Electronic Energy Partitioning in Photodissociation

Despite the apparent simplicity of photodissociation in diatomic molecules, some ofthe essential physics of this process is not understood when there is fine structure inthe atomic photofragments. Previous theories cannot treat the branching ratios andangular distributions of the individual fine structure sublevels. We have developed acomplete quantum mechanical theory of the effects of nonadiabatic couplings and ofelectronic angular momentum on the fine structure branching ratios, angular distributions,and polarization in diatomic photodissociation. When the photofragments separatewith large relative kinetic energy, simple limiting expressions can be obtained forbranching ratios and the symmetry parameters which characterize fragment angulardistributions and polarized fluorescence from excited fragments. Information aboutthe symmetry of the molecular states involved in the optical transition which dissociatesthe molecule may be deduced from fine structure branching ratios and asymmetryparameters in the high energy limit. At low relative kinetic energies where non-adiabaticcouplings are crucial, cross sections and asymmetry parameters exhibit interestingbehavior which intimately reflect the shape of the dissociative molecular surfaces. Weemploy the example of sodium hydride photodissociation to produceP 2excited sodiumatoms as a model system because of the availability of ab initio potential curves andoscillator strength matrix elements. The low energy photodissociation cross sectionand angular distributions are shown to exhibit resonances which arise in part due tonon-adiabatic spin–orbitand Coriolis couplings. Their energy dependence can thereforebe utilized to probe the nature of potential curves which are not directly pumped inoptical absorption processes and may therefore provide a unique spectroscopic meansfor measuring properties of these “dark” states.