Adsorption and reactivity of NO on Cu(111): A synchrotron infrared reflection absorption spectroscopic study

Infrared reflection absorption spectroscopy of NO adsorbed on Cu(111) has been performed in the frequency range 200-2500 cm(-1). At low temperatures (T < 120 K), two different states were identified with increasing coverage: adsorption followed by dissociation. At coverages up to one monolayer, NO molecules adsorb on identical sites, which are suggested to be of threefold symmetry, with an upright geometry. Two ordered overlayers are formed in turn: p(3 x 3) and (root 7 x root 7)R19.1 degrees. The infrared spectra show two absorption bands: the internal stretching made of NO, which shifts upwards in frequency with increasing coverage, and a low frequency anti-absorption band. The anti-absorption band is assigned, based on its isotopic frequency dependence, to the hindered rotation of the NO molecules. After completion of the monolayer, the NO molecules react and adsorbed N2O molecules are found on the surface bound through the oxygen atom (Cu-ON2 stretching mode at 352 cm(-1)) with their molecular axis (O-N-N) parallel to the surface plane. Desorption of the adsorbed N2O occurs at 120 K. NO dimers, which are the reaction intermediates in the formation of N2O from NO, have been clearly identified.