Far-infrared absorption in liquid methane

Abstract

We present a simple theory of the far-infrared collision-induced absorption spectrum of liquid methane. We assume that CIA arises through octopolar and hexadecapolar induction. By neglecting the small anisotropy in the intermolecular potential, we can represent the spectral profile as convolutions of single-molecule free rotation spectra with intermolecular 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 (the calculation of these moments is described in an appendix). In this way we avoid the need to make questionable assumptions about the system dynamics. Our theory contains no adjustable parameters, yet the computed lineshapes are in excellent agreement with the available experimental data.