Geometry of (22)S/Cu(001) determined with use of angle-resolved-photoemission extended fine structure

We have measured the adsorption geometry for the (2×{}2)S/Cu(001) surface, including the reconstruction and relaxation of the copper substrate. Multiple-scattering spherical-wave calculations were compared with angle-resolved-photoemission extended-fine-structure data and these results were then compared with a Fourier analysis of the data. The sulfur atoms are located in the fourfold hollow site, with a S--Cu bond distance of 2.26(1) A{}. With (1/4) monolayer ``p(2×{}2)'' sulfur coverage the symmetry of the copper substrate is necessarily lowered from (1×{}1) to (2×{}2). Accordingly, the sulfur adsorption induces reconstruction and relaxation of the two topmost Cu layers. All the movement occurs in Cu₅S moieties, as if the surface were forming Cu₅S clusters embedded in the bulk. The first-layer Cu atoms are shifted {}0.06 A{} towards the third Cu layer and 0.05(2) A{} laterally towards S. The second-layer Cu atoms under S atoms mimic the surface texture, in that they are 0.13 A{} farther from the third layer than the bulk spacing would predict. Second-layer Cu atoms under fourfold open sites are close to positions predicted by the bulk spacing. Details of the data analysis and extraction of surface-structural parameters are presented.