On the density expansion of spectral lineshapes

Abstract

We compare two types of expansions (in the density of perturbers) of collisionally broadened spectral lineshapes. The first is the more conventional frequency-domain expansion of Fano whereas the second is made in the time domain. In the impact limit they both predict a simple lorentzian line shape but otherwise they yield very different results upon trunction. Our calculations indicate that the time-domain expansion is more adequate and should be preferred.