THE LOCAL GEOMETRY OF CHALCOGEN ATOMS ON PD(100) - THE LOW-COVERAGE PHASES OF O AND S

The local geometrical structures of the p(2 x 2) and c(2 x 2) phases o f S/Pd(100) as well as of the p(2 x 2) structure of atomic oxygen on t he same surface were determined by LEED-IV analysis using the energy r ange between 35 and 350 eV. The total energy range available for analy sis varied between 2900 eV for the p (2 x 2) structures and 1750 eV fu r the c(2 x 2) structure. Pendry R-factors between 0.15 and 0.19 were obtained for the best fit geometries. Ln all cases the adsorbate atoms were found to stay above the first substrate layer and to occupy the fourfold coordinated hollow site. Bond lengths to the first substrate neighbors are close to the sum of covalent radii for both S structures , but significantly longer (by 0.10 Angstrom) for oxygen. Whereas late ral shifts and rotations of groups of atoms in the substrate layers tu rn out to be below the detection limits, significant buckling of the s econd substrate layer was found for all three systems, being largest f or oxygen (0.12 Angstrom), The unusually extended first to second laye r distance found already for the clean Pd(100) surface remains virtual ly unchanged in all structures and is not compensated by a contraction between the second and third layers.