Oxidation of the group IBmetals studied by X-ray and ultraviolet photoelectron spectroscopy. Part 1.—The surface oxidation of polycrystalline copper

Abstract

He I and He II(α and β) spectra of clean polycrystalline copper are reported and discussed. The oxidation of clean copper surfaces by both ground state molecular and microwave excited oxygen has been studied using X-ray and ultraviolet photoelectron spectroscopic (XPS and UPS) technique (including estimation of work function changes) in combination in the same apparatus. XPS measurements on bulk copper (I) and copper (II) oxides have also been made, and are compared with earlier work. The room-temperature chemisorption of oxygen on copper is deduced to be a dissociative process leading, for up to about a monolayer uptake, to a Cu^I surface oxide, which on further exposure to oxygen is converted to a Cu^II surface oxide as a further adsorbed overlayer builds up. Incorporation of oxygen thereafter is quite rapid. The electronic structures of the surface oxides are similar to those of the corresponding bulk oxides. The He II spectra are found to reflect changes in the density of states resulting from oxidation much more directly than do the He I spectra: the spectra also provide evidence, in support of earlier work, that u.v.-stimulated primary photoelectrons, unlike those ejected by X-radiation, only escape in significant numbers from the uppermost layers (≲ 10 Å) of the solid.