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 oxygen precoverages, Theta(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 Theta(O)+Theta(NO)=1. On the well-ordered (2x2)-3O oxygen layer (Theta(O)=0.75 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 (2x2) hole structure within the (2x2)-3O mesh conserving the same symmetry. For this well-ordered Ru(001)-(2x2)-(NO+3O) layer the external and internal NO stretching modes show downward dispersions of 16 and 23 cm(-1), respectively, from the Gamma point to the (K) over 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 internal 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 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.