Coverage-dependent changes in the adsorption geometries of ordered benzene layers on Ru(0 0 0 1)

With its five ordered phases of adsorbed benzene (three commensurate and two incommensurate) the Ru(0 0 0 1) surface offers the unique possibility to study the local adsorption geometry of this molecule on a metal surface over a wide coverage range. We have performed low energy electron diffraction I–V structure analyses of all three commensurate superstructures p(23×23)R30° (Θideal=0.083 ML), c(23×4)rect (Θideal=0.125 ML), and p(7×7)R19° (Θideal=0.143 ML). Despite the different lateral distances and lattice symmetries, the geometry of the adsorption complex, i.e. the benzene molecule together with the three Ru atoms forming the hcp adsorption site underneath, is exactly the same in all three phases. We find a significant crown-like buckling distortion of the C6 ring of about 0.1 Å and a small expansion of less than 0.1 Å which is within the error bar of the structure determination. The vertical distance between the carbon atoms and the first layer Ru atoms is 2.11 Å. The average vertical distance d12 between the first and the second Ru layer decreases slightly from 2.09 to 2.07 Å (±0.05 Å) with increasing coverage whereas d23 increases from 2.13 to 2.16 Å (±0.07 Å). The largest influence of variations in the benzene coverage is seen for the Ru atoms which are not in direct contact with the benzene molecules.