CO ADSORPTION ON UNRECONSTRUCTED AND RECONSTRUCTED RH(110) SURFACES - LEED AND XPS STUDIES

The adsorption of CO on the (1 x 1) unreconstructed and (1 x 2) reconstructed Rh(110) surfaces has been studied by means of XPS and LEED. The O 1s spectra of CO were used for measuring the coverage and determining the binding sites of the adsorbed molecules. It has been found that on both surfaces CO adsorption occurs in two bonding configurations characterized by O 1s binding energies at 530.8 and 531.9 eV. They are assigned to occupation of bridge and on-top sites, respectively. On the (1 x 1) surface initially the on-top sites are occupied yielding an ordered c(2 x 2) layer at a coverage of 0.5 ML. At higher coverage CO changes its bonding configuration and a (2 x 1)p2mg structure is formed near saturation when the coverage approaches 1 ML. It is suggested that the (2 x 1)p2mg structure consists of zig-zag chains of bridging tilted CO. When CO adsorbs on the (1 x 2) surface simultaneous occupation of on-top and bridge sites is observed. Molecules in both bonding configurations contribute to the observed (2 x 2), c(2 x 4) and (2 x 2)p2mg structures formed on the (1 x 2) surface. The intensities of the CO O 1s spectra indicate that the amount of CO adsorbed on the (1 x 1) and (1 x 2) surfaces is the same. Comparable amounts of CO adsorbed on the [110BAR] rows and (111) microfacets of the (1 x 2) surface are suggested in order to explain the observed (2 x 2), c(2 x 4) and (2 x 2)p2mg structures.