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) a nd 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, br idge and the 2 threefold sites (fcc and hcp site) were tested. The cal culated IV-curves for the hcp site show by far the best agreement with the experimental data already for the unrelaxed Ru surface so that to p, bridge and fcc sites can be safely ruled out for both structures. F or the p(2 X 2) structure small, but significant relaxations (less-tha n-or-equal-to 0.07 angstrom) of first and second Ru surface layers in both lateral and vertical directions were found which, however, mainta in the full point symmetry of the substrate surface. Especially rotati ons of groups of atoms within the layers were tested and found to be v ery unlikely. No significant buckling or lateral shifts were detectabl e for the p(square-root 3 X square-root 3)R30-degrees phase. Only slig ht contractions by 0.02 angstrom of first and second Ru layers were fo und. The S-Ru bond length of 2.28 angstrom is virtually identical in b oth structures. The determined site agrees with that found recently by us with STM and is also in agreement with the symmetry requirements o f continuous order-disorder phase transitions found for these two stru ctures. 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.