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 of S/Pd(100) as well as of the p(2 x 2) structure of atomic oxygen on the same surface were determined by LEED-IV analysis using the energy range between 35 and 350 eV. The total energy range available for analysis varied between 2900 eV for the p (2 x 2) structures and 1750 eV fur 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 lateral shifts and rotations of groups of atoms in the substrate layers turn out to be below the detection limits, significant buckling of the second substrate layer was found for all three systems, being largest for oxygen (0.12 Angstrom), The unusually extended first to second layer distance found already for the clean Pd(100) surface remains virtually unchanged in all structures and is not compensated by a contraction between the second and third layers.