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 (2×1)-O and (2×2)-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 (2×1)-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 (2×1)-(O+NO) structure. All adsorbates sit in hcp 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 v 1 NO species sitting in hcp and fcc 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 ( d 12=2.22 Å), while that from the second to the third layer contracted ( d 23=2.08 Å). The oxygen parameters are virtually unchanged from those of the (2×1)-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 (2×2)-(2O+NO) layer contains a honeycomb layer of O atoms, half of them having switched to fcc 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 v 2 species of the pure NO layer. There is a clear difference between the hcp and fcc Os ( z e=1.20 Å and 1.39 Å, respectively); they can also be distinguished in XPS. The average distances d 12 and d 23 are less changed than in the (2×1)-(O+NO) layer, but there is considerable buckling. Starting from a (2×2)-O layer with O on hcp sites, one or two NO molecules can be incorporated per unit mesh. In the (2×2)-(O+NO) layer, the NO sits upright on the top site, with essentially v 2 parameters. In the (2×2)-(O+2NO) layer, one NO is roughly identical, and the second sits on the fcc site with slightly changed parameters compared to v 1 ( r e=1.22 Å, z e=1.39 Å). We discuss conclusions from this systematic series of