Pw. Browning et al., PROBING DYNAMICS IN THE FRANCK-CONDON AND EXIT CHANNEL REGIONS OF DISSOCIATING H2S - EMISSION-SPECTRA UPON TUNABLE EXCITATION FROM 199-203 NM, The Journal of chemical physics, 101(7), 1994, pp. 5652-5664
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.