Maxwell versus Non-Maxwell Velocity Distributions for Molecules Emitted from a Liquid Surface

Abstract

The nature of the velocity distribution for molecules emitted from a liquid surface is considered, and the angular dependence of the sticking coefficient for molecules striking the liquid surface is calculated with allowance for the presence of thermally excited capillary waves. Of the various forms of Maxwell distribution that have been assumed to apply to the molecules emitted from a gas−liquid interface, the speed distribution for molecular effusion is most appropriate, but the available experimental data do not allow us to discriminate between this function and “floating” forms of the ordinary Maxwell distribution of speeds. The distribution function for effusion, which is the one normally used in studies of the gas−solid interface, includes a cos θ factor, where θ is the angle between the velocity vector and the normal to the surface. Our work on the kinetics of emission from the liquid surface indicates that the factor should be cos2 θ for molecules emitted from the liquid. This angular factor has a...