Nonadiabatic Coupling in Fine-Structure Transitions of Alkali Atoms Induced by Diatomic Molecules

Fine-structure transitions of alkali atoms induced by collisions with diatomic molecules are shown to be strongly influenced by a dynamical feature that is completely absent when the collision partner is a rare gas. The effect is a nonadiabatic coupling caused by the anisotropy of the molecule. Calculations for $\mathrm{Rb}\left(5\mathrm{}{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}\right)$ + ${\mathrm{H}}_2$ and for $\mathrm{Rb}\left(5\mathrm{}{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}\right)$ + He are in good agreement with recent experiments and explain the dramatic difference between these two systems.