Electronic Energy Partitioning in Photodissociation

Abstract

Despite the apparent simplicity of photodissociation in diatomic molecules, some of the essential physics of this process is not understood when there is fine structure in the atomic photofragments. Previous theories cannot treat the branching ratios and angular distributions of the individual fine structure sublevels. We have developed a complete quantum mechanical theory of the effects of nonadiabatic couplings and of electronic angular momentum on the fine structure branching ratios, angular distributions, and polarization in diatomic photodissociation. When the photofragments separate with large relative kinetic energy, simple limiting expressions can be obtained for branching ratios and the symmetry parameters which characterize fragment angular distributions and polarized fluorescence from excited fragments. Information about the symmetry of the molecular states involved in the optical transition which dissociates the molecule may be deduced from fine structure branching ratios and asymmetry parameters in the high energy limit. At low relative kinetic energies where non-adiabatic couplings are crucial, cross sections and asymmetry parameters exhibit interesting behavior which intimately reflect the shape of the dissociative molecular surfaces. We employ the example of sodium hydride photodissociation to produce P2 excited sodium atoms as a model system because of the availability of ab initio potential curves and oscillator strength matrix elements. The low energy photodissociation cross section and angular distributions are shown to exhibit resonances which arise in part due to non-adiabatic spin–orbitand Coriolis couplings. Their energy dependence can therefore be utilized to probe the nature of potential curves which are not directly pumped in optical absorption processes and may therefore provide a unique spectroscopic means for measuring properties of these “dark” states.