Short range force effects in semiclassical molecular line broadening calculations

A semiclassical theory of the width and shift of isolated infrared and Raman lines in the gas phase is developed within the impact approximation. A parabolic trajectory model determined by the isotropic part of the interaction potential allows a satisfactory treatment to be made of the close collisions leading to an analytical expression for the elastic collision cross section. A numerical test of this theory has been made for HCl-Ar by comparing the present results to those of previous infinite order treatments using numerical curved classical trajectories. Extension to the diatom-diatom collisions is then made by expressing the anisotropic potential using an atom-atom interaction model which takes both the long and short range contributions into account. Numerical applications have been performed for the Raman line widths of pure N2, CO2 and CO and for the infrared line widths of pure CO and of CO perturbed by N2 and CO2. A good quantitative agreement with experiments is obtained for all the considered cases and a correct variation of the broadening coefficient with the rotational quantum number is achieved in opposition to the previous results. A consistent variation of the line broadening with temperature is also obtained even for high rotational levels.