COLLISION-INDUCED ABSORPTION WITH EXCHANGE EFFECTS AND ANISOTROPIC INTERACTIONS: THEORY AND APPLICATION TO H2_H2 and N2_N2.

Abstract

Collision-induced absorption spectra can be calculated quantum mechanically and from first principles. However, such calculations are usually performed in the approximation of an isotropic interaction potential and neglecting exchange effects. We present theory for including exchange and anisotropic interactions in the calculation of collision-induced absorption spectra, and apply this method to the $mathrm{H}_2-mathrm{H}_2$ and $mathrm{N}_2-mathrm{N}_2$ systems. For $mathrm{H}_2-mathrm{H}_2$, the isotropic interaction approximation is generally accurate, although significant effects of anisotropic interactions are observed in the far wing of the spectrum. For $mathrm{N}_2-mathrm{N}_2$, anisotropic interactions increase the line strength at low energy by two orders of magnitude. The agreement with experimental data is reasonable in the isotropic interaction approximation, and improves when the full anisotropic potential is considered. The effect of the interaction anisotropy decreases at higher energy, which validates the usual isotropic interaction approximation as a high-temperature approximation for the calculation of collision-induced absorption spectra.