Nonadiabatic Theory of Fine-Structure Branching Cross Sections for Na-He, Na-Ne, and Na-Ar Optical Collisions

The nonadiabatic close-coupled theory of atomic collisions in a radiation field is generalized to include electron spin and is used to consider the weak-field Na–rare-gas (RG) optical collision Na(2S1/2)+RG+nhν μNa(2Pj)+RG+(n-1)hν. The effects of detuning and incident energy on the branching into the atomic Na 3p2P3/2 and 3p2P1/2 states are examined. The cross sections σ(j) are found to have a strong asymmetry between red and blue detuning as well as a complex threshold and resonance structure dependence on energy. A partial cross-section analysis of σ(j) shows a significant difference between contributions from states of e and f molecular parity. The theoretically calculated detuning dependence of the branching ratio into each fine-structure state is in good agreement with available experimental data for Na-Ar, Na-Ne, and Na-He, as well as the total absorption coefficient for the production of Na 3p atoms. The fine-structure branching ratio for thermal energy collisions shows considerable variation with a rare-gas collision partner, due to the different interaction potentials. For sufficiently high collision energy, the branching approaches a recoil limit which is independent of collision partner