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Abstract:
We have employed high resolution electron energy loss spectroscopy (HREELS), angle resolved photoelectron spectroscopy (ARUPS), and near edge X-ray absorption fine structure (NEXAFS) measurements to study the adsorption of nitric oxide (NO) on a clean and on oxygen precovered Ni(100) surfaces at T = 95 K. The adsorption behaviour on both the clean and the oxygen precovered surfaces is very complex. On the clean surface adsorption at low coverage starts in hollow sites with the NO axis oriented perpendicular to the surface. Consecutively, bridge sites are populated with both perpendicular and bent NO molecules. Finally, terminally bound, linear NO adsorbs on the surface. On the oxygen precovered surfaces we find the same adsorbate sites. Depending on whether we have chosen p(2×2) or c(2×2) oxygen precoverage we find a smaller percentage of molecules adsorbed in hollow sites, because oxygen occupies these sites in both layers but twice as many hollow sites in a c(2×2) layer. A particularly interesting observation concerns an oxygen influenced site in which the NO molecules are adsorbed with a bent orientation. This is corroborated via NEXAFS measurements. This NO species is more strongly bound to the surface than on the clean Ni surface. A section is included in the paper where we discuss some general aspects of NO bonding towards metal atoms in a linear versus bent orientation and the influence of coadsorbed species on the orientation of the molecular axis.
