Collision-induced absorption in the ν2 fundamental band of CH4. II. Dependence on the perturber gas

Abstract

The integrated intensities of the collision-induced enhancement spectra of the ν2 band of CH4 perturbed by rare gases and linear molecules (N2, H2, and CO2) are calculated theoretically using the quadrupole transition moment obtained from an analysis of CH4–Ar spectra. In addition to the isotropic quadrupole mechanism responsible for the enhancement in CH4-rare gases, there is additional absorption arising from the anisotropic quadrupole mechanism in the case of molecular perturbers. This latter effect involves the matrix element of the anisotropic polarizability for the ν2 transition in CH4 that is available from the analysis of the depolarized Raman intensity measurements. Overall, the theoretical values for the slope of the enhancement spectra with respect to the perturber density are in reasonably good agreement with the experimental results, thus confirming that the collision-induced absorption arises primarily through the quadrupolar induction mechanism.