THE GEOMETRY OF ORDERED BENZENE LAYERS ON RU(001)

We have measured LEED-IV-curves for all known commensurate benzene str uctures on Ru(001), carefully excluding contamination and radiation da mage 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 com pared 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 de viation from planarity which leads to good adjustment to the threefold symmetry of the site. We argue that this site selection and slight di stortion of the molecule might be connected selfconsistently with the decreased pi-character of the chemisorbed molecule. Very small deviati ons from the gas phase value of the C-C bond length and of the distanc es of the C-atoms from the molecular center are found which are not ou tside the error bars, as is a possible small rotation of the ring in r esponse to the neighbouring molecules. The main adsorbate induced modi fication of the substrate is a very large almost uniform contraction o f the distance between the first and the second Ru layer of 0.05 Angst rom compared to the clean surface which is the largest found on this s urface so far. The very similar experimental IV-curves of the other or dered benzene structures on this surface suggest that they have basica lly the same local geometries.