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 $\mathrm{Sr}\left({}_{}{}^1{}_{}{}^{}S\right)+\mathrm{Ar}+\hslash \omega \to \mathrm{Sr}\left({}_{}{}^1{}_{}{}^{}P\right)+\mathrm{Ar}$, cross sections on the order of ${10}^{-14\mathrm{}}$ ${\mathrm{cm}}^2$ are obtained for $\hslash (\omega -{\omega }_0)\sim 17$ ${\mathrm{cm}}^{-1\mathrm{}}$ over a large range of the Rabi frequency. Depolarization predominates at high field strengths.

• Received 6 May 1977

DOI:https://doi.org/10.1103/PhysRevA.18.1363