Energy curves and moments for PbHe and PbXe

Collision induced absorption at either the pump or Stokes wavelength could reduce the efficiency of Raman scattering of XeCl pump radiation in Pb vapor. Calculation of the collision induced absorption requires a knowledge of the different energy curves between the ground and upper states and the transition moment as a function of the internuclear distance. These curves were obtained in a first-order configuration interaction calculation of the PbHe, PbHe ⁺, PbXe, and PbXe⁺ valence states. The atomic cores of Pb and Xe were replaced by relativistic effective core potentials so as to make the calculations tractable. The van der Waals interaction was also estimated by a variational perturbation calculation. These results were transformed to an Ω representation using an empirically determined spin-orbit operator. Using these theoretical data we estimate the collision induced absorption cross sections to be 10^-23 and 10^-21 cm² per atmosphere of Xe for the first and second Stokes lines in the case of a 308.0 nm pump. Cross sections for He will be about an order of magnitude smaller due to the smaller transition moments. The energy defect for the pump wavelength is estimated to be so large that no significant absorption is expected for either PbXe or PbHe collision partners. There are also no calculated curve crossings at an energy lower than 0.25 eV, which indicates a very small probability for deactivation of Pb(³P₂) by rare gas collision.