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 (Δø) measurements. With increasing oxygen precoverage, the CO TPD spectrum changes due to interaction with the O layer. In the low oxygen precoverage regime (up to θ_O=0.25 ML), a vibrational CO state (v(CO)=2050 cm^-1) adsorbed in linear configuration is observed. For a (2 × 2) oxygen precoverage the two coadsorbates are completely mixed and form an ordered (2 × 2)(CO + O) layer at saturation, with a single linear CO state. At higher oxygen coverages, a second vibrational CO state (v(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 + O regions with low (θ_O=0.25 ML) and high (θ_O=0.5 ML) local oxygen coverages. For θ_O=0.5 ML, CO saturation at 120 K leads to crowded CO in the empty space of the (2 × 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 O and (2 × 2)CO, i.e. (2 × 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 O layer.