Oxygen adsorption, reconstruction, and thin oxide film formation on clean metal surfaces: Ni, Fe, W and Mo

Abstract

A kinetic investigation is reported of the interaction of molecular oxygen with nickel, iron, tungsten and molybdenum films deposited in ultra-high vacua, covering the pressure range 10 −9 to 10 −3 torr, and temperatures between 77 and 373°K. On each of the metals and under all conditions examined the sticking probability s for oxygen on the clean surface was found to be very close to unity (> 0.99). The sticking probability-coverage profile for nickel shows a minimum at one quarter of a monolayer, which is more pronounced at higher temperatures, and is associated with a phase change of the adsorbate: up to this coverage oxygen is non-dissociatively bound on the surface. Adsorption on the high-coverage phase proceeds with a higher value of s, whence the minimum in the s-profile. At a monolayer coverage s drops to < 10 −4 before a slow process sets in, involving a further substantial uptake of oxygen: the rate of this process is independent of temperature in the range 195 to 290°K, and proportional to the pressure. On iron, only a monolayer uptake is obtained at temperatures < 190° K, but at room temperature the equivalent of 3 monolayers are sorbed when s has fallen to 10 −4. Five distinguishable and reproducible stages are noted in the s-profile, the discontinuities being associated with phase changes in the surface layer. The final stage of the process has an activation energy of 20 ± 5 kcal mole , and the rate is proportional to the pressure ( P < 10 −4 torr). Four stages are noted in the adsorption of oxygen on tungsten and molybdenum films; the final stage on tungsten has an activation energy of 5 kcal mole , and the rate is again proportional to the pressure ( P < 10 −3 torr). About 2 monolayers are taken up at room temperature before s falls to 10 −7. Desorption spectra are obtained to characterise low adsorption heat states on these films. In all cases the populations in these y states are low by comparison with, for example, those found for nitrogen on tungsten 24,25 or titanium 21).