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combination to measure the nascent rotational and vibrational surface chemical reactions.Is distributions of excited C02 molecules created by a variety of chemically interesting processes such as photodiss~ciation,~~ Acknowledgment. This work was performed a t Columbia electronic quenching,16inelastic scattering,” and bimolecular and University and supported by the National Science Foundation under Grants CHE-80-23747 and CHE-85-17460 and by the Joint (15) ONeill, J. A.; Kreutz, T. G.; Flynn, G. W., accepted for publication Services Electronics Program (U.S. Army, U S . Navy, and U S . in J. Chem. Phys. Wood, C. F.; O’Neill, J. A.; Flynn, G. W. Chem. Phys. Lett. 1984. 209. 317. Air Force) under Contract DAAG29-85-K-0049. Equipment (16) Biady, B. B.; Spector, G. B.; Chia, L.; Flynn, G. W. J . Chem. Phys. support was provided by the Department of Energy under Contract 1987,88, 3245. DE-AC02-78ER04940. (17) ONeill, J. A.; Wang, C. X.;Cai, J. Y.; Flynn, G. W.; Weston, Jr., R. E. J . Chem. Phvs. 1986. 85. 4195. ONeill. J. A.: Cai. J. Y.: Flvnn. G. W.; Weston, Jr., R.E. J . Chem. Phys. 1986,84, 50. ’Chu; J. 0.: Wbod; C. F.; Flynn, G. W.; Weston, Jr., R. E. J . Chem. Phys. 1984, 80, 1703; 1984, 81, 5533. Hewitt, S. A.; Hershberger, J. F.; Flynn, G. W.; Weston, Jr., R. E. J. Chem. Phys. 1987, 87, 1894.
(18) Kreutz, T. G.; O’Neill, J. A.; Flynn, G. W.; Brown, L. S.; Bernasek, S. L., manuscript in preparation.
Emission Spectrum of Dissociating H2S K. Kleinermanns,* E. Linnebach, and R. Suntz Physikalisch- Chemisches Institut der Ruprecht- Karls- Universitat Heidelberg, Im Neuenheimer Feld 253, 6900 Heidelberg, FRG (Received: April 15, 1987; In Final Form: July 20, 1987)
The photoemission spectrum of H2S excited at 193 nm, near the maximum of its first absorption band, contains long progressions of stretch and stretch-bend combination bands but no progression solely of the bend vibration in the region 193-320 nm. The results are interpreted in terms of the dissociation dynamics at very short and at intermediate times.
Introduction Emission spectroscopy of photodissociating molecules provides new insight into the short-time dynamics of bond ruptures. The separation of photofragments is like half a molecular collision and represents a bridge to a more detailed understanding of full collision reactions. As in conventional Raman spectroscopy, the frequency shift of the emitted bands relative to the excitinfline corresponds to the ground-state fundamental, overtone, and combination vibrational energies. Imre et al.’ have observed unusually extended vibrational progressions from both ozone and methyl iodide photodissociated at 266 nm. High quality ground-state energy surfaces can be constructed from the precise knowledge of a sufficient number of vibrational energies. The relative intensities of the various bands contain information about the changes of bond angles and bond distances while the fragments separate. In simple terms, a vertical optical transition takes place to a high (stretch or bend) overtone at a later stage of the dissociation, where bond distances and (eventually) angles have changed relative to the electronic ground-state equilibrium values. In the lower state this conformation corresponds to a lengthened bond and the molecule responds by vibrating. This correlation between the temporal course of the dissociation and the transition to a specific vibrational overtone is not unequivocal if the transient species revisits the Franck-Condon region or other previously visited parts of the surface at a later stage of the dissociation. Hence the spectrum can be interpreted much more easily if the dissociation is direct, Le., occurs on the time scale of a vibrational period (typically lo-‘* s) without performing complicated Lissajous figures. The theoretical understanding of photoemission from repulsive surfaces is still in its infancy. One way to extract dynamical information from the spectra is to use the time-dependent formulation of photon emission developed by Heller et al.2 Here, the pump laser transfers the ground-state vibrational wavepacket to the Franck-Condon region of the upper surface. The wave(1) Imre, D.; Kinsey, J. L.; Sinha, A,; Krenos, J. J. Phys. Chem. 1984,88, 3956. ( 2 ) Heller, E. J.; Sundberg, R. L.; Tanner, D. J. Phys. Chem. 1982, 86, 1822.
