The ordered adsorbed CO layer of (2 • 1) p2mg symmetry on a Ni(110) surface was studied by X-ray photoelectron diffraction over a polar angle range O = +_ 85 ~ and for four different azimuthal orientations. Forward scattering enhancement peaks in the C ls photoelectron intensity were detected at various angles, being due to intramolecular as well as intermolecular scattering. In particular, strong intermoleeular scattering peaks were found at + 68 ~ along the [001] azimuthal direction. The anisotropy in intensity is about 46% compared to 67% for the intramolecular scattering peak at +21 ~ 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.

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