AN STM, AES AND LEED STUDY OF THE SEGREGATED SULFUR ON PD(111)

At elevated temperatures, 700 K, sulfur segregates to the Pd(111) surf ace. The coverages as measured by Auger electron spectroscopy (AES) ar e in the range of 0.1 to 0.25 monolayers. The LEED pattern represents a clear (root 7 x root 7) R 19.1-S degrees structure. With the scannin g tunneling microscope (STM) the root 7 structure is found on parts of the surface only. The position of the adsorbed S atoms is the fee thr eefold hollow site. Beside the root 7 structure, part of the surface i s covered by a disordered layer of sulfur. A sizeable part of the surf ace is covered by sulfur in a (2 x 2) structure. The adsorbate in that structure is mesoscopically ordered in triangular 2D clusters with a rather narrow size distribution. The triangles contain between 15 to 6 6 sulfur atoms. The [110] edges of the Pd(111) surface are 'decorated' with sulfur, also in a (2 x 2) structure. These structures run parall el to the steps with a width of 4 to 5 atoms.