Cu{001}c(2×2)-Pd: An ordered surface alloy

A quantitative low-energy electron diffraction (LEED) intensity analysis shows that the room-temperature reaction of Pd atoms with a Cu{001} surface produces a surface alloy, Cu{100}c(2×{}2)-Pd, analogous to Cu{001}c(2×{}2)-Au. The structure of Cu{001}c(2×{}2)-Pd is almost planar, with the Pd atoms located 0.02±{}0.03 A{} outwards from the Cu atoms, and an interlayer distance from the Cu atoms to second (100% Cu) layer equal to the Cu{001} bulk interlayer spacing (1.807 A{}). Although there are no ordered Pd atoms beyond the surface mixed layer, there is some evidence for disordered Pd either above or below the alloy layer. Angle-resolved photoemission spectra show that the valence band of the surface alloy is characterized by a well-defined Cu 3d--derived band and by the appearance of features due to Pd. The dispersion and the photon-energy dependence of these features suggest their probable origins and are consistent with the alloy character of Cu{001}c(2×{}2)-Pd.