ADSORBATE-INDUCED RELAXATIONS OF S/RU(0001) - P(2X2) AND P(ROOT-3X-ROOT-3) R30-DEGREES STRUCTURES

LEED-IV analyses were carried out for S/Ru(0001) ordered in p(2 X 2) and p(square-root 3 X square-root 3)R30-degrees structures using 8 non-equivalent orders of diffraction and a total energy range between 1520 eV (p(square-root 3 X square-root 3)R30-degrees) and 1700 eV. Top, bridge and the 2 threefold sites (fcc and hcp site) were tested. The calculated IV-curves for the hcp site show by far the best agreement with the experimental data already for the unrelaxed Ru surface so that top, bridge and fcc sites can be safely ruled out for both structures. For the p(2 X 2) structure small, but significant relaxations (less-than-or-equal-to 0.07 angstrom) of first and second Ru surface layers in both lateral and vertical directions were found which, however, maintain the full point symmetry of the substrate surface. Especially rotations of groups of atoms within the layers were tested and found to be very unlikely. No significant buckling or lateral shifts were detectable for the p(square-root 3 X square-root 3)R30-degrees phase. Only slight contractions by 0.02 angstrom of first and second Ru layers were found. The S-Ru bond length of 2.28 angstrom is virtually identical in both structures. The determined site agrees with that found recently by us with STM and is also in agreement with the symmetry requirements of continuous order-disorder phase transitions found for these two structures. Identical geometries as for the pure phases were found in the coexistence region of the p(2 X 2) and p(square-root 3 X square-root 3)R30-degrees phases.