Frequency Separation for Relaxation Problems
  The modern era in spectral line broadening began with the understanding that the slow(quasistatic) ion and fast(impact) electron perturbers could be treated separately. The problem remained of unifying these two theoretical limits. A scheme for this unification is presented here that has at its foundation a fundamental observation that is supported by analytical theory and is further demonstrated by computer simulation. The fundamental observation is that the ions and electrons can be separated most of the time, and that a frequency separation within each perturber subsystem can be used for unification. That is, the rigorous inclusion of slow, but not necessarily static ions together with the correct impact ion perturbations, will produce valid ionic line shapes. We show that a frequency separation may be effected to exactly include the fast modulation limit in a variety of modern methods that can deal with the intermediate regime between the fast and slow frequency limits of the perturbation. 
S.Alexiou (Universite Paris VI/LULI Ecole Polytechnique)