ADSORPTION OF CO ON OXYGEN-COVERED RU(001)

The adsorption of CO on oxygen precovered Ru(001) has been studied with correlated HREELS, TPD, LEED, and work-function (DELTAphi) measurements. With increasing oxygen precoverage, the CO TPD spectrum changes due to interaction with the 0 layer. In the low oxygen precoverage regime (up to theta(O) = 0.25 ML), a vibrational CO state (nu(CO) = 2050 cm-1) adsorbed in linear configuration is observed. For a (2 X 2) oxygen precoverage the two coadsorbates are completely mixed and form an ordered (2 X 2)(CO + O) layer at saturation, with a single linear CO state. At higher oxygen coverages, a second vibrational CO state (nu(CO) = 1850 cm-1) is observed in bridge configuration. This form is characterized by desorption peaks at 200-250 K and 285-310 K, respectively, and is correlated with the coexistence of mixed CO + 0 regions with low (theta(O) = 0.25 ML) and high (theta(O) = 0.5 ML) local oxygen coverages. For theta(O) = 0.5 ML, CO saturation at 120 K leads to crowded CO in the empty space of the (2 X 1) layer, containing both bridge and linear CO; annealing to 300 K desorbs about 35% CO, and the residual CO together with the O atoms rearrange to a common mixed superlattice with honeycomb 0 and (2 X 2)CO, i.e. (2 x 2)(2O + CO). Differences between CO adsorption on ordered and disordered oxygen precoverages are observed below 300 K, where bridge CO is formed at lower oxygen coverage than for the ordered 0 layer.