Four-state model of optical collisions: Sr + Ar

The semiclassical theory of electronic excitation of an atom in a strong nonresonant laser field due to a collision with another atom is presented. It is shown that in the presence of asymptotic degeneracy of the excited state the Landau-Zener two-state model is inaccurate, the exact solution of a two-state model is inaccurate, and both differ qualitatively from the accurate solutions of the equations when all (four) states are included. With parameters chosen to model the process Sr(/sup 1/S) + Ar + h..omega.. ..-->.. Sr(/sup 1/P) + Ar, cross sections on the order of 10/sup -14/ cm/sup 2/ are obtained for h (..omega.. - ..omega../sub 0/) approx. 17 cm/sup -1/ over a large range of the Rabi frequency. Depolarization predominates at high field strengths.