Particle-photon radiative interactions and thermalization

TL;DR

This paper investigates how radiative interactions influence the velocity distribution of molecules, deriving a fluctuation-dissipation theorem and showing deviations from Gaussian behavior due to radiative effects.

Contribution

It introduces a radiative fluctuation-dissipation theorem and analytically demonstrates non-Gaussian velocity distributions caused by radiative interactions.

Findings

Velocity distribution deviates from Gaussian without molecular collisions.

Maxwellian distribution is recovered at small radiative friction.

Analytical expression for kurtosis shows non-Gaussian behavior.

Abstract

We analyze the statistical properties of radiative transitions for a molecular system possessing discrete, equally spaced, energy levels, interacting with thermal radiation at constant temperature. A radiative fluctuation-dissipation theorem is derived and the particle velocity distribution analyzed. It is shown analytically that, neglecting molecular collisions, the velocity distribution function cannot be Gaussian, as the equilibrium value for the kurtosis $\kappa$ is different from $\kappa=3$. A Maxwellian velocity distribution can be recovered in the limit of small radiative friction.