Surface-structure determination using anomalous x-ray scattering: Underpotential deposition of copper on Pt(111)

Using anomalous x-ray-diffraction methods we have investigated the underpotential deposition of Cu on Pt(111) in the presence of Cl and Br. The initial adsorption of Cu results in an ordered structure with an incommensurate hexagonal unit cell. Changes in the in-plane scattering intensities at x-ray energies close to the Cu K edge indicate that the absorbed layer must include both Cu and the halide. The data are described by a bilayer structure very similar to the (111) planes of CuCl.