Adsorption and Diffusion of Oxygen on Tungsten

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Abstract
The surface diffusion and adsorption of oxygen on tungsten have been studied by a field emission technique, by utilizing the high sticking coefficient, and negligible vapor pressure of O2 at 4.2–20°K. Three types of migration were noted. At ∼27°K diffusion occurs with a sharp boundary and an activation energy Ed≈0.9 kcal. This is believed to be migration of physically adsorbed O2 over the chemisorbed layer, with precipitation on the bare surface. The distance traversed by the boundary was found to be limited by desorption above ∼40°K. A study of this effect yielded an estimate of 2.3 kcal for the heat of adsorption, Ha, in the physisorbed layer. At 400–500°K and high coverages, diffusion occurs within the monolayer with activation energies of 23–25 kcal. Boundaries moving radially outward from the close‐packed faces are observed, and believed to result from precipitation of O atoms in trap sites on the rough regions of the surface. At low coverages boundary free diffusion with Ed≃30 kcal is observed and interpreted as migration from trap sites. The latter are believed to constitute ∼60% of all sites. Activation entropies for the various diffusion processes are estimated to vary from ∼7–13 eu. It was found that 80% of the chemisorbed layer can be formed without appreciable activation at 27°K. The layer is stable up to 700°K, where desorption and oxidation start. The latter is markedly dependent on the amount of oxygen on the surface. At 300°K there is a slow oxygen uptake probably associated with incipient oxidation.