INTERACTION OF S WITH W(001)

Sulfur chemisorbed on W(001) has been studied by soft X-ray and ultraviolet photoelectron spectroscopy (UPS), Auger electron spectroscopy and work function measurements. Two distinct S 2p photoemission states have been identified as a function of sulfur coverage and sample annealing temperature. Below 0.5 ML, virtually all of the sulfur exists in a single state. Above 0.5 ML, a second state emerges at a lower binding energy. At 1 ML of sulfur, the two states have comparable intensity if the sample is annealed to 600 K. However, if the sample is annealed to 900 K nearly all of the sulfur resides in the low binding energy state. The high and low binding energy states have been associated with sulfur adsorbed in hollow and bridge sites, respectively. Possible reasons for the change in binding energy for sulfur adsorbed in the two different sites include an initial state (charge transfer) effect and a change in reference potential due to a change in height above the surface in the two types of adsorption site. The UPS, work function change and S(LMM) Auger lineshape results all indicate a change in S-W bonding at sulfur coverages above 0.5 ML.