Two-photon collisional redistribution of radiation

We study collisional redistribution in the presence of two weakly exciting laser fields, including the effects due to degeneracy of the radiator states. A general expression for the total redistributed intensity is derived valid for arbitrary detunings and polarizations of the exciting laser fields. In particular, this expression contains all single- and sequential-collisional contributions, which are equally important under certain circumstances. We also point out the similarities and differences between the redistributed intensity as calculated in this paper and collisionally aided radiative excitation cross sections, which have been studied previously.