Kinetic theory of vibrational relaxation in a radiation field: The optic-acoustic effect

A microscopic derivation is presented of the rate equations governing vibrational relaxation occurring in the optic-acoustic effect. Detailed expressions applicable to the spectrophone experiment are given both for an excitation source consisting of a broadband radiation field and for laserdriven systems. It is clear from the present treatment that no real advantage accrues from the use of laser excitation sources in the standard spectrophone experiment, due to the resultant strong dependence of the driving force itself on the mechanical chopper frequency. For broadband radiation field the dependence on the chopper frequency is removed and the standard result containing the Einstein coefficient of induced absorption is recovered. The spectrophone response for the simplest case of a two-level system is given explicitly and its similarity to phenomenologically derived expressions is pointed out.The work of F.R.M. was supported in part by a grant from the National Research Council of Canada. The work of A.T. was supported by the Foundation for Fundamental Research on Matter, which is sponsored by the Netherlands Organization for the Advancement of Pure Research.