Relaxation Processes and Molecular Motion in Liquids and Gases

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Abstract
In this paper we extend the time correlation function approach to infrared absorption in an organic liquid or dense gas. We include an interaction energy (coupling) between the ir active vibrational mode and the coupled translational–rotational degrees of freedom to account for collision-induced de-excitation of the vibration (damping). The additional time dependence of the dipole correlation function is a damped exponential and modifies the line shape obtained by earlier treatments. The appearance of a damping term also changes the interpretation of the first few terms in the short-time expansion of the correlation function, which are related to the molecular moment of inertia and the mean-square torque. Our treatment is based on many-body perturbation theory (renormalization of the vibrational energy to second order in the coupling) and can be extended to more general cases.