Infrared spectra of diatomic polar molecules in rare-gas liquids. I. Spectral theory

A theoretical model for the infrared (0–1) band of dilute solutions of diatomic polar molecules in nonpolar rare-gas liquids is presented. The model is based on decomposition of the rotational motion of the diatomic molecule into two limiting cases, according to the Bratos model: quasifree rotation and rotational diffusion. Contribution to the infrared absorption coefficient due to quasifree rotation is analyzed within a non-Markovian formalism using a stochastic directing intermolecular field (DIF) model to describe the diatomic molecule–solvent interaction. The P and R branches appear as a consequence of the quasifree contribution, which is also important in the Q-branch region. The contribution due to rotational diffusion is calculated making use of the Debye model and is mainly significant in the Q-branch region.