Electronic Energy Partitioning in Photodissociation

Sherwin J Singer,Yehuda B Band,Karl F Freed

doi:10.1155/lc.3.57

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.

Highlights

One of the important decay channels of excited molecular states involves decomposition
At fragment translational energies far in excess of the magnitude of non-adiabatic coupling matrix elements, transition amplitudes to atomic fragment eigenstates are given in terms of amplitudes to molecular excited states (Eq 3.1)
When only one molecular excited state participates in the photodissociation in the recoil limit, the various observable branching ratios and anisotropies may be completely evaluated without consideration of the molecular potential curves themselves and without any further dynamical calculations

Summary

INTRODUCTION

One of the important decay channels of excited molecular states involves decomposition. Treatment of electronic energy partitioning in diatomic molecule photodissociation is important because the effect of non-adiabatic interactions is significant whenever the photofragments have non-vanishing angular momentum. In many cases this high energy approximation allows prediction of the photofragment angular distributions and polarization of fluorescence emitted by excited fragments based solely upon the symmetry of the molecular states involved in the optical excitation of the molecule and without the need for any dynamical calculations.

PHYSICAL MOTIVATION OF THE THEORY

HIGH ENERGY LIMIT

EXACT QUANTUM CALCULATIONS

CONCLUSIONS