A PHOTOELECTRON DIFFRACTION STUDY OF ORDERED STRUCTURES IN THE CHEMISORPTION SYSTEM PD(111)-CO

The adsorption system Pd{lll}-CO gives rise to a series of at least se venteen, sometimes complicated, ordered overlayers. The (root 3 x root 3)R30 degrees and c(4 x 2) phases are formed at coverages of theta=0. 33 ML and 0.50 ML, respectively. The results from surface vibrational spectroscopy have hitherto been interpreted in terms of CO adsorbing o n threefold symmetric hollow sites in the (root 3 x root 3)R30 degrees phase, but occupying bridge sites in the c(4 x 2) phase. We show in a quantitative photoelectron diffraction study in the scanned energy mo de that. whereas hollow sites are indeed occupied in the (root 3x root 3)R30 degrees structure, CO adsorbs in a mixture of fee and hcp hollo ws in the c(4 x 2) phase. Several structural parameters, in particular the Pd-C layer spacings, have been determined. The result is essentia lly identical to that obtained for the Ni {111}c(4 x 2)-CO system. It is also inferred that at coverages between 0.33 and 0.50 ML ''domain w all'', or weakly incommensurate, phases occur in which the percentage of occupied hcp sites increases monotonically as the coverage is raise d. (C) 1998 Elsevier Science B.V. All rights reserved.