J . Phys. Chem. 1991, 95, 7994-8004
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Incklent Frequency Dependence and Polarization Properties of the CH31 Raman Spectrum G . E.Galica>t Bruce R. Johnson,+*flJ. L. Kinsey,**t*t*L and M. 0. HaletJl Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 021 39, and Department of Chemistry and Rice Quantum Institute, Rice University, P.O. Box 1892, Houston, Texas 77251 (Received: March 1 1 , 1991; In Final Form: May 2, 1991)
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The Raman excitatiog profiles of the 3, (n I4) and 2,3, (n 5 3) vibrational bands in methyl iodide are measured for excitation in the region of the A R dissociative continuum. All spectra are calibrated to an external standard, thereby providing calibrated Raman cross-section information. There are significant deviations of the measured Raman excitation profiles from the theoretical predictions of the simplest "short time" theory of dissociative resonance Raman spectroscopy. The experimental evidence suggests that off-resonance interference in the excited state is responsible for the anomalous Raman excitation profiles. Calculations are made to account for this interference in order to extract the derivatives of the 'QO+ surface responsible for most of the resonant scattering. From these arise conclusions with respect to the composition of the A absorption band.
I. Introduction The study of the mechanisms of chemical reactions is of central importance in the field of chemistry.14 Over the last several years, significant progress has been made in achieving a more detailed and intimate description of reaction dynamics for small molecules. One direction taken, for instance, is in statetestate measurements, where the impact parameters available for reaction have been restricted or tightly determined in recent experiments.s-21 In particular, resonance Raman spectroscopy has been shown22-28 to provide a uniquely detailed description of the early stages of the "half-reaction" involved in photodissociation of 03,CH31, CD31, and other molecules. The dissociative resonance Raman spectroscopy of CH31 is the subject of this investigation. CH31 is of fundamental significance in the scheme of photodissociation studies. The first Raman experimentz3in our laboratory revealed that the initial stage of CH31 dissociation in the UV is dominated by simple breaking of the C-I bond (v3 in the ground state), followed a bit later by an opening of the CH, umbrella angle (u2) from its initial nearly tetrahedral angle to its final planar angle. Little activity is seen in other modes, in accord with the two-mode model of the dissociation originally presented by Shapiro and B e r s ~ h n . ~ ~ The time-dependent interpretation of Raman scattering30 emerges as the natural framework in which to interpret the experiments since the spectra are determined by dissociation dynamics occurring on the femtosecond time scale. Simple Fourier transformation provides the needed link between quantum dynamics, which is frequently easier to interpret, and the frequency-domain spectra, which can be easier to measure. For direct photodissociations, the "short-time" approximation in Raman ~ c a t t e r i n g ~provides l - ~ ~ particularly useful analytical expressions for the absorption and Raman cross sections in terms of low-order derivatives of the excited potential energy surface (PES) with respect to vibrational coordinates. Such developments represent a sort of time-dependent analogue of the use of the normal-mode approximation in time-independent applications, and provide a valuable starting point for our Raman analyses. It seems to be true that nothing is ever as simple as it first appears with CH31. In the simplest scenario where the absorption and Raman spectra are primarily determined by the derivative of the PES with respect to the C-I stretch, the short-time theory Massachusetts Institute of Technology. $Rice University. #Current address: Visidyne, Inc., IO Corporate Place, South Bedford Street, Burlington, MA 01803. *Current address: Department of Chemistry and Rice Quantum Institute, Rice University, P.O. Box 1892, Houston, TX 7725 1. I Current address: Spectra Technology, 2755 Northup Way, Bellevue, WA 98004.
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predicts that the intensity of the 32 overtone should be precisely ?r/4 times the fundamental 3,. However, in our earlier investi-
(1) Levy, M. R. Prog. React. Kinet. 1979, 10, 1. (2) a r e , R. N.; Bernstein, R. B. Phys. Today 1980, 1 1 , 43. (3) Leone. S.R. Annu. Rev. Phys. Chem. 1984.35, 109.
