Oxidation of cerium and titanium studied by photoelectron spectroscopy

Cerium and titanium are examples of reactive metals that form protective oxide films on their surfaces upon exposure to oxygen. In order to study the building up of these oxide films we have undertaken photoemission measurements with X-ray and ultraviolet radiation (ESCA and UPS). Cerium and titanium levels were studied using ESCA on freshly in situ evaporated metal films. These levels, as well as the O 1s level, were studied after different oxygen exposures. In order to vary the probing depth, spectra have been recorded at two different electron emission angles. All measurements were performed at room temperature. Effects of oxygen exposures upon core levels were quite different in the two metals. In the Ce 3d spectrum a strong peak related to the oxide was observed at an exposure of only 2 L. Such a strong peak was not observed in the Ti 2p spectrum even after an exposure of 3000 L. The valence band spectrum of Ti as observed both by UPS and ESCA measurements, however, changed significantly at much smaller exposures.