Molecular orientation of CO adsorbed on clean and Al-modified Ru(0 0 0 1) surfaces

Chemisorption of CO on the bare and Al precovered Ru(0 0 0 1) surface as well as on the Al2Ru surface alloy, was studied by means of angle-resolved photoelectron spectroscopy using unpolarised He II radiation. Whereas CO molecules stand upright on the clean Ru(0 0 0 1) surface, as deduced by applying the symmetry selection rules of photoemission, they adsorb in a tilted geometry on the Al-precovered Ru(0 0 0 1) surface. On an Al2Ru surface alloy, produced by annealing the Al-covered Ru(0 0 0 1) surface up to 1100 K, the CO molecules are again found to stand upright, as on the clean Ru(0 0 0 1) surface. The Al atoms, incorporated into the topmost Ru layer, do not have a significant influence on the orientation of the axis of the CO molecules.