Chemisorption and Oxidation: Oxygen on Tungsten

Abstract

Oxygen chemisorption and the extent of oxidation on a tungsten ribbon at room temperature were studied by flash desorption using line-of-sight detection with a time-of-flight mass spectrometer. The only chemical species in the adlayer are oxygen atoms and the monotungstic oxides: WO, WO2, and WO3. These oxides are desorbed with an energy of ∼100 kcal. In agreement with earlier predictions, oxygen desorbs as atoms. Neither molecular oxygen nor polytungstic oxides were ever observed from a ribbon flashed after a room-temperature exposure to oxygen. However, flowing oxygen on tungsten at surface temperatures > 1400°K does produce the polytungstic oxides. An order-of-magnitude estimate of the adlayer composition at room temperature suggests that at saturation there is approximately one monolayer of oxygen atoms in a random configuration with a few tenths of a per cent each of the monotungstic oxides. Consequently, oxidation is not a significant surface process on clean tungsten at room temperature.