SUBMONOLAYER FILMS OF AU ON CU(001) STUDIED BY PHOTOELECTRON DIFFRACTION

Ultrathin films of one monolayer (ML) or less of Au on Cu(001) have been prepared by in situ deposition at 300 K in order to investigate the formation of a surface alloy. The local order within these films was determined by X-ray photoelectron diffraction, measuring full solid angle Au 4d5/2 intensity distributions. For depositions of between 0.2 and 0.6 ML we confirm that a surface alloy is formed, with a mixed c(2 X 2) Au/Cu layer on top. In contrast to earlier low-energy electron diffraction measurements we find a measurable amount of substitutional Au in the 2nd and 3rd layer. At a coverage of 1.2 ML all azimuthal structure in the angular distribution is lost, consistent with the growth of an incommensurate pseudo-hexagonal Au overlayer with no short-range orientational order. Still, the presence of narrow rings of higher intensity reveals the presence of subsurface Au.