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Scherer, N. F.; Sipes, C.; Bernstein, R. B.; Zewail, A. H. J . Chem. Phys. 1990, 92, 5239. (5) Buelow, S.; Radhakrishnan, G.; Cantanzarite, J.; Wittig, C. J . Chem. Phys. 1985,83, 444. (6) Radhakrishnan, G.; Buelow, S.; Wittig, C. J . Chem. Phys. 1986, 84, 127. (7) Leach, C. A.; Tsekouras, A. A.; Vaccaro, P. H.; Zare, R. D. Faraday Discuss. Chem. Soc., in press.
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(8) Brooks, P. R.; Curl, R. F., Jr.; McGuire, T. C. Ber. Bunsen-Ges.Phys. Chem. 1982,86,401. (9) Gyeen, W. R.; Lukasik, J.; Willison, J. R.; Wright, M. D.; Young, J. F.; Hams, S. E. Phys. Rev. Lett. 1979, 42, 970. (10) Klieber, P. D.; Lyyra, A. M.; Sando, K. M.; Heneghan, S. P.; Stwalley, W. C. Phys. Rev. Lerr. 1985, 54, 2003. (1 I ) Polak-Dingels, P.; Keller, J.; Weiner, J.; Gauthier, J. C.; Bras, N. Phys. Rev. A 1981, 24, 1107. (12) Arrowsmith. P.; Bartoszek, F. E.; Bly, S. H. P.; Carrington, T., Jr.; Charters, P. E.; Polanyi, J. C. J . Chem. Phys. 1980, 73, 5895. (13) Foth, H.; Polanyi, J. C.; Telle, H. H. J . Phys. Chem. 1982,86, 5027. (14) Hering, P.; Brooks, P. R.; Curl, R. F.; Judson, R. S.; Lowe, R. S. Phys. Rev. Leu. 1980, 44, 687. (15) McGuire, T. C.; Brooks, P. R.; Curl, R. F., Jr. Phys. Reu. Lett. 1983, 50, 1918. (16) Barnes, M. D.; Brooks, P. R.; Curl, R. F., Jr.; Harland, P. W. J . Chem. Phvs. 1991, 94, 5245. (17) Giieneisen, H;P.; Xue-Jing, H.; Kompa, K. L. Chem. Phys. Lett. 1981,82,421. (18) Wilcomb, B. E.; Burnham, R. E. J . Chem. Phys. 1981, 74, 6784. (19) Khundkar, L. R.; Knee, J. L.; Zewail, A. H. J . Chem. Phys. 1987, 87, 17. (20) Dantus, M.; Rosker, M. J.; Zewail, A. H. J . Chem. Phys. 1987,87, 2395. (21) Scherer, N. F.;Khundkar, L. R.; Bernstein, R. B.; Zewail, A. H. J. Chem. Phys. 1987,87, 1451. (22) Imre, D. G.; Kinsey, J. L.; Field, R. W.; Katayama, D. H. J. Phys. Chem. 1982, 86, 2564. (23) Imre, D. G.; Kinsey, J. L.; Sinha, A.; Krenos, J. J . Phys. Chem. 1984, 88, 3956. (24) Hale, M. 0.;Galica, G. E.; Glogover, S. G.; Kinsey, J. L. J. Phys. Chem. 1986, 90,4997. (25) Lao, K. Q.; Person, M. D.; Xayariboun, P.; Butler, L. J. J . Chem. Phys. 1990, 92, 823. (26) Markel, F.; Myers, A. B. Chem. Phys. Lett. 1990, 167, 175. (27) Myers, A. B.; Markel, F. Chem. Phys. 1990, 149, 21. (28) Phillips, D. L.; Myers, A. B. J . Chem. Phys. 1991, 95, 226. (29) Shapiro, M.; Bersohn, R. J . Chem. Phys. 1980, 73, 3810. (30) Lee,S. Y.;Heller, E. J. J . Chem. Phys. 1979, 71, 4777. (31) Heller, E. J.; Sundberg, R. L.; Tannor, D. J. J . Phys. Chem. 1982, 86, 1822.
