Oxygen-Induced Changes Of The Au30pd70(110) Surface Structure And Composition Under Increasing O-2 Pressure

The structure and composition of the Au30Pd70(110) surface were followed by grazing incidence X-ray diffraction under increasing oxygen pressure from ultrahigh vacuum (UHV) to 500 mbar at moderate temperatures (300-420 K). These measurements were complemented by Auger electron spectroscopy and environmental scanning tunneling microscopy (STM) images with atomic resolution up to 500 mbar. After the cleaning and preparation procedures in UHV, the Au30Pd70(110) surface is (1 X 1) with a quasipure topmost layer of segregated gold. Under oxygen pressure, Pd segregation occurs and progressively enriches the near-surface region. The surface evolves through different states that slightly differ from those of pure Pd(110). First, the (1 X 2) missing-row reconstruction was induced by oxygen adsorption that is stable in a large range of pressures depending on the temperature. Actually, STM images show regular vacancies every two atoms along the dense [110] rows. Then, for higher oxygen pressures, a transition phase appears before the formation of an oxidized pure Pd film growing in the [100](pdo) direction.