Angle-resolved-photoemission extended-fine-structure spectroscopy investigation of c(2×2)S/Ni(011)

Measurements of the extended fine structure in the photoemission intensity from the S(1s) core level were performed for a c(2 x 2) overlayer of sulfur on Ni(011). Four experimental geometries were employed, making this the most complete angle-resolved-photoemission extended-fine-structure (ARPEFS) study to date. Surface structural information was extracted from the ARPEFS using a combination of Fourier-transform techniques and comparisons to multiple-scattering calculations. The results of this analysis are in excellent agreement with previous studies of this system, indicating that S adsorbs in a rectangular hollow site 2.20 plus/minus 0.02 A above a second-layer Ni atom. Evidence is presented for a buckling of the second Ni layer, giving an expansion in the separation between the first Ni layer and the second-layer Ni atoms covered by S atoms of 11% from the bulk value, while second-layer Ni atoms not covered in the c(2 x 2) structure assume essentially bulk positions. Also examined in detail are the effects of multiple scattering on the extraction of this structural information from ARPEFS and results for surfaces with S coverages > 0.5 monolayer are presented. 33 ref.--AA