A photoelectron diffraction study of ordered structures in the chemisorption system Pd{111}-CO

The adsorption system Pd{111}-CO gives rise to a series of at least seventeen, sometimes complicated, ordered overlayers. The (3x3) R30° and c(4×2) phases are formed at coverages of θ=0.33 ML and 0.50 ML, respectively. The results from surface vibrational spectroscopy have hitherto been interpreted in terms of CO adsorbing on threefold symmetric hollow sites in the (3x3)R30° phase, but occupying bridge sites in the c(4×2) phase. We show in a quantitative photoelectron diffraction study in the scanned energy mode that, whereas hollow sites are indeed occupied in the (3x3)R30° structure, CO adsorbs in a mixture of fcc and hcp hollows in the c(4×2) phase. Several structural parameters, in particular the Pd-C layer spacings, have been determined. The result is essentially identical to that obtained for the Ni{111}c(4×2)-CO system. It is also inferred that at coverages between 0.33 and 0.50 ML “domain wall”, or weakly incommensurate, phases occur in which the percentage of occupied hcp sites increases monotonically as the coverage is raised