Adsorption of NO on the Rh(111) surface has been studied in the monolayer, bilayer, and multilayer regimes with near edge x-ray absorption fine structure (NEXAFS) spectroscopy. NO dimer layers are formed on a chemisorbed monomer layer. The polarization dependence in the NEXAFS spectra of the dimer components has contradicted the previous assignments. To determine the structure of the NO dimer layers from the polarization analysis of the NEXAFS spectra, ab initio configuration interaction calculations have been carried out for some low-lying core excited states of the weakly bound NO dimer with cis-ONNO planar geometry. It is revealed that the NO dimers in the multilayer are standing with the N-N bond perpendicular to the surface, while in the second layer they are rather lying on the first monomer layer.
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