Far infrared absorption in liquid methane-argon mixtures

Abstract

We present a simple theory of the far-infrared collision-induced absorption spectrum of dilute solutions of methane in liquid argon. We assume that CIA arises through octopolar and hexadecapolar induction of a dipole on the solvent argon atoms by the solute methane molecules. By neglecting the small anisotropy in the CH₄-Ar intermolecular potential, we can express the spectral profile as convolutions of CH₄ free rotation spectra with intermolecular CH₄-Ar translational components. Information theory is used to estimate the ‘ least biased ’ form for the latter, based on our knowledge of their zeroth and second spectral moments. In this way we avoid the need to make questionable assumptions about the system dynamics. Our theory contains no adjustable parameters, yet yields results in excellent agreement with experiment.