Boltzmann–Langevin equation and light scattering by dilute gases

Abstract

The Boltzmann–Langevin equation, which is the linear Boltzmann equation supplemented by a fluctuating force term, is used as a starting point to analyze the density correlation function in a dilute gas. By referring to the fluctuation–dissipation theorem, an equivalence of this approach and the conventional treatment based on the kinetic equation is established. For molecular gases, a quasi-classical description is employed, permitting one to incorporate internal degrees of freedom into the theory. With the help of Zwanzig's projection operator formalism, a new simple computational procedure is developed. Its versatility is illustrated by the explicit calculation of the scattered light spectrum from H2 and HD for two specific models.