Theory of diatomic molecule photodissociation: Electronic angular momentum influence on fragment and fluorescence cross sections

Abstract

We present a general theory of the photodissociation of diatomic molecules in the presence of nonadiabatic interactions between dissociative electronic states. Nonadiabatic couplings exert a profound influence on the photodissociation process when the molecule dissociates to atoms with nonvanishing electronic angular momentum, even if there are no ordinary curve crossings. Nonadiabatic interactions in this latter situation couple adiabatic molecular states which would otherwise be degenerate at infinite internuclear separation. Methods are developed for properly including nonadiabatic interactions in an exact or approximate calculation of photodissociation transition amplitudes for the production of the atomic fragments in particular fine structure levels. We derive differential cross sections for photofragment production and general expressions describing the detection of fragments by any secondary process, such as spontaneous emission, occurring during or after the breakup of the diatom. These expressions are then specialized to fragmentation of a diatom into atoms in fine structure states. Detection of photofragments, fluorescence from atoms produced in excited electronic states, or coincidence detection of both fragments and fluorescence, are considered in detail. An approximate treatment of nonadiabatic interactions is developed which is valid when the fragments recede with relative kinetic energy much greater than the magnitude of the nonadiabatic coupling matrix elements. In some cases, the effects of nonadiabatic interactions are still observable in the high kinetic energy limit. Finally, we discuss how the effect of nonadiabatic interactions may be observed experimentally, and how this can be used to amplify the information to be gained from a photodissociation experiment. In particular, it is shown how the polarization of the atomic fragments contains information fingerprinting the participating intermediate dissociative molecular electronic states.