Far infrared absorption of the gaseous CH4–Ar mixture

Abstract

We present the first measurements of the rototranslational absorption spectrum of the CH4–Ar gaseous system in the far infrared region 40–600 cm−1. Measurements were made at 296, 243, and 195 K at densities as low as possible; the binary absorption spectrum of the CH4–Ar pair was then obtained. We performed a line-shape analysis of these spectra following the procedure previously introduced for pure methane. We find that the experimental data can be described at high frequencies by using ab initio values of the octopole and hexadecapole multipole moments and anisotropic overlap intensities very close to those used for pure methane. At low frequencies, the computed spectrum is significantly lower than the experimental data at any temperature. We thus include in our analysis a low-frequency spectral contribution associated with the isotropic overlap induction mechanism. Good fits of the experimental data are then obtain at each temperature over all the spectral range.