Emission spectroscopy of dissociating H2S: Influence of nonadiabatic coupling

Abstract

The emission spectroscopy of H2S excited in the first absorption band around 195 nm is investigated theoretically using ab initio potential energy surfaces (PES) and transition dipole moment functions. As shown in our previous studies, the photodissociation involves two excited electronic states, one which is binding and another one which is dissociative. The nonadiabatic coupling between these two states is very strong and described in a diabatic representation in which only the binding state is optically excited while the dissociative state is dark. As in the case of H2O excited in the 165 nm band, the emission spectrum shows a long progression of stretching states up to seven HS vibrational quanta. In contrast to water, however, some weak activity in the bending mode is also observed. Most remarkable is a prominent wavelength dependence which is attributed to the strong nonadiabatic coupling between the two excited electronic states. The agreement with experimental data is only fair; the essential features of the measurements are qualitatively reproduced, finer details such as the wavelength dependence are, however, not well described. It is concluded that more accurate ab initio input data are required in order to reproduce all details of the measured emission spectra.