STRUCTURAL REARRANGEMENT BY COADSORPTION - A LEED-IV DETERMINATION OFTHE RU(001)-P(2X2)(2O+CO) STRUCTURE

This LEED IV analysis of the coadsorbate structure consisting of 0.5 m onolayers (ML) of O and about half as much CO corroborates prior sugge stions that the coadsorption of CO restructures the (2 x 1)O structure , with all O in hcp sites, into a honeycomb structure of the O atoms w hich now occupy both fcc and hcp sites; the CO molecule is adsorbed on top of the Ru atom in the center of the O hexagon. Any row structure for the O atoms in the coadsorbate layer can be clearly excluded, even though one starts from the (2 x 1)O row structure and adsorption of C O proceeds at low temperature. The intramolecular distance d(C-O) = 1. 15 +/- 0.04 angstrom is essentially unaltered by adsorption, and the b ond length to the underlying Ru atom is d(C-Ru) = 1.98 +/- 0.08 angstr om. The bond lengths found for the two different oxygen atoms in the u nit cell are identical at d(O(fcc)-Ru) = d(O(hcp)-Ru) = 2.05 +/- 0.16 angstrom; they are similar to the bond lengths of pure oxygen adsorbed on Ru(001). We find buckling of the first substrate layer, which lead s to a partial expansion of the first layer distance of the substrate: All Ru atoms within this layer (with the exception of those underneat h the CO molecules) are shifted outwards by 0.02 +/- 0.02 angstrom wit h respect to their bulk positions; in contrast, a contraction of the f irst layer distance of -2% has been found earlier for the clean surfac e. Additionally, the Ru atoms around the fcc site are shifted towards this site by an amount of 0.08 +/- 0.08 angstrom; there is no change o f the second layer. The structure analysis was performed by convention al search procedures and by Tensor LEED. A comparison of the time requ irements of these two procedures yielded an advantage of factor 6 for Tensor LEED.