SULFIDATION OF W(001) STUDIED BY LOW-ENERGY ION-SCATTERING

Low-energy alkali Li+ ion scattering, low-energy diffraction and Auger electron spectroscopy were used to study the structure of sulfided W( 001). The c(2 x 2)-S/W(001) structure, found at coverages less than 0. 5 ML, is determined to be an ordered overlayer of S atoms bounded in u ndistorted four-fold hollows with perpendicular height of 0.102 nm. Th e maximum second-layer rippling is less than 0.002 nm and the S overla yer relaxes the first-second W interlayer spacing to the bulk value to within 0.004 nm. For coverages exceeding that of the c(2 x 2)-S/W(001 ), W lattice vacancies are induced at the surface by annealing at and above 900 K. The number of vacancies increases with coverage reaching 0.2 ML at S saturation. The saturation structure, characterized by a p (square-root 5 x square-root 5)R26.5-degrees-S/W(001) LEED pattern is associated with 0.8 ML of S located atop second-layer W atoms. No evid ence is found for S adsorbed in deep-hollow sites resulting from W vac ancies. The results are compared to previous LEED, SXPS, work function and X-ray diffraction results from W(001) and Mo(001) surfaces.