Comparison of Spectral Line Profiles in Hard and Soft Collision Models.

The qualitative and quantitative effects on the line profile of hard and soft velocity-changing large- and small-angle scattering collisions of absorbing molecules, respectively, are considered. It is shown that large-angle scattering significantly contributes to the diffusion profile model, known as the "soft" collision model, as compared to that in the hard collision model. The difference between these traditional models lies only in the mathematical representation of the collision integral, either in integral or differential form, and the following analytical expressions for the profiles. Simple approximate formulas for the profile are derived and tested, which simultaneously take into account hard and soft collisions. [ABSTRACT FROM AUTHOR]