Unified theory for collisional dephasing in strong radiation fields. a generalisation of the Karplus-Schwinger formula

An expansion is presented for the absorption lineshape of a two-level system in a strong radiation field being perturbed by dephasing collisions, within the binary collision approximation. No dynamical assumptions regarding the time scale of the collisions (relative to the broadening) need to be made and the present theory interpolates smoothly between the impact and the quasistatic limits. The present solution reduces in weak radiation fields to the linear unified theory of spectral lineshapes and in the impact limit (fast collisions) the authors recover the Karplus-Schwinger formula. The only molecular information necessary for the exact evaluation of the absorption lineshape in a strong field is the same single-particle correlation function g( tau ) which appears in the linear (weak field) unified theory.