Collision-induced infrared absorption by H2-He complexes: Accounting for the anisotropy of the interaction.

Abstract

We give the expressions for the zeroth and first spectral moments of collision-induced absorption (CIA) in the infrared by atom-diatom pairs. The perturbation treatment takes into account both the anisotropy of the interaction potential and its dependence on the vibrational coordinate. With the help of reliable ab initio induced dipole and potential surfaces of the ${\mathrm{H}}_{2}$-He collisional complex, we compute these expressions numerically. Under conditions for which laboratory measurements of CIA exist, accounting for the anisotropy lowers absorption, the more so the greater the anisotropy (especially of the repulsive core) is. Moreover, systems that possess relatively strong overlap-induced dipole components show more substantial anisotropy corrections. Specifically, while for ${\mathrm{H}}_{2}$ pairs in the rototranslational CIA band we see only small anisotropy corrections, in the vibrational bands the corrections are more significant, especially at the higher temperatures. ${\mathrm{H}}_{2\mathrm{\ensuremath{-}}}$He complexes show even more substantial corrections. In a comparison of a recent measurement of the absorption profile in hydrogen-helium mixtures in the ${\mathrm{H}}_{2}$ fundamental band with quantum line shape calculations, the effects of the anisotropic interaction are actually discernible.