Photodissociation of water. II. Wave packet calculations for the photofragmentation of H2O and D2O in the B̃ band

Abstract

A complete three-dimensional quantum mechanical description of the photodissociation of water in the B̃ band, starting from its rotational ground state, is presented. In order to include B̃-X̃ vibronic coupling and the B̃-Ã Renner–Teller coupling, diabatic electronic states have been constructed from adiabatic electronic states and matrix elements of the electronic angular momentum operators, following the procedure developed by A. J. Dobbyn and P. J. Knowles [Mol. Phys. 91, 1107 (1997)], using the ab initio results discussed in the preceding paper. The dynamics is studied using wave packet methods, and the evolution of the time-dependent wave function is discussed in detail. Results for the H2O and D2O absorption spectra, OH(A)/OH(X) and OD(A)/OD(X) branching ratios, and rovibrational distributions of the OH and OD fragments are presented and compared with available experimental data. The present theoretical results agree at least qualitatively with the experiments. The calculations show that the absorption spectrum and the product state distributions are strongly influenced by long-lived resonances on the adiabatic B̃ state. It is also shown that molecular rotation plays an important role in the photofragmentation process, due to both the Renner–Teller B̃-X̃ mixing, and the strong effect of out-of-plane molecular rotations (K>0) on the dynamics at near linear HOH and HHO geometries.