Collision-induced absorption and anisotropy of the intermolecular potential

A scheme is developed for quantum mechanical calculations of binary collisioninduced spectra which permits full inclusion of rotovibrational molecular degrees of freedom. A close-coupling scheme which includes the radiation in the Hamiltonian is used. The collision-induced absorption spectra of interacting atom–diatom and diatom–diatom pairs are investigated. The inclusion of the anisotropy of the intermolecular potential introduces couplings among the rotational levels of the diatomic molecules. Previous calculations of collision-induced spectra have almost exclusively been done using the isotropic potential approximation and we present an extensive investigation of the validity of that approximation. Absorption spectra in the rotational and fundamental bands of H2, induced by collisions with He, H, Ar, and H2 are calculated for various temperatures. In all of these, except for H2–H, the anisotropy of the intermolecular potential affect the absorption by 5-10% in certain parts of the spectra. Comparisons with the available measurements show very good agreement of the shapes of the spectral profiles, although the absolute intensities differ by up to 10% in some cases. These remaining differences between theory and measurements appear to be random and are generally smaller than the differences among comparable measurements. In the H2–H spectra the effect of the anisotropy of the potential turns out to be almost negligible at the temperature for which a full coupled quantum calculation was done. This is supported by spectral moment calculations. The smallness of the effect is believed to stem from the short range character of the anisotropic potential components for H2–H. Collision-induced absorption spectra of gaseous mixtures of deuterium hydride and helium in the rotational and fundamental bands of HD are calculated at a temperature of 77 K. The computed absorption profile agree with a measurement taken in the HD fundamental band. We also consider the interference phenomena of the HD permanent dipole with the HD–He interaction-induced dipole by computing the wings of various R(j) lines and of the P1(1) line in the single, binary collision limit. Agreement between theory and measurements is observed in the low-helium-density limit of the measured absorption line shapes.