46 Low-Energy Electron Diffraction Studies of Oxygen Adsorption and Oxide Formation on a (100) Crystal Face of Nickel Cleaned Under High-Vacuum Conditions

Abstract

After many treatments of the nickel crystal by outgassing in high-vacuum, argon-ion bombardment, and annealing, an essentially clean surface is obtained as indicated by a diffraction pattern characteristic of a nickel lattice only. At room temperature, oxygen first adsorbs on this surface as a double-spaced, face-centered chemisorbed monolayer. This monolayer is complete after a pressure × time exposure of 2 × 10 −6 mm. Hg-min. As the exposure is increased, further irreversible adsorption occurs as an amorphous covering layer. Above an exposure of about 10 −5 mm. Hg-min., a nickel oxide layer is formed at room temperature. If the nickel crystal is initially covered with a chemisorbed monolayer of carbon with a double-spaced, face-centered, square array, no appreciable effect is observed due to oxygen adsorption at exposures less than 4 × 10 −6 mm. Hg-min. At exposures above 10 −5 mm. Hg.-min., nickel oxide is formed as in the case of clean nickel. Both the chemisorbed oxygen and nickel oxide are removed by heating at 250-300°, thus indicating that the oxygen diffuses into the nickel.