A systematic investigation of the geometrical structures of four oxygen/nitric oxide coadsorbate layers on Ru(001)

LEED IV analysis has been used to determine the detailed geometries of four well-defined ordered coadsorbate structures which can be formed by the interaction of NO with (2 x 1)-O and (2 x 2)-O layers on Ru(001), and which have been characterized previously by various surface spectroscopies in this laboratory. New high-resolution XPS measurements are also reported which provide exact information on the coverages and chemical types of the species concerned, information which is helpful for the selection of model structures. Post-adsorbing NO below 150 K onto the well-developed (2 x 1)-O row structure leads to a layer with equal amounts of O and NO consisting of alternating rows of oxygen atoms and NO molecules, i.e. a (2 x 1)-(O + NO) structure. All adsorbates sit in hep sites. In terms of geometry as well as electronic and bonding properties, the NO (upright orientation, NO bond length r_e= 1.20 Å, Ru-N vertical layer distance z_e = 1.32 Å) is essentially identical to the electronegative V₁ NO species sitting in hep and fee sites in the pure NO layer reported previously [M. Stichler, D. Menzel, Surf. Sci. 391 (1997) 47]. The first-to-second Ru layer distance is considerably expanded (d₁₂ = 2.22 Å), while that from the second to the third layer contracted (d₂₃ = 2.08 Å). The oxygen parameters are virtually unchanged from those of the (2 x 1)-O layer. Heating this layer to 300-450 K leads to the desorption of half the NO and restructuring of the residual coverage. The resulting well-ordered (2 x 2)-(2O + NO) layer contains a honeycomb layer of O atoms, half of them having switched to fee sites. The remaining NO molecules sit upright on the top sites surrounded by the O honeycombs, and have properties (r_e = 1.12 Å, z_e = 1.76 Å) which are very similar to the electropositive v₂ species of the pure NO layer. There is a clear difference between the hep and fee Os (z_e = 1.20 Å and 1.39 Å, respectively); they can also be distinguished in XPS. The average distances d₁₂ and d₂₃ are less changed than in the (2 x 1)-(O + NO) layer, but there is considerable buckling. Starting from a (2 x 2)-O layer with O on hcp sites, one or two NO molecules can be incorporated per unit mesh. In the (2 x 2)-(O + NO) layer, the NO sits upright on the top site, with essentially v₂ parameters. In the (2 x 2)-(O + 2NO) layer, one NO is roughly identical, and the second sits on the fcc site with slightly changed parameters compared to v₁ (r_e = 1.22 Å, z_e = 1.39 Å). We discuss conclusions from this systematic series of structures for the reliability of geometry determinations by quantitative LEED, and for the chemistry of these layers. Using Badger's rule we can estimate the internal bond orders of the NO molecules, 2.0-2.2 for the v₁(O)-NO and 2.5-2.7 for the v₂(O)-NO species, which are in line with expectations from a simple frontier orbital argument.