A quasiharmonic model calculation for two non-Markovian far-infrared spectra of diatomic polar molecules in a rare-gas liquid. I. Spectral theory

Abstract

A model consisting in a quantum rigid rotor interacting with a quasiharmonic medium is presented to obtain the far-infrared spectra of diatomic polar molecules dissolved in rare-gas liquids. The absorption coefficient is calculated within the framework of two non-Markovian approaches called TTOC (total time ordered cumulants) and PTOC (partial time ordered cumulants). Both non-Markovian spectra contain the same correlation functions of the diatomic–medium interaction, which is expressed in terms of a Legendre polynomial series truncated up to second order. These correlation functions are calculated in paper II. The absorption cofficients appear as a sum of two spectral contributions. The first one, called secular contribution, is given as a juxtaposition of basic resonances, different in each of two non-Markovian approaches, associated to the absorption lines j→j+1. The second one, called interference contribution, takes into account the existence of a overlapping line effect. The contributions to the absorption coefficients from the P1 and P2 Legendre polynomials of the interaction are explicitly reported.