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photoelectron diffraction; chemisorption; surface structure; morphology; roughness and topography; nickel oxides; carbon monoxide; ammonia; single crystal epitaxy
Abstract:
The local adsorption structures of CO and NH3 on NiO(I 0 D) have,been determined by C Is and N 1s scanned-energy mode photoelectron diffraction. CO adsorbs atop Ni surface atoms through the C atom in an essentially perpendicular geometry (tilt angle 12 +/- 12degrees) with a C-Ni nearest-neighbour distance of 2.07 +/- 0.02 Angstrom. NH3 also adsorbs atop Ni surface atoms with a N-Ni distance of 2.06 +/- 0.02 Angstrom. These bondlengths are only very slightly longer than the comparable values for adsorption on metallic Ni surfaces. By contrast theoretical values obtained from total energy calculations, which exist for CO adsorption on NiO(I 00) (2.46 Angstrom and 2.86 Angstrom) are very much longer than the experimental value. Similar discrepancies exist for the N-Ni nearest-neighbour bondlength for NO adsorbed on NiO(1 0 0). Combined with the published measurements of the desorption energies, which also exceed the calculated bonding energies, these results indicate a significant failure of current theoretical treatments to provide an effective description of molecular adsorbate bonding on NiO(1 0 0).
