Collisional broadening of CO₂ IR lines. II. Calculations

The ability of available theoretical models in describing broadening mechanisms is tested for the CO₂-O₂, CO ₂-CO₂, and CO₂-N₂ systems. It is shown that the Anderson-Tsao-Curnutte theory is inaccurate since short-range forces can contribute significantly to broadening. We use the approach of Robert and Bonamy, but the usual expansion of the atom-atom potential to the fourth order around the intermolecular distance appears insufficient at short distances for these particular systems. We propose a better representation of the radial dependence of the atom-atom potential, while keeping the previous analytical expression of the cross section. Satisfactory results are obtained for both the rotational quantum number dependence of room-temperature CO₂-O ₂, CO₂-CO₂, and CO₂-N₂ half-widths and the evolution of CO₂-N₂ broadening with temperature. It is shown that the isotropic part of the potential involved in the trajectory calculation must be coherently deduced from the atom-atom interaction potential.