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packet then moves along the steepest descent of the surface. For example, the excitation of H 2 0in its first absorption band leads initially to motion along the symmetric stretch normal coordinate, because the slope of the potential along this coordinate is Only after a substantial (symmetric) extension of the O H bond does the gradient along the antisymmetric stretch increase and the motion proceed along that coordinate. At large bond distances, a linear configuration is energetically more favorable than the strongly bent one in the Franck-Condon region. Thus at a later stage of dissociation, a force is exerted on the wavepacket so that it moves along the bending normal coordinate. During its motion, the wavepacket may spread due to a change in force constant of a specific normal coordinate.2 The overlap between the moving wavepacket and the vibrational levels of the electronic ground state determines the emission intensities through a half Fourier transform.2 Another theoretical approach is to calculate emission cross sections by using the time-independent Kramer-Heisenberg-Dirac expression. This involves computing the transition dipole matrix elements between the vibrational eigenfunctions of the excited and ground states and requires knowledge of large parts of the upper-state potential surface. This perturbation theoretical approach was used to calculate the “continuum resonance Raman spectrum” of bromine for comparison with experiments4 Finally, a quasi-classical approach can be conceived5whereby the emission cross section for a particular transition depends on how long a trajectory spends at a specific point in configuration space and how large the amplitudes of the (electronic ground state) vibrational eigenfunctions are at that point. In this paper we report the photoemission spectrum of H2S excited at 193 nm. The photodissociation of H2S in the first absorption band between 170 and 250 nm is already quite well ~~
(3) Staemmler, V.;Palma, A. Chem. Phys. 1985, 93, 95. (4) Baierl, P. Dissertation, University of Munich, 1978. For other investigations on the emission spectra of photodissociating halogens see: Rousseau, D.L.; Williams, P. F.J. Chem. Pbys. 1976, 64, 3519. Change, H.; Hwang, D. M. J . RamanSpectrosc. 1978, 7, 253. Frank, C. J. Chem. Educ. 1981, 58, 343. (5) Arrowsmith, P.; Bartoszek, P. E.; Bly, S. H. P.; Carrington, T.; Charters, P. E.; Polanyi, J. C. J . Chem. Phys. 1980, 73, 5895. Arrowsmith, P.; Bly, S . H. P.; Charters, P. E.; Polanyi, J. C. J . Chem. Phys. 1983, 79, 283.
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studied. The products are exclusively H and SH in their electronic ground states. Hawkins and Houston6-' have measured the internal state distribution of the HS and DS product radicals using laser-induced fluorescence following dissociation at 193 nm. They observed nearly Boltzmann rotational distributions with only a small part of the excess energy channeled into product rotation. The SH rotational distribution is very similar to the OH distribution from dissociation of H 2 0 at 157 nm in its first absorption band.* In fact, as in the case of H 2 0 dissociation, the SH rotational distribution from H2S photodissocation can be described well by a simple Franck-Condon p i c t ~ r ewhere ,~ the population of the different rotational levels is given by the square of the projection of the parent molecule vibrational ground-state eigenfunction onto the product radical rotational eigenfunctions. Hence in neither case do the rotational distributions provide direct information about the excited-state potential surface. Considerable SH vibrational excitation was observed by van Veen et al. in time-of-flight studies of the H product atom upon dissociation at 193, 222, and 248 nm.lo At 193 nm, vibrational excitation up to SH, u = 5 has been observed with almost 10% of the available energy channeled into vibration.1° The H atom angular distribution gives an anisotropy parameter @ = 0.48 f 0.01 at 193 nm.lo Hence the transition dipole moment at this excitation wavelength must be perpendicular to the H2S plane and the dissociation direct." At 222 and 248 nm, the p-values -0.42 and -0.33 indicate somewhat longer excited-state lifetimes s,l0 again assuming perpendicular transitions, of around 1.8 X at these wavelengths. Ab initio calculations show two excited states of 'B1 and 'A2 symmetry in the energy region of the first absorption band of H2S.I2 Essentially only the Franck-Condon region of these two surfaces has been calculated so that theory so far does not indicate whether both surfaces are repulsive in the dissociation coordinate or whether one of them is bound. Only the perpendicular transition from the 'A, ground state of H2S to the 'B, state is electronically allowed. Excitation to the lA2 state is vibronically allowed if an odd number of antisymmetric stretch quanta is excited. In C2, symmetry, these vibrations are of B2 type; hence the integrand of the transition dipole is totally symmetric for a perpendicular transition. The emission spectrum of dissociating H2S should give more detailed information about extended parts of the excited-state potential and the interplay of the stretching and bending motion on that surface, especially if comparison with theory is possible.
