Speed-dependent collisional width and shift parameters in spectral profiles

Abstract

The absorption or emission line shape that one generally associates with low-density atomic systems is termed a Voigt profile. When collisions are occurring within the atomic system, the Voigt profile is modified to include collisional width and shift parameters. However, the speed dependence of these width and shift parameters is most often ignored, even though such a speed dependence is predicted by simple collision theories. An expression for the spectral profile, termed a Speed Dependent Voigt Profile (SDVP), will be derived, taking into account speed-dependent shift and width parameters which are calculated on the basis of an active atom-perturber interaction V ∝ R-n (R is the internuclear separation). We shall show that an analysis of line shapes in terms of simple Voigt profiles (rather than the SDVP) may lead to considerable errors in the determination of characteristic atomic parameters, especially in systems where the perturber to active atom mass ratio is large. It will also be noted that the speed-dependent shift leads to an asymmetrical profile, and such a profile will be displayed.