CO adsorption on Pd(111): a high-resolution core level photoemission and electron energy loss spectroscopy study

By combining high-resolution X-ray photoelectron and electron energy loss s pectroscopies a comprehensive analysis of the adsorption of CO on Pd(1 1 1) at 300 K has been performed. The characteristic fingerprints for various C O-Pd(1 1 1) bonding configurations have been identified from the decomposit ion analysis of the adsorbate C 1s and the substrate Pd 3d(5/2) core-level photoemission spectra obtained after CO adsorption at 120 K. The c(4 x 2) s tructure at 0.5 monolayer (ML) and the (2 x 2)-3CO structure at 0.75 ML for med at low temperature have been used for calibration purposes. The core-le vel results are consistent with CO adsorbing in a mixture of fee and hcp th reefold hollow sites in the c(4 x 2) structure and of hollow and on-top sit es in the (2 x 2) structure, as reported in the literature. For CO adsorpti on at 300 K, a different site occupation is evidenced by the presence of tw o components in the C 1s and Pd 3d(5/2) core-level and C-O stretching vibra tion lineshapes. At coverages up to 0.1 ML only fee threefold hollow sites in a (root3 x root3)R30 degrees structure are occupied. The population of t hese sites continues with increasing the CO coverage up to 0.32 ML, but in addition adsorption of CO in bridge sites takes place. The latter sites bec ome predominant at the 300 K saturation coverage of 0.5 ML, and are conside red to form domain boundaries in a poorly ordered (root3 x root3)R30 degree s phase with split low energy electron diffraction reflexes. Between 0.32 a nd 0.5 ML the domain boundary regions grow at the expense of the (root3 x r oot3)R30 degrees domains. (C) 2000 Elsevier Science B.V. All rights reserve d.