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 derailed geometries of four well-defined ordered coadsorbate structures which can be formed by the int eraction of NO with (2 x 1)-O and (2 x 2)-O layers on Ru(001), and which ha ve been characterized previously by various surface spectroscopies in this laboratory. New high-resolution XPS measurements are also reported which pr ovide exact information on the coverages and chemical types of the species concerned, information which is helpful for the selection of model structur es. Post-adsorbing NO below 150 K onto the well-developed (2 x 1)-O row str ucture leads to a layer with equal amounts of O and NO consisting of altern ating rows of oxygen atoms and NO molecules, i.e, a (2 x 1)-(O + NO) struct ure. All adsorbates sir in hcp sites. In terms of geometry as well as elect ronic and bonding properties, the NO (upright orientation, NO bond length r (c) = 1.20 Angstrom, Ru-N vertical layer distance z(e) = 1.32 Angstrom) 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. Me nzel, Surf. Sci. 391 (1997) 47]. The first-to-second Ru layer distance is c onsiderably expanded (d(12)=2.22 Angstrom), while that from the second to t he third layer contracted (d(23)=2.08 Angstrom). The oxygen parameters are virtually unchanged from those of the (2 x 1)-O layer. Heating this layer t o 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 cont ains a honeycomb layer of O atoms, half of them having switched to fee site s. The remaining NO molecules sit upright on the top sites surrounded by th e O honeycombs, and have properties (r(e) = 1.12 Angstrom, z(e) = 1.76 Angs trom) which are very similar to the electropositive v(2) species of the pur e NO layer. There is a clear difference between the hcp and Fee Os (z(e) = 1.20 Angstrom and 1.39 Angstrom, respectively); they can also be distinguis hed in XPS. The average distances d(12) and d(23) are less changed than in the (2 x 1)-(O + NO) layer, but there is considerable buckling. Starting fr om a (2 x 2)-O layer with O on hcp sites, one or two NO molecules can be in corporated per unit mesh. In the (2 x 2)-(O + NO) layer, the NO sits uprigh t on the top site, with essentially v(2) parameters. In the (2 x 2)-(O + 2N O) layer, one NO is roughly identical, and the second sits on the fee site with slightly changed parameters compared to v(1) (r(e) = 1.22 Angstrom, z( e) = 1.39 Angstrom). We discuss conclusions from this systematic series of structures for the reliability of geometry determinations by quantitative L EED, and for the chemistry of these layers. Using Badger's rule we can esti mate the internal bond orders of the NO molecules. 2.0-2.2 for the v(1)(O)- NO and 2.5-2.7 for the v(2)(O)-NO species, which are in line with expectati ons from a simple frontier orbital argument. (C) 1999 Elsevier Science B,V. All rights resented.