Diffusional Contribution to the Total Flow from a Knudsen Cell

Abstract
It has been recognized for sometime that the geometry of a Knudsen effusion cell influences the kinetics of the molecular effusion of the vapor from within the cell. However, no treatment has been reported of the contribution to the effusion current made by the surface diffusion out through the orifice of the effusate adsorbed on the walls within the cell. The applicable integro‐differential equations which describe the steady‐state kinetics for both surface diffusion and vapor effusion out through the orifice have been solved approximately. The derived relations predict that under specified conditions the diffusion flux for the orifice dimensions commonly employed may contribute an appreciable fraction of the total weight loss of the cell during an effusion experiment. If not taken into account, this diffusional contribution would yield equilibrium vapor pressures in excess of the actual values.