THE GEOMETRY OF ORDERED BENZENE LAYERS ON RU(001)

We have measured LEED-IV-curves for all known commensurate benzene structures on Ru(001), carefully excluding contamination and radiation damage effects, and have carried out a detailed analysis of the p(root 7 x root 7)R19.2 degrees-structure. We find that the molecule is bound on the hcp-site in C-3v(sigma(v)) orientation, at an average distance from the metal of 2.12 Angstrom. The distortion of the molecule as compared to its free state is very small: the molecular radius is almost identical to that in the gas phase, but there is a small crown-like deviation from planarity which leads to good adjustment to the threefold symmetry of the site. We argue that this site selection and slight distortion of the molecule might be connected selfconsistently with the decreased pi-character of the chemisorbed molecule. Very small deviations from the gas phase value of the C-C bond length and of the distances of the C-atoms from the molecular center are found which are not outside the error bars, as is a possible small rotation of the ring in response to the neighbouring molecules. The main adsorbate induced modification of the substrate is a very large almost uniform contraction of the distance between the first and the second Ru layer of 0.05 Angstrom compared to the clean surface which is the largest found on this surface so far. The very similar experimental IV-curves of the other ordered benzene structures on this surface suggest that they have basically the same local geometries.