Vapor–Solid Interactions and the Effusion Oven. II. Spatial Distribution of an Effusion Flux

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Abstract
Under molecular‐flow conditions, the spatial distribution of molecules emerging from an effusion channel characterizes the nature of the vapor–solid interactions that occur in the transport process. Theoretical distributions evaluated by Clausing with a “diffuse‐reflection” interaction model have proven inadequate in accounting for the experimental distributions produced with channels of appreciable length. With a coupled surface‐ and vapor‐transport effusion model which considers the possible mobility of an adsorbed molecule, theoretical distributions can be evaluated in terms of the channel geometry and the binding energy of the adsorbed species. A comparison between the theoretical distributions and three separate experimental studies is presented. In general, the quantitative agreement obtained is good, and supports the applicability of the coupled transport model to the effusion process. Both theory and experiment indicate that many of the published results of effusion studies are in error because of the application of Clausing effusion parameters.