HETEROGENEOUS RECOMBINATION OF O ATOMS ON METAL SURFACES

Abstract

A study on recombination of neutral oxygen atoms on the surface of different metals is presented. The source of oxygen atoms was a weakly ionized highly dissociated oxygen plasma created in an inductively coupled radio-frequency discharge. Ionized particles as well as excited molecules were effectively recombined and de-excited on the walls of a noncatalytic tube between the discharge and the experimental chamber, so the gas in the latter consisted only of well-thermalized neutral molecules and atoms. The density of oxygen atoms in the experimental chamber was measured with a catalytic probe. Depending on discharge parameters, the O density was between 2×1020 and 5×1021 m-3. Thin foils of different metals were mounted in the experimental chamber and exposed to oxygen atoms. Due to heterogeneous surface recombination of oxygen atoms on the surface of the samples, the metal temperature was increased well above the ambient temperature. The recombination coefficient was calculated from the foil temperature using physical formalism. Among the materials tested the highest recombination coefficient of 0.41 was found for pure polycrystalline iron. The recombination coefficient for flat and well-oxidized nickel, copper, stainless steel, and niobium were found to be 0.27, 0.23, 0.07, and 0.09, respectively, while the recombination coefficient for nanostructured niobium was 0.8. The accuracy of these values was estimated to be about 30%.