Adsorption of CO and NO on NiO(111) thin films epitaxially grown on Ni(111) substrate has been studied by infrared-visible sum frequency generation (SFG) and the results were compared with those of infrared reflection absorption spectroscopy. From SFG measurements, the C-O stretching band of adsorbed CO was observed at 2144 cm-1 for both p- and s-polarized visible (532 nm) light whereas the adsorbed NO gave the N-O stretching band at 1800 cm-1 only for the p-polarized visible light. These observations suggested that the CO molecule was inclined to the surface whereas the tilt angle of NO from the surface normal was smaller than that of CO. The adsorption sites of CO and NO molecules are located on the slopes of trigonal microfacets formed by the reconstruction of the NiO(111) surface. CO adsorbed on Ni(111) instead of NiO(111) was also examined: the SFG signal corresponding to the C-O stretching mode of linearly bonded CO was observed at 2076 cm-1 only for the p-polarized visible light, but that of the bridge-bonded one (at saturation coverage) was not detected by SFG.
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