INTERACTION OF OXYGEN WITH AL(111) STUDIED BY SCANNING-TUNNELING-MICROSCOPY

The interaction of oxygen with Al(111) was studied by scanning tunneli ng microscopy (STM). Chemisorbed oxygen and surface oxides can be dist inguished in STM images, where for moderate tunnel currents and indepe ndent of the bias voltage the former are imaged as depressions, while the latter appear as protrusions. An absolute coverage scale was estab lished by counting O adatoms. The initial sticking coefficient is dete rmined to s(o)=0.005. Upon chemisorption at 300 K the O adlayer is cha racterized by randomly distributed, immobile, individual O adatoms and , for higher coverages, by small (1 X 1) O islands which consist of fe w adatoms only. From the random distribution of the thermalized O adat oms at low coverages a mobile atomic precursor species is concluded to exist, which results from an internal energy transfer during dissocia tive adsorption. These ''hot adatoms'' ''fly apart'' by at least 80 an gstrom, before their excess energy is dissipated. A model is derived w hich explains the unusual island nucleation scheme by trapping of the hot adatoms at already thermalized oxygen atoms. Oxidation starts long before saturation of the (1 X 1) O adlayer, at coverages around THETA (O) congruent-to 0.2. For a wide coverage range bare and O(ad) covered surfaces coexist with the surface oxide phase. Upon further oxygen up take both chemisorbed and oxide phase grow in coverage. Oxide nucleati on takes place at the interface of O(ad) islands and bare surface, wit h a slight preference for nucleation at upper terrace step edges. Furt her oxide formation progresses by nucleation of additional oxide grain s rather than by growth of existing ones, until the surface is filled up with a layer of small oxide particles of about 20 angstrom in diame ter. At very large exposures up to 5 X 10(5) L they cover the entire s urface as a relatively smooth, amorphous layer of aluminum oxide. The difference in Al atom density between Al metal and surface oxide is ac commodated by short range processes, with no indication for any long r ange Al mass transport. Based on our data we discuss a simpler two ste p model for the interaction of oxygen with Al(111), without making use of an additional subsurface oxygen species. The complex spectroscopic data for the O/Al(111) system are rationalized by the wide coexistenc e range of bare and O(ad) covered surface with surface oxide and by di fferences in the electronic and vibronic properties of the surface ato ms depending on the number of neighboring O adatoms in the small O(ad) islands.