X-RAY PHOTOELECTRON DIFFRACTION OF NI(110)-2 X 1-CO P2MG LAYER INTERMOLECULAR SCATTERING

The ordered adsorbed CO layer of (2 x 1) p2mg symmetry on a Ni(110) surface was studied by X-ray photoelectron diffraction over a polar angle range THETA = +/- 85-degrees and for four different azimuthal orientations. Forward scattering enhancement peaks in the C 1s photoelectron intensity were detected at various angles, being due to intramolecular as well as intermolecular scattering. In particular, strong intermolecular scattering peaks were found at +/- 68-degrees along the [001] azimuthal direction. The anisotropy in intensity is about 46% compared to 67% for the intramolecular scattering peak at +/- 21-degrees. Multiple scattering calculations for different CO adsorbate configurations and binding sites on Ni(110) were performed. A comparison with the experimental data, corrected for the instrument function, supports the on-top binding site for CO and a linear Ni-C-O configuration. Thus, XPD has been used for the first time to evaluate adsorbate registry, in addition to the bond and molecule orientation.