Synchrotron radiation core level photoemission investigation of the initial stages of oxidation of Al(111)

Synchrotron radiation photoemission from the Al 2p levels shows that oxygen adsorption on Al(111) leads to at least four different chemically shifted peaks in addition to the metallic substrate peak. Three of these, associated with the chemisorption stage, are discussed in terms of a local bonding model and are attributed to emission from surface Al atoms bonded to one, two or three chemisorbed O atoms occupying 3-fold coordinated hollow sites. The fourth peak is the usual surface oxide peak, although its exact energy is found to be dependent on the stage of oxidation and whether this is achieved by exposure to molecular oxygen or water. Much lower occupation of the chemisorption states is found if the clean Al(111) surface is exposed to water instead of oxygen.