Structure of submonolayer gold on silicon (111) from x-ray standing-wave triangulation

The three-dimensional registry of a submonolayer Au film adsorbed on a Si(111) surface was measured using the x-ray standing-wave method. Au photoelectron yields were monitored as the (111), (220), and (111{}) reflections from the substrate were scanned. These results were combined with symmetry considerations in order to triangulate the Au position relative to the bulk crystal structure. An unusual adsorption site, one which is embedded in the substrate and bridges two Si atoms in the lower half of the (111) double layer, agrees well with the data.