Surface structure of {S}/{Ni(111) }-(√3 × √3) R30° determined by back-reflection X-ray standing-wave method

The surface structure of {S}/{Ni(111) }-(√3 × √3) R30° has been studied by means of the back-reflection X-ray standing wave (SW) method. The SW profiles of the surface S atoms were measured for the (111) and (11 1̄) reflections by monitoring the S-K fluorescence yield. The results of the (111) reflection in combination with previous SEXAFS results have revealed that the surface Ni layer is vertically relaxed outward by 0.12 ± 0.05 Å. The (11 1̄) data have clarified that 80 ± 5% of the S adsorb on fcc hollow sites which are directly above the third-layer Ni atom, while the rest (20 ± 5%) occupy the hcp hollow sites above the second-layer Ni atom. These results are discussed in comparison with several previous investigations.