8004
polarization theory as described by Ziegler et al.,3s*36augmented 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.
0022-3654/9 1 /2095-8004%02.50/0 @ 199 1 American Chemical Society
Photodissociation of Acetylene at 193.3 nm
0.s
The Journal of Physical Chemistry, Voi. 95, No. 21, 1991 8005
TABLE I: Quantum Yields for Phatoprocemesin Acetylene A, nm 4CCHtH "CCtHH 4HCCH** source 0.68 this work 193.3 0.26
J
184.9 147.0
0
20
4b
60
80
100
intrnrity (mJlcm?)
Figure 1. H atom fluorescence intensity versus excimer laser fluence in mJ/cm2. C2H could be dissociated efficiently with a second photon. In these two investigations the bond energy was deduced from the assumption that the fastest H atom was produced simultaneously with an internally cold C2Hradical. The bond energy would then be just the photon energy minus the relative translational energy. On this basis Wodtke and Lee found a bond energy of 132 f 2 kcal/mol whereas Segall et aL2 found 127.5 f 1.5 kcal/mol. Subsequent measurements by other methods' have converged on the value of 131.3 f 0.7 kcal/mol. Baldwin et al." showed that the experiment of Segall et al. led to a slightly low value because of a small amount of secondary dissociation of the C2H by a second 193-nm photon. The H atom from this process is faster than that from the first dissociation because the second C-H bond is as much as I5 kcal/mol weaker than the first. The fast H atom would add a small tail to the MPI excitation curve, thus exaggerating the maximum kinetic energy release in the first dissociation. The dissociated C2H has considerable internal energy. This has been verified by diode laser absorption spectroscopy$ unresolved infrared e m i s s i ~ n ,and ~ FTIR emission experiments6 Excitation is found in all three modes,_and indeed there is extensive mixing of the vibra_tionally excited X(22+) levels with the electronically excited A211 level at 3692 cm-I. This paper presents measurements by laser-induced fluorescence (LTF) of (1) quantum yield for dissociating into hydrogen atoms and ethynyl radicals, (2) anisotropy effects when dissociating with polarized light, (3) average translational energies, and (4) H / D branching ratios in the photodissociation of HCCD.
Experimental Section The method for studying the LIF of H atoms has been previously described.' The H / D ratio for the photolysis of HCCD was determined by measuring the areas under the H and D LIF excitation curves. Typical pressures of acetylene were in the 8-25 mTorr range, and the delay time between the photolysis and probe lasers was 100 ns. As shown in Figure 1, the H signal from the photolysis of pure C2H2was linear with the ArF laser intensity. Furthermore, the H / D ratios were measured for a range of photolysis laser intensities (15-1 15 mJ/cm2 per pulse) with no change in the results. In principle, the measurement of the branching ratio in the photolysis of HCCD is straightforward. However, the HCCD purchased from MSD isotopes contained 17% C2H2. Therefore, (3) Erwin, K. M.; Gronert, S.; Barlow, S.E.;Gilles, M. K.; Harrison, A. G.; Bierbaum, V. M.; DePuy, C. M.; Lineberger, - W. C.:Ellision. G. B. J . Am. Chem. Soc., in press. (4) Kanarmori, H.; Seki, K.; Hirota, E. J . Chem. Phys. 1987, 87. , 73. Kanamori, H.; Hirota, E. Ibid. 1988, 88, 6699; 1989, 89, 3962. (5) Shokooki, F.; Watson, T. A.; Reisler, H.; Kong, F.; Renlund, A. M.; Wittig, C. J. Phys. Chem. 1986. 90. 5695. (61 Fletcher, T.R., Leone, S . R. J . Chem. Phys. 1989, 90, 871. (7) Johnston, G. W.; Katz, B.; Tsukiyama, K.; Bersohn, R. J . Phys. Chem.
1987, 91. 5445.
0.10
0.06 0.3
