Vibrational characterization of a high-densityRu(001)−(2×2)−(NO+3O)phase

Abstract

High-density $(\mathrm{NO}+\mathrm{O})$ coadsorbate layers on Ru(001) have been studied for oxygen precoverages, ${\ensuremath{\Theta}}_{\mathrm{O}},$ between 0.5 and 1 ML by means of high-resolution electron energy loss spectroscopy and temperature programmed desorption. In this oxygen coverage range NO adsorption is possible on any remaining hcp threefold-coordinated site, to a saturation coverage of ${\ensuremath{\Theta}}_{\mathrm{O}}+{\ensuremath{\Theta}}_{\mathrm{NO}}=1.$ On the well-ordered $(2\ifmmode\times\else\texttimes\fi{}2)\ensuremath{-}3\mathrm{O}$ oxygen layer $({\ensuremath{\Theta}}_{\mathrm{O}}=0.75\mathrm{ML})$ NO molecules adsorb at 90 K with a high sticking coefficient close to unity up to the saturation coverage of 0.25 ML. The NO sublayer is ordered as the NO's occupy the threefold sites of the $(2\ifmmode\times\else\texttimes\fi{}2)$ hole structure within the $(2\ifmmode\times\else\texttimes\fi{}2)\ensuremath{-}3\mathrm{O}$ mesh conserving the same symmetry. For this well-ordered $\mathrm{Ru}(001)\ensuremath{-}(2\ifmmode\times\else\texttimes\fi{}2)\ensuremath{-}(\mathrm{NO}+3\mathrm{O})$ layer the external and internal NO stretching modes show downward dispersions of 16 and $23{\mathrm{cm}}^{\ensuremath{-}1},$ respectively, from the $\overline{\ensuremath{\Gamma}}$ point to the ${K}^{\ensuremath{'}}$ point at the boundary of the surface Brillouin zone. The dispersion of the internal mode can be completely described by dynamical dipole-dipole coupling. This coupling is also dominant for the external mode dispersion for which additional substrate-mediated contributions exist. Based on this understanding of the dynamical coupling the chemical shift of the NO internal and external stretch is determined for various $(\mathrm{NO}+\mathrm{O})$ structures. It can be related to the occupation of nearest- and next-nearest-neighbor sites. The internal mode shows chemical shifts between 12 and $30{\mathrm{cm}}^{\ensuremath{-}1}$ per neighboring NO or O but is insensitive to the structure beyond the nearest neighbors. For the external mode significant chemical shifts due to the occupation of the next-nearest-neighbor sites have been determined.