Experimental Section From H atom aqgular distribution measurements, the lifetime of H2S excited at 193 nm can be estimated to be s or less,1° and hence the reemission efficiency is very small (say assuming a radiative lifetime of lo-* s). The unfocused ArF excimer laser (EMG 200, Lambda Physik) delivers around 10'' photons per cm2. The absorption cross section of H2S at 193 nm is 7 X cm2, and hence nearly all the molecules in the light beam absorb a photon at the 0.2 mbar of H2S in our experiment. Therefore we expect a sufficient number of emitted photons, even at low fluorescence quantum yields and high imaging losses through the spectrometer. On the other hand, laser stray light should be a problem because of the high photon flux and the divergence of our stable resonator excimer laser. As Figure 1 shows optimized light baffles consisting of AI/MgF,-coated, highly reflective skimmers, as used in molecular beam experiments, serve to suppress the scattered light. The light further away from the laser beam center is mirrored into the exit baffle system instead ( 6 ) Hawkins, W . G.; Houston, P. L. J . Chem. Phys. 1980, 73, 297. (7) Hawkins, W. G.; Houston, P. L. J . Chem. Phys. 1982, 76, 729. (8) Andresen, P.; Ondrey, G. S.;Titze, B.; Rothe, E. W. J . Chem. Phys. 1984, 80, 2548. (9) Schinke, R.; Engel, V. J . Chem. Phys. 1985, 83, 4522. (10) van Veen, G. N. A.; Mohamed, K. A.; Baller, T.; De Vries, A. E. Chem. Phys. 1983, 74, 261. ( 1 1 ) Zare, R. N. Mol. Photochem. 1972, 4, 1 . (12) Shih, S.; Peyerimhoff, S. D.; Buenker, R. J. Chem. Phys. 1976, 17, 391.
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Figure 1. Experimental arrangement for studying emission spectra of dissociating molecules.
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Figure 2. Low-resolution emission spectrum of dissociating H2S (0.2 mbar, 298 K) obtained by excitation at 193 nm. For band assignment see text. The spectrum is the result of a single scan and is not corrected for changes in laser energy and system response. The structure labeled with stars was not reproducible in other scans.
of being reflected into the view region of the photomultiplier. This arrangement suppresses scattered light almost 2 orders of magnitude better than traditional "pinhole" baffles. In the experiment, H2S flowed through the cell at 0.2 mbar pressure. Substantial amounts of H2Swere necessary for the fast flow experiment. To avoid polluting the environment, the pump exhaust was bubbled through two 10-L tanks filled with a saturated aqueous copper sulfate solution. In this way, H2S is precipitated quantitatively as copper sulfide. The side arms and the fluorescence window are purged with helium to prevent excessive absorption of the pump beam, reabsorption of the emitted light, and buildup of photolysis products. The fluorescence light is collected with a quartz lens, imaged onto the entrance slit of a 0.35-m monochromator (McPherson 270, f/6) and detected by a cooled photomultiplier (EM1 9789QB). The photomultiplier current is measured by a boxcar integration system (SR250, Stanford Research) with a gatewidth equal to the full pulse width (ca. 40 ns) of the ArF laser. To reduce electrical interference by the laser discharge, the whole electronic system is triggered optically and housed in a Faraday cavity. The slit width of the monochromator was usually set between 100 and 300 gm (corresponding roughly to 0.3-0.6 nm fwhm spectral bandwidth). It is, however, the natural ArF excimer laser bandwidth of ca. 0.7 nm which seems to limit the resolution so that no progress was made by working at higher monochromator resolution. This spectral resolution has to be improved further to be able to distinguish between transitions to the symmetric and antisymmetric stretch modes of HIS ( ~ ( 1 0 0 = ) 2611 cm-I, i.e., 203.6 nm and ~ ( 0 0 1 )= 2684 cm-', Le., 203.9 nmI3).
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Figure 3. Band intensities, relative to the u1,3band, for the emission spectrum of H2S excited at 193 nm. The relative intensities were determined by peak height measurement of four different scans and are corrected for changes in laser energy, and system response. The uncertainty of the intensity determination is &30%. Weak bands with greater uncertainty are not given here.
Figure 2 was not chosen here because of a particularly good signal to noise ratio, but because it is one of the few scans where the whole spectrum 193-320 nm was taken within a single experimental run. Usually, to allow for longer integration under experimentally stable conditions, only parts of the spectrum were taken and normalized to one another. In this way the data shown in Figure 3 were obtained. The relative intensities given there are corrected for changes in laser energy, and system response.
