Probing dynamics in the Franck–Condon and exit channel regions of dissociating H2S: Emission spectra upon tunable excitation from 199–203 nm

Abstract

This study uses emission spectroscopy of H2S at excitation energies near 200 nm to probe the dissociation dynamics from a conical intersection in the Franck–Condon region to the H+SH product exit channel. Photoexcitation accesses these coupled surfaces near the transition state region of the lower adiabat, a potential surface for the excited state H+SH→HS+H reaction. Excitation wavelengths from 199–203 nm tune through the first of the resonances in the absorption spectrum assigned to recurrences in the motion along the symmetric stretch orthogonal to the reaction coordinate and also access energies just above and at the conical intersection. We disperse the emission from the dissociating molecules at each of five excitation wavelengths in this region to probe several features of the reaction dynamics on the coupled potential energy surfaces. The resulting emission spectra cover the range of final vibrational eigenstates from 500 to 11 000 cm−1 above the initial ground vibrational state for all five excitation wavelengths, and go out to 16 500 cm−1 for the 199 and 201 nm excitation wavelengths. The resulting spectra, when considered in conjunction with recent scattering calculations by Heumann and Schinke on ab initio potential energy surfaces for this system, evidence a progression of emission features to low vibrational eigenstates in the SH stretch that result from coupling of the nuclear motion from the bound to the dissociative region of the potential energy surfaces. This emission, into local mode eigenstates such as 00+1, 11+0, 11+1, 21+0, 21+1, evidences the antisymmetric dissociative motion and bending induced near the conical intersection, and dominates the spectrum at excitation wavelengths only near 200 nm. We analyze the excitation wavelength dependence of these features and also of the n0+0 progression for n≥4, which reflect the exit channel dynamics. The excitation wavelength dependence shows that while the emission spectra do not reveal any dynamics unique to scattering states that access a symmetric stretch 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 distribution shows that v=0 products are strongly favored at excitation wavelengths near the conical intersection.