Spectral moments for the collision-induced rotovibrational absorption bands of nonpolar gases and mixtures (H2-He).

Abstract

Expressions for the zeroth, first, and second translational spectral moments are reported for the rotovibrational collision-induced absorption (RVCIA) spectra of atom-diatom pairs, which take into account the dependence of the intermolecular interaction potential on the vibrational excitation of the molecule involved. From these, numerical results are obtained for the fundamental CIA band of ${\mathrm{H}}_{2}$He. Consistency with a previous ab initio line-shape computation is observed which has also accounted for this dependence. However, especially at the higher temperatures considered (150--300 K), the first and second moments differ significantly, by up to 30%, from values calculated from existing, commonly used sum formulas which neglect the vibrational-state dependence of the interaction potential. The work thus corrects an existing inexactitude of the moment expressions in general use. When these expressions are combined with suitable model profile functions, a simple and yet very accurate modeling of RVCIA spectra is possible, with rms deviations from exact quantum profiles of no more than a few percent.