Chemisorption geometry of NO on Rh(111) by X-ray photoelectron diffraction

The adlayer structure of (2 x 2)-3NO on Rh(111) has been investigated by high-energy, scanned-angle X-ray photoelectron diffraction (XPD) in conjunction with single scattering cluster theory and R-factor analysis. In addition, a plausible adlayer registry, consistent with previously published HREELS data, is deduced by means of physical reasoning. The resulting structural model consists of NO bound to atop, three-fold hollow fee, and three-fold hollow hcp sites through the N atom with an NO bond length of 1.15 Angstrom. NO molecules at all three sites within the adlayer are oriented normal to the surface. The Z coordinates (Z being defined as perpendicular to the surface) of atop and hollow-site NO molecules relative to the top layer of Rh atoms differ by 0.5 Angstrom, with hollow-site NO being bound more closely to the substrate. This structure differs considerably from one proposed previously by a LEED I-V analysis in both the site occupancy and the spacing of NO molecules within the adlayer.