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 pha se of S/Ru(0001) was determined by LEED-IV analysis using the energy r ange between 30 and 400 eV. Twenty non-equivalent beams with a total e nergy range of 5110 eV were available for analysis. A group of 3 S ato ms is found to be located close to nearest neighbor hcp sites, whereas the 4th atom in the unit cell occupies an fee site. Significant later al shifts of the S atoms close to the hcp sites, with respect to the i deal geometric positions on a non-relaxed substrate, include both radi al and rotational movements. They make the lateral distances between a ll S atoms almost equal (within 0.1 Angstrom). Buckling within the fir st Ru layer is strictly correlated with the coordination number of S a toms, so that the difference in z-positions between singly and triply coordinated Ru atoms amounts to 0.39 +/- 0.02 Angstrom, that between s ingly and doubly coordinated atoms to 0.18 Angstrom. Buckling within t he sulfur layer turned out to be very small (0.03 Angstrom). Surprisin gly, the buckling in the second Ru layer is already at the limit of de tectability (0.03 +/- 0.03 Angstrom). Lateral shifts within the substr ate layers turn out not to be significant. The net result of all posit ional relaxations are nearly identical bond lengths of S to its neares t neighbor substrate atoms (2.27 +/- 0.06 Angstrom). This finding toge ther with the quick disappearance of relaxations in deeper substrate l ayers gives evidence for the local nature of S bond formation on this surface.