Potential energy surfaces for the interactions of Rb and Cs atoms with methane

Diode pumped alkali vapor lasers (DPAL's) rely on collisional energy transfer to move population from the optically pumped level (P-2(3/2)) to the upper level of the laser transition (P-2(1/2)). Small hydrocarbon molecules such as methane and ethane have proved to be very effective transfer agents. It is thought that the primary mechanism is electronic-to-rotational (E-R) energy transfer, but this has yet to be proven. In the present study we use electronic structure calculations to determine the potential energy surfaces for Rb(P-2) and Cs(P-2) interacting with methane. These surfaces may be used to estimate the probabilities for transfer by E-R exchange and by surface hopping at the seams of intersection between potential energy surfaces. We have also examined the influence that interactions with CH4 have on the P-2-S-2 transition moments. This is a preliminary step towards understanding the enhanced far-wing optical absorption cross-sections that have been observed in alkali metal/rare gas/hydrocarbon mixtures.