Interaction of oxygen with polycrystalline cobalt studied by inverse-photoemission spectroscopy

We report a room-temperature UV inverse-photoemission study of the early oxidation stages of polycrystalline cobalt within the photon energy range 12–25 eV for exposures between 1 and ${10}^4$ Langmuir (L) (1 L=${10}^{\mathrm{-}6}$ Torr s). The decomposition of each spectrum related to intermediate exposures in terms of bulk Co and oxidized Co, together with the analysis of the relative intensities for the different contributions, shows the stable formation of CoO at the interface for exposures ≥30 L, whereas for lower exposures the oxide stoichiometry depends on the amount of oxygen atoms. In the 5–7-L range, possible ${\mathrm{Co}}_3$ ${\mathrm{O}}_4$ formation takes place consistently with a decreased interdiffusion in this regime as compared to the higher exposures regime. An estimation of the oxide film thickness at the maximum exposure is performed giving a surface oxide thickness of about 20 Å.