0 1991 American Chemical Society
CH31 Raman Spectrum g a t i ~ of n ~the ~ Raman spectra with a fixed excitation frequency of 266 nm, it was noted that the fundamental intensity was actually lower than that of the overtone. From these data it was impassible to identify the source of the discrepancy since this could include any combination of effects from PES curvature and anharmonicity, variation with nuclear geometry of the transition moment, and interference from resonant and off-resonant electronic states. This situation has prompted us to investigate the Raman spectra at a variety of excitation wavelengths throughout the UV region. From this study has arisen clear evidence that there is interference in the fundamental from at least one other electronic state. This was not apparent from the broad and featureless absorption spectrum. While it has long been known that the Raman excitation profile (REP) can provide uniquely sensitive information about excited electronic states in Raman spectroscopy with bound intermediate states," it is also true to a certain extent in the case of a direct dissociation. A polarization analysis was made of the emitted light at selected excitation wavelengths in order to ascertain the symmetries of the electronic states involved. This analysis follows essentially the quantum mechanical developments given by Ziegler et al.35*36 for light scattering from a symmetric top molecule with C3,symmetry. After averaging over rotational degrees of freedom, the spectra and depolarization ratios are given in terms of the quantum mechanical Placzek invariant^.,^ The Placzek invariants are readily expressed in terms of familiar integrals over vibrational dynamics. In this way, the short-time approximation can be embedded within a fully polyatomic model for the polarization analysis. Since our experiments were performed, other polarization experiments and analyses of similar nature have been made. Lao et al.25have also studied the vapor-phase Raman intensities and depolarization ratios for the 3, series with excitation at 266 nm, in this case for n I12. In the region where this experiment and our own overlap (n I4 for 266-nm excitation), the results are consistent and indicate that the absorption and emission steps are both governed predominantly by a transition moment parallel to the molecular axis (i.e., the state labeled ,QO+;see next section). For higher n, there is apparently an increasing contribution of perpendicular transition moment components to the emission, which is interpreted in terms of nonadiabatic transitions to the neighboring ' Q , surface before emission. CH3126927 and higher alkyl iodidesZ*have also been studied in solution very recently. The polarization analysis used by Phillips and Myers is very similar to our own, and the conclusions they reach with respect to the nature of the interference in the 3, Raman band for the higher alkyl iodides are in substantial agreement. In the following, relevant background material from experimental and theoretical studies of CH,I is summarized. The experiments to obtain the REPS are then described, followed by a brief description of the theory behind the analysis of the depolarization ratios and Raman cross sections. The REP and polarization results are then given and modeled by using the short-time approximation as it relates to the one- and two-mode descriptions of CH,I. Derivatives of the potential surface and averaged parameters related to off-resonant electronic states are then given.
The Journal of Physical Chemistry, Vol. 95, No. 21, 1991 7995
Frequency (cm-') Figure 1. Measured (-) and tit (---) methyl iodide absorption cross sections adapted from Gedanken and R ~ w e . ' ~
nm), B (190 nm), and C ( 1 or n > 1,33 which could not alter all of the combination bands in the required manner. The absorption cross section can also be determined within the above models. The calculated cross sections are shown in Figure 10 with the experimental spectrum determined by Gedanken and R ~ w e The . ~ ~absolute normalization is predicted from the Raman
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Sension, R. J.; Strauss, H.L. J . Chem. Phys. 1986, 85, 3791.
