Atomic arrangement in thec(2×2)structure of Cl on Ag(100)

A LEED (low-energy electron diffraction) structure analysis is described of the $c(2\mathrm{}\times 2)$ superstructure obtained by reacting a clean Ag(100) surface with Cl. Two of the structural models considered in the analysis produce encouraging agreement between theory and experiment for some LEED beams: the overlayer model with Cl adsorbed on the fourfold symmetrical hollows of the Ag(100) surface in the $c(2\mathrm{}\times 2)$ arrangement and the mixed-layer model with non-coplanar Ag and Cl atoms. The full analysis of 21 beams at three different angles of incidence and one azimuth, however, leads to rejection of the mixed-layer model and adoption of the simple overlayer model. Refinement of the structure with regard to several nonstructural parameters reveals an interference between the choice of the potential for the adsorbed Cl and the value of the distance $d_z^S$ between overlayer and first substrate layer. For the proposed structure, $d_z^S=1.72\mathrm{}$ Å, corresponding to a hardsphere Cl radius of 1.23 Å, but range of acceptable $d_z^S$ values extends from 1.57 to 1.78 Å, depending on the Cl potential used in the intensity calculations.