Results and Discussion A typical emission spectrum of H2S excited at 193.3 nm is shown in Figure 2. The spectrum is dominated by the fundamentals, a progression in the stretch vibration and combinations of the bend and stretch modes. The spectrum was simulated by calculating the vibrational energy levels with the usual expression for a nonlinear triatomic molecule having no degenerate vibrat i o n ~ .Five ~ ~ of the anharmonic constants xI, could be obtained from a simulation of the infrared overtone spectrum of H2S.I3 Combination bands between u, and u2 are missing in the infrared and Raman spectrum so that x12had to be fitted to our emission spectrum. Best agreement was obtained for small values of x12 near zero or just a few wavenumbers positive. The assignment given in Figure 2 was facilitated by the regular spacing of the four bands between 240 and 265 nm of 1200 f 100 cm-’ which indicates that a bending progression (u2 = 1290 cm-’) is combined with an overtone of the stretch vibration. The four bands between 275 and 305 num exhibit a similar regular spacing and probably also arise from a bending progression, combined with still higher stretch excitation. However, a definite assignment of these bands cannot be given here because of the low resolution of the measurements, as discussed above, and the already high state density a t these energies. Figure 3 shows the average band intensities, ~ of four different scans in the assignable relative to the v ] ,band, range. The spectrum shows that initially the intensities of the stretch mode overtones decrease slowly and monotonically in the progression, but regain intensity in combination with the bend mode u2 further along in the progression. The absence of a progression solely in the bend vibration is noteworthy; higher bend quanta are only excited in combination with higher stretch quanta. A definitive and quantitative interpretation of these results requires ab initio calculation of the excited-state surface(s) of H2S and a full dynamical treatment, Le., the solution of the Kram(13) Herzberg, G. Infrared and Raman Spectra; Van Nostrand: New York, 1966; p 283. (14) Herzberg, G.Infrared and Raman Spectra; Van Nostrand: New York, 1966; p 206, formula CII, 268.
er-Heisenberg-Dirac expression. As discussed in the Introduction, there are qualitative similarities between the photodissociation of H2S at 193 nm and that of the isovalent H 2 0 at 157 nm in their first absorption bands. For HzO, the surface of the lowest excited singlet state is well characterized on an ab initio b a s k 3 We therefore begin the discussion here. At the ”vertical” starting point of the H20dissociation, in the equilibrium bond angle of the ground and the excited state are the same (close to 105’) while there is a large gradient of the potential in the direction of the O H bond stretch. At the Franck-Condon point, the gradient of the symmetric stretch motion is largest and hence trajectories start predominantly with a symmetric stretch of the O H bonds and only cross to the “dissociative”antisymmetric motion at larger bond distances. The angle dependence part of the upper surface is rather isotropic at intermediate distances while a t large H O H separation, the energetically favored bond angle is 180’ (linear H O H geometries). If we assume that, similar to H 2 0 , the H2Sequilibrium bond angle of 92’ in the ground state is approximately the same in the Franck-Condon region of the excited state and opens only at large SH bond distances, then the spectrum shows a progression in stretch quanta due to vertical transitions to the ground-state surface while the dissociation proceeds and the bond distances increase. We do not observe a progression solely in bend quanta, because the optimum bond angles of the excited- and ground-state surfaces are approximately the same at smaller bond distances. Only when the bond distances have already lengthened considerably (resulting in vertical transitions to larger bond distances on the ground-state surface, Le., to excitation of higher stretch overtones) does the bond angle open up (leading to vertical transitions to larger bond angles on the ground-state surface, i.e., to excitation of higher bend overtones). Hence, high bend excitation should only occur in combination with stretch excitation, as observed experimentally. Now the SH prodwt rotational distribution observed does suggest that both the electronic ground- and excited-state surfaces have roughly the same equilibrium angles. As pointed out at the beginning of this paper, the SH rotational state distributions can be described well by Franck-Condon calculations and thus simply reflect the angular behavior of the ground-state wave function of H2S and not the anisotropy of the excited-state potential. In classical terms, the H and SH fragments separate without exerting a large torque after electronic excitation, because dissociative trajectories sample only isotropic regions of the angle dependent part of the upper surface over that range of distances where the fragments still “feel” each other. In contrast, the emission spectrum of dissociating HIS also samples parts of the upper surface where the bond angles have opened up considerably and vertical transitions to high bend quanta take place. Of course, this qualitative interpretation can only be preliminary. The assumption of short-time dynamics made in this analysis may not be justified. Despite an upper limit of the H2S lifetime on the time scale of vibrations (ca. s at 193 nmIo), a revisiting of the Franck-Condon region “later” in the disscoation cannot be excluded. Also, the analysis may be complicated by vibronic coupling and nonadiabatic interaction between the two upper surfaces involved and by predissociation (although the data available suggest more of a direct dissociation mechanism, at least upon excitation of H2S at 193 nm). For definite confirmation, the upper H2Ssurface(s) have to be calculated and the dynamics treated completely. Future experimental work is directed toward measuring the H2S emission spectrum at higher spectral resolution, in order to discriminate between symmetric and antisymmetric stretch excitation, and at different excitation wavelengths in the first absorption band.
Acknowledgment. We thank Prof. Dr. J. Wolfrum for his continuous interest in this work and encouragement and F. Kimm for assistance in late stages of the experiment. We gratefully acknowledge the financial support of the Deutsche Forschungsgemeinschaft.
