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

With its five ordered phases of adsorbed benzene (three commensurate and tw o incommensurate) the Ru(0001) surface offers the unique possibility to stu dy 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 (i deal) = 0.125 ML), and p(root7 x root7)R19 degrees (Theta (ideal) = 0.143 M L). Despite the different lateral distances and lattice symmetries, the geo metry of the adsorption complex, i.e. the benzene molecule together with th e three Ru atoms forming the hcp adsorption site underneath, is exactly the same in all three phases. We find a significant crown-like buckling distor tion of the Co ring of about 0.1 Angstrom and a small expansion of less tha n 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 ato ms 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 t he benzene coverage is seen for the Ru atoms which are not in direct contac t with the benzene molecules. (C) 2001 Elsevier Science B.V. All rights res erved.