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polarization theory as described by Ziegler et al.,3s*36 augmented by an approximate treatment of off-resonant interference in the
Figure 10. Calculated absorption spectra based on the one-dimensional model fit to the 3, REP (---) and the two-dimensional model fit to all of the REPS(--), and measured spectrum of Gedanken and ROW^'^ (-).
calibrations using eqs 21,34, and 35 without any arbitrary scaling. Even if the normalization is slightly off, it is clear that the oneand two-dimensional models do nor reproduce the experimental spectrum on the high-energy side of the band, lending support to the assignment of Gedanken and Rowe that a significant fraction of the integrated absorption spectrum is carried by another state, presumed to be the lQ1.In fact, if the normalization for the two-dimensional absorption spectrum was 5% higher, it would be essentially identical with the 3Q0+component given in ref 49. This would also explain the asymmetry to the high-energy side of the 2,3, REPs. This point is currently under investigation.
VII. Conclusion A detailed experimental determination has been made of the lowest resonance Raman bands of CH31, each calibrated to the intensity in the N2 fundamental, at a number of different excitation wavelengths throughout the UV. Polarization analysis of the scattered light has been used to analyze the symmetry properties of the excited electronic states, verifying that the AI symmetry 3Q0+ state dominates the resonance Raman scattering in the 3, progression for 1 In I 4 and the &3, progression for 1 In I 3. The minimum level of theory required for the analysis of these specific bands has used the short-time approximation of Heller
wavelength dependence of the scattering cross section. This has allowed us to correct our earlier impression (based on results at a single excitation wavelength) that the short-time theory was inaccurate. Rather, just as for Raman spectroscopy where the intermediate state is not dissociative, the fundamentals prove to be particularly susceptible to influence by distant electronic levels. The influence of the dynamics in the absorption and Raman spectra is indeed limited to a few femtoseconds. However, determination of this dynamics and the underlying derivatives of the excited-state surfaces requires either taking account of offresonant features in the fundamentals or else the use of overtones where this influence is minimized. Our results are consistent with Gedanken and Rowe’s finding49 that a significant part of the absorption to the blue of the maximum is not due to 3Q0+.The natural attribution of the missing intensity is to lQI, and this state appears to also give a minor contribution to (at least) the Raman combination-band REPs. This assignment, however, still flies in the face of the interpretation of the observed product angular distributions.
Acknowledgment. Appreciation is extended to Professors E. J. Heller, A. B. Myers, L. D. Ziegler, S. R. Leone, and the late R. B. Bernstein for very helpful conversations during the course of this work. Support from N S F Grants CHE86-10343, CHE89-10975, and CHE89-09777 and from the Robert A. Welch Research Foundation is gratefully acknowledged. Registry No. CHJ, 74-88-4.
Photodissociation of Acetylene at 193.3 nm S.Satyapal and R. Bersohn* Department of Chemistry, Columbia University, New York, New York 10027 (Received: October 18, 1990) The laser-induced fluorescence of H and D products has been used to study the photodissociation dynamics of C2H2,C2D2, HCCD, and CH3CCDat 193.3 nm. The quantum yield for the production of H atoms from acetylene by a one-photon process is determined to be 0.26 f 0.04. The average translational energies of D atoms from C2D2and CH3CCD were 8.0 f 1.4 and 9.5 f 1.5 kcal/mol. No H atoms were detected as photoproducts of CH3CCD. These facts show that dissociation is faster than internal vibrational redistribution. No anisotropy is found in the Doppler profile as a function of polarization of the photolysis light. The atomic H/D ratio of HCCD is 2.1 & 0.2 at 193 nm and 0.93 & 0.10 at 157 nm. The low ratio at 193 nm argues against tunneling as a dissociation mechanism. Photodissociation of Acetylene The photodissociation mechanism for acetylene in its first few absorption bands is still, after a considerable number of investigations, only partially understood. Measurements have been made using 193-nm light of the translational energy distribution by molecular beam time-of-flight measurements1 and by sub-
Doppler spectroscopy of the hydrogen atoma2 Wodtke and Lee1 pointed Out that the products C2H and H were the major channel, accounting for at least 85% of the dissociation processes, and that (1) Wodtke, A. M.;Lee, Y . T.J . Phys. Chem. 1985,89, 4744. ( 2 ) Segall, J.; Lavi, R.;Wen, Y.;Wittig, C. J. Phys. Chem. 1989,93, 7287.
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