Wall collision lifetimes of contained gases

Abstract Wall collisions can significantly reduce the lifetime of contained gases in sufficiently small gas cells. Existing methods for calculating the lifetime between wall collisions are either indirect approximations, or only account for the smallest dimension of the gas cell. A general gas kinetic method for calculating collisional lifetimes of contained gases is presented. This methodology accounts for both collisions with the gas cell walls and collisions with other gas constituents. Furthermore, this procedure is applicable for arbitrary gas cells geometries and gas distributions. Lifetimes and fraction of wall collisions expected are calculated for three idealized gas cells with uniform particle distributions: between two infinite parallel plates, within a sphere, and within an infinitely long cylinder. The lifetimes vary significantly from values approximated using spectral line broadening. Comparisons are made between uniform and radially Gaussian gas distributions within the sphere and cylinder geometries. Wall collision lifetimes are shown to depend on the confining geometry, not just the cell’s smallest dimension. Both wall and internal gas collisions contribute significantly to gas lifetimes when the smallest cell dimension is on the order of the inter-particle mean free path.