THE STRUCTURES OF SULFUR ON PD(111) STUDIED BY X-RAY STANDING WAVE-FIELD ABSORPTION AND SURFACE EXAFS

The surface structures of (root 3 x root 3)R30 degrees-S and (root 7 x root 7)R19.1 degrees-S on Pd(lll) have been investigated by normal in cidence X-ray standing wave (NIXSW) absorption and surface extended X- ray absorption fine structure (SEXAFS). NIXSW measurements show that t he most likely site of S adsorption in the (root 3 x root 3)R30 degree s phase is the threefold ''fcc'' hollow. The location of the S atoms a t the ''fcc'' hollow site is consistent with S adsorption on the neigh bouring fee (111) transition metal surfaces. SEXAFS analysis revealed a S-Pd nearest neighbour bond distance of 2.28+/-0.04 Angstrom. The re sults for the (root 7 x root 7)R19.1 degrees phase suggest that the st ructure involves a mixed S-Pd overlayer, with the S-Pd vertical layer spacing equal to the Pd bulk (111) spacing. (C) 1998 Elsevier Science B.V. All rights reserved.