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.

Communication: State-to-state differential cross sections for H2O($\tilde B$B̃) photodissociation

“…As discussed before, these undulating peaks in the absorption spectrum stem from wavepacket recurrence in the Franck-Condon region. 21,22,31 In Fig. 2, we compare our calculated DCSs with those measured in a recent experiment.…”

Quantum Mechanical Studies of Photodissociation Dynamics Using Accurate Global Potential Energy Surfaces

“…78 With these accurate PESs available, rigorous quantum mechanical dynamics calculations have been performed, taking simultaneously into account all three states and the couplings between them and all nuclear degrees of freedom. 80 These calculations, for the first time, allowed detailed comparisons with the many experimental data and provided valuable insight into the complex dissociation dynamics. In the following we discuss some of the highlights of the comparisons between experiment and theory.…”

“…80 For low energies above the OH(A) threshold, the branching ratio is small for OH(A) and almost unity for OH(X). However, as the excess energy increases, the first one increases and the latter decreases.…”

“…However, there is a long tail with small probability extending to very high vibrational states, n ≈ 10, in good agreement with the calculations. 80 There are more interesting dynamical details in the photodissociation of water in the second band. Because of lack of space we have to refer to the original literature.…”

A Comprehensive Study of Infrared OH Prompt Emission in Two Comets. II. Implications for Unimolecular Dissociation of H2O

“…The latter problem has played a principal role in unimolecular dissociation studies, and it has received significant experimental and theoretical attention (e.g., Carrington 1964;Yamashita 1975;Andresen et al 1984;Häusler et al 1987;Engel et al 1988;Harich et al 2000;Nizkorodov et al 2003;Mordaunt et al 1994;Dixon 1995;Dixon et al 1999;van Harrevelt & van Hemert 2000). Our purpose is to augment the existing laboratory database for OH Ã rotational distributions with a set of measurements of emission from this photodissociation product of cometary H 2 O.…”