INITIAL STEPS OF ALKALI-METAL-PROMOTED OXIDATION OF THE AL(111) SURFACE

We investigate the initial Na-promoted oxidation of an Al(111) surface at room temperature by core-level photoemission spectroscopy using sy nchrotron radiation. We bring the first evidence that, in the initial state, adsorbed oxygen atoms break the Na-Al bonds. This leads to the formation of an intermediate oxidation state involving both Na and Al sites with oxygen atom in a bridge-type site. This bonding configurati on is shown to result from the Na/A1 intermixing and appears as a prec ursor step playing a key role in the observed promoted oxidation. It i s also likely to explain the formation, upon annealing, of Al oxides h aving a higher oxygen content than Al2O3.