THE STRUCTURE OF DENSE SULFUR LAYERS ON RU(0001) .2. THE (ROOT-7X-ROOT-7)R19.1-DEGREES STRUCTURE

The geometrical structure of the (root 7 X root 7)R19.1 degrees-4S phase of S/Ru(0001) was determined by LEED-IV analysis using the energy range between 30 and 400 eV. Twenty non-equivalent beams with a total energy range of 5110 eV were available for analysis. A group of 3 S atoms is found to be located close to nearest neighbor hcp sites, whereas the 4th atom in the unit cell occupies an fee site. Significant lateral shifts of the S atoms close to the hcp sites, with respect to the ideal geometric positions on a non-relaxed substrate, include both radial and rotational movements. They make the lateral distances between all S atoms almost equal (within 0.1 Angstrom). Buckling within the first Ru layer is strictly correlated with the coordination number of S atoms, so that the difference in z-positions between singly and triply coordinated Ru atoms amounts to 0.39 +/- 0.02 Angstrom, that between singly and doubly coordinated atoms to 0.18 Angstrom. Buckling within the sulfur layer turned out to be very small (0.03 Angstrom). Surprisingly, the buckling in the second Ru layer is already at the limit of detectability (0.03 +/- 0.03 Angstrom). Lateral shifts within the substrate layers turn out not to be significant. The net result of all positional relaxations are nearly identical bond lengths of S to its nearest neighbor substrate atoms (2.27 +/- 0.06 Angstrom). This finding together with the quick disappearance of relaxations in deeper substrate layers gives evidence for the local nature of S bond formation on this surface.