On the velocity distribution function of spontaneously evaporating atoms

TL;DR

This study uses numerical solutions of the Enskog-Vlasov equation to accurately model the velocity distribution of atoms evaporating into near-vacuum, revealing anisotropic temperatures and drift effects caused by interface collisions.

Contribution

It introduces a refined approximation of the velocity distribution function incorporating drift velocity and temperature anisotropy, based on EV equation solutions.

Findings

Half-Maxwellian with drift and anisotropic temperatures fits the data well.

Drift velocity and temperature anisotropy decrease with lower bulk temperature.

Collisions at the interface cause deviations favoring lower normal velocities.

Abstract

Numerical solutions of the Enskog-Vlasov (EV) equation are used to determine the velocity distribution function of atoms spontaneously evaporating into near-vacuum conditions. It is found that an accurate approximation is provided by a half-Maxwellian including a drift velocity combined with different characteristic temperatures for the velocity components normal and parallel to the liquid-vapor interface. The drift velocity and the temperature anisotropy reduce as the liquid bulk temperature decreases but persist for relatively low temperatures corresponding to a vapor behaviour which is only slightly non-ideal. Deviations from the undrifted isotropic half-Maxwellian are shown to be consequences of collisions in the liquid-vapor interface which preferentially backscatter atoms with lower normal-velocity component.