SURFACE AND INTERFACIAL ALLOYS OF PD WITH CU(100) - STRUCTURE, PHOTOEMISSION AND CO CHEMISORPTION

We have studied the alloy structures formed by the deposition of 0-4 monolayers (ML) of Pd on Cu(100) at 300 K. c(2 x 2) surface alloy and p4g interfacial alloy structures are seen at 0.5 and 1.0 ML Pd coverages, respectively. Valence band spectra resemble those of dilute Pd-Cu bulk alloys below 0.5 ML Pd coverage. At higher coverages they become increasingly Pd-like. The Pd (Cu) core level binding energies increase (decrease) in the alloys. CO chemisorption on Pd sites is weakened relative to Pd(100), but the desorption temperature of CO from Cu does not change. The distribution of Pd-CO and Cu-CO adsorption sites is discussed in terms of the surface morphology of the alloys. For CO adsorption on Cu(100) we show that the sticking coefficient decreases significantly with CO coverage, even below 0.5 ML. The additional CO adsorbed on formation of the compressed overlayer fills a low-temperature desorption state.