give interferogramo as a function of time following the COz photolysis pulse. During the recording of an interferogram, the CO, laser energy values followed a roughly normal distribution, with a FWHM of - 5% of the mean value. Over this small range of laser energies, the intensities of the component spectral features were found to have very similar dependences on the laser energy, as illustrated in Figure 5 for COz and H F emissions observed when CF,HCl is photolyzed in the presence of oxygen atoms. Hence, the variation of the total emission as a function of laser energy (which for this almost saturated IRMPD process is less than linear) can be used to normalize the interferograms for energy output variations. Although corrections are small, the variation in the interferogram away from the centerburst are also small, and the quality of the spectra are improved by this normalization. Figure 6 illustrates interfer-

I 1 7- , I 1

'1

Figure 5. Variation of the infrared emission intensity for vibrationally excited CO, and HF as a function of the CO, laser fluence. Both display similar functional forms over the spread of values around the normal operating value, and these data were used to normalize the interferograms for the 5% variation in laser fluence during the course of an experiment.

advances in photochemistry...advances in photochemistry volume 18 editors david h. volman department...

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