Frequency separation method 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. A scheme for unifying these two theoretical limits is presented that has at its foundation the observation, supported by analytical theory and by computer simulation, that ions and electrons can be separated most of the time, and that a frequency separation within each perturber subsystem can be used for unification. 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.