Coverage-dependent changes in the adsorption geometries of ordered benzene layers on Ru(0001)

With its five ordered phases of adsorbed benzene (three commensurate and two incommensurate) the Ru(0001) 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(2 root3 x 2 root3)R30 degrees (Theta (ideal) = 0.083 ML). c(2 root3 x 4)rect (Theta (ideal) = 0.125 ML), and p(root7 x root7)R19 degrees (Theta (ideal) = 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 Co ring of about 0.1 Angstrom and a small expansion of less than 0.1 Angstrom 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 Angstrom. The average vertical distance d(12) between the first and the second Ru layer decreases slightly from 2.09 to 2.07 Angstrom (+/-0.05 Angstrom) with increasing coverage whereas d(23) increases from 2.13 to 2.16 Angstrom (+/-0.07 Angstrom). 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. (C) 2001 Elsevier Science B.V. All rights reserved.