Collisional broadening of infra-red absorption lines

Abstract

The Lorentz half-widths of collision-broadened lines in the rotation-vibration bands of diatomic molecules vary with line number |m| in the P and R branches. The observed variation of half-width for lines in the 0 → 1 and 0 → 2 bands of CO and HCl are interpreted in terms of a simple fitting procedure. One dominant source of line broadening is assumed to consist of diabatic hard collisions involving transitions from each rotational level to all higher rotational levels; the effectiveness of this process, which varies from line to line, is described in terms of an empirically adjusted collision cross section, a maximum collision parameter related to independently measured molecular properties, and upon the availability of the required energy and angular momentum in molecular collisions. The second source of line broadening, assumed to be the same for all lines, includes all other types of collisions and is represented by a single empirically adjusted cross section. The simple fitting procedure is applied successfully to self-broadening of CO lines and to foreign-gas broadening of CO and HCl; for HCl self-broadening an additional cross section for resonant-dipole processes must be included. Possible applications of the fitting procedure to HBr, HF, and CO₂ are discussed. The simple procedure presented represents an approximation that may prove useful pending the development of readily applicable complete theories based on first principles.