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Abstract

The local adsorption geometries of the (2×2)-1O, (2×2)-2O, (2×2)-(O+CO) and (2×2)-(O+2CO) phases on the Rh(111) surface have been investigated by analysing low-energy electron diffraction (LEED) intensity data. In all cases, the oxygen atoms were found to occupy the threefold fcc site and the topmost layer spacing d₁₂ of Rh(111) was found to be expanded by about 3%. Additional experiments with disordered oxygen overlayers prepared at low adsorption temperatures show a monotonic increase of d₁₂ with oxygen coverage. An O coverage of ϑo≈0.1 already lifts the contraction of 0.02 Å of the clean surface. This finding is remarkable as for Ru(0001) a substantial expansion of the first layer spacing (≈3%) is observed only at very high O coverages, e.g. for the (1×1)-O structure. For the high-coverage Rh(111)-(2×2)-2O phase a honeycomb arrangement of the O atoms can clearly be ruled out. The (2×2)-2O has rather to be regarded as a (2×1)-1O phase. In the mixed (2×2)-(O+CO) overlayer the CO molecule occupies the on-top position which is also expected from a reduced back-donation of (substrate) electron charge density due to coadsorbed O atoms. The presence of CO molecules weakens the Rh-O bond. Altogether, the structural findings nicely reflect the competition of CO and O for electron charge density at the Rh(111) surface. In the (2×2)-(O+2CO) phase 60% of available hcp sites and 100% of available on-top sites in the (2×2) unit cell are occupied.