Vibrational characterization of a high-density Ru(001)-(2X2)-(NO+3O) phase

High-density (NO+O) coadsorbate layers on Ru(001) have been studied for oxy gen precoverages, Theta(O), between 0.5 and 1 ML by means of high-resolutio n electron energy loss spectroscopy and temperature programmed desorption. In this oxygen coverage range NO adsorption is possible on any remaining hc p threefold-coordinated site, to a saturation coverage of Theta(O)+Theta(NO )=1. On the well-ordered (2x2)-3O oxygen layer (Theta(O)=0.75 ML) NO molecu les adsorb at 90 K with a high sticking coefficient close to unity up to th e saturation coverage of 0.25 ML. The NO sublayer is ordered as the NO's oc cupy the threefold sites of the (2x2) hole structure within the (2x2)-3O me sh conserving the same symmetry. For this well-ordered Ru(001)-(2x2)-(NO+3O ) layer the external and internal NO stretching modes show downward dispers ions of 16 and 23 cm(-1), respectively, from the Gamma point to the (K) ove r bar' 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 intern al and external stretch is determined for various (NO+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 cm(-1) per neighb oring NO or O but is insensitive to the structure beyond the nearest neighb ors. For the external mode significant chemical shifts due to the occupatio n of the next-nearest-neighbor sites have been determined.