The Accommodation Coefficient Technique Applied to a Study of Molecular Oxygen Film on Tungsten

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Abstract
The accommodation coefficient method of Roberts was used to measure the adsorption isotherms of the upper molecular oxygen film on tungsten at 25 and 100°C and at pressures from 9×10−9 to 1×10−4 mm Hg. The isotherms were found to fit Wang's theoretical isotherm for a mobile film with repulsive interaction. An entropy of adsorption was calculated from the surface pressure by Hill's method and found to be about four entropy units at low film concentrations. The large discrepancy between calculated and theoretical entropies suggests either or both irreversible effects and nonlinearity of the relation between accommodation coefficients and surface coverage. The minima which are exhibited during the growth with time of the adsorbed film at 100°C may be direct evidence for the presence of holes in the immobile first layer which are filled by activated diffusion from the upper layer.

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