ADSORPTION SITE DETERMINATION FOR OXYGEN ON AL(111) USING NORMAL INCIDENCE STANDING X-RAY WAVE-FIELD ABSORPTION

The structure of the chemisorption phase of oxygen on Al(111) has been studied using normal incidence standing X-ray wavefield absorption (NISXW) at the (111) and the (111BAR) Bragg reflection conditions. The resulting O-Al layer spacings in these two directions gives a measure of both the absorbate-substrate layer spacing perpendicular to the surface, and the adsorption site. At the lowest coverages of oxygen, the results indicate that the adsorbed oxygen atoms occupy "fcc" hollow sites (i.e. directly above Al atoms in the third layer of the substrate) at a layer spacing of 0.70 angstrom. This result agrees well with previous SEXAFS results and with later LEED work, and reinforces the view that early LEED results giving much larger layer spacings were in error. The new results also indicate that this site is occupied at the lowest coverages of the chemisorbed phase, and that there is no evidence for a previously proposed precursor underlayer. Similar NISXW measurements taken from a surface containing a significant fraction of the subsequent surface oxide phase indicate that the oxygen atoms in this structure retain the same coordination, as well as bondlength, relative to the Al substrate, and favour a model in which the majority of the overlayer oxygen atoms of the chemisorbed phase retain their overlayer sites.