A LEED analysis of the Rh(100)-(2 × 2)-S surface structure

The (2 x 2) structure formed by adsorbing H2S onto clean (100)Rh was analyzed; a sharp (2 x 2) LEED pattern with good contrast could be obtained by exposing cleaned (100)Rh to 10-8 torr H2S for 1 min, followed by annealing at 300 deg C for 1 min. Diffracted beam intensities at normal incidence were detremined at 2 eV intervals from 40 to 200 eV and I(E) curves were recorded for four integral-order and five fractional-order beams. I(E) curves were calculated with the layer-doubling method for various trial surface geometries in which only sulphur was present in an overlayer. A band structure potential was used for the atomic regions in the substrate and superposition potential for those in the overlayer. Three types of structure, all corresponding to a quarter monolayer, were tested: (A) each S atom on top of a single substrate atom; (B) each S atom 'bridging' two adjacent substrate atoms; and (C) each S atom adsorbed in a symmetrical hollow of four-fold symmetry. The best correspondence of the structure in calculated and experimental I(E) curves was for structure (C) with topmost Rh--S spacing 1.29 A and VOr = --13.6 eV. Min. reliability index was 0.33. This conclusion was supported by a visual analysis. 19 ref.--E.J.S.