Theory of collisionally aided radiative excitation

Abstract

The excitation of a two-level active atom by a radiation field in the presence of a low-density perturber bath is studied. In particular, the collisional enhancement of the absorption cross section is investigated. Features of the absorption-line profiles are predicted on physical grounds using a dressed-atom approach and verified by detailed numerical calculations for attractive, repulsive, and Lennard-Jones-type interatomic potentials. Complete spectra of the absorption cross section both as a function of detuning and as a function of field strength are given. In the weak-field limit, the results are compared with those of traditional pressure-broadening theory and to the recent experimental results of Carlsten et al. In the strong-field limit, the results display a saturation behavior consistent with the predictions of Lisitsa and Yakovlenko.