Branching and anisotropy of barrier tunneling and fluorescent decay in H2. II. Theory

A quantum-mechanical treatment describing photofragmentation of a homonuclear molecule is presented. The treatment is developed to describe results on photofragmentation in molecular hydrogen [preceding paper, Phys. Rev. A 54,522 (1996)] in which photoexcitation takes place to an excited state that can dissociate using different channels, all of which are accompanied by fluorescence. The kinetic energy of the photofragments, which is directly related to the fluorescence spectrum, and their anisotropy were measured. The theoretical treatment incorporates electronic-state mixing due to nonadiabatic effects in the excited state. Fluorescence may occur as molecular fluorescence, that is, before dissociation, or atomic fluorescence, after dissociation has taken place. Both are treated simultaneously and the branching is determined. A remarkable result is the occurrence of interference when molecular fluorescence is degenerate with atomic fluorescence. It is shown that in case of nonadiabatic effects the anisotropy of the photofragments can depend on the decay channel; the actual anisotropy does not represent the anisotropy of the ensemble of excited molecules directly.