Transition between the Quasistatic and Impact Limits in Spectral Line Broadening

A kinetic description of line broadening is used to express the conventional approximation schemes as limits of a more general point of view. A profile function is given in terms of a generalized collision rate. Central to the calculation of this rate is a perturbed phase function that accounts for possible strong coupling between perturbers and the particle coupled to the light. An approximate expression for this phase function is given which continuously describes the transition between the two limits.