PROBING DYNAMICS IN THE FRANCK-CONDON AND EXIT CHANNEL REGIONS OF DISSOCIATING H2S - EMISSION-SPECTRA UPON TUNABLE EXCITATION FROM 199-203 NM

This study uses emission spectroscopy of H2S at excitation energies ne ar 200 nm to probe the dissociation dynamics from a conical intersecti on in the Franck-Condon region to the H+SH product exit channel. Photo excitation accesses these coupled surfaces near the transition state r egion of the lower adiabat, a potential surface for the excited state H+SH-->HS -->H reaction. Excitation wavelengths from 199-203 nm tune t hrough the first of the resonances in the absorption spectrum assigned to recurrences in the motion along the symmetric stretch orthogonal t o the reaction coordinate and also access energies just above and at t he conical intersection. We disperse the emission from the dissociatin g molecules at each of five excitation wavelengths in this region to p robe several features of the reaction dynamics on the coupled potentia l energy surfaces. The resulting emission spectra cover the range of f inal vibrational eigenstates from 500 to 11 000 cm(-1) above the initi al ground vibrational state for all five excitation wavelengths, and g o out to 16 500 cm(-1) for the 199 and 201 nm excitation wavelengths. The resulting spectra, when considered in conjunction with recent scat tering calculations by Heumann and Schinke on ab initio potential ener gy surfaces for this system, evidence a progression of emission featur es to low vibrational eigenstates in the SH stretch that result from c oupling of the nuclear motion from the bound to the dissociative regio n of the potential energy surfaces. This emission, into local mode eig enstates such as 00(+)1, 11(+)0, 11(+)1, 21(+)0, 21(+)1, evidences the antisymmetric dissociative motion and bending induced near the conica l intersection, and dominates the spectrum at excitation wavelengths o nly near 200 nm. We analyze the excitation wavelength dependence of th ese features and also of the n0(+)0 progression for n greater than or equal to 4, which reflect the exit channel dynamics. The excitation wa velength dependence shows that while the emission spectra do not revea l any dynamics unique to scattering states that access a symmetric str etch resonance in the Franck-Condon region, they do reveal the energy location of and the dynamics at the conical intersection. A reanalysis of other workers' measurements of the SH product vibrational state di stribution shows that upsilon=0 products are strongly favored at excit ation wavelengths near the conical intersection.