Angle-scanned photoelectron diffraction: From clean surfaces to complex adsorption systems

We exploit the capabilities of photoelectron diffraction (PED) to provide quantitative information on the local structure of the first layers of clean and adsorb ate-covered surface systems. Selected studies of low-energy PED are presented, highlighting the advantages of the angle-scanning approach. In the first experiment, we evaluate the clean Rh(110) surface layer relaxation by employing the PED of the Rh 3d(5/2) surface component. The resulting relaxation is in good accord with previous low energy electron diffraction data. In the second experiment, a system lacking long-range order is examined, namely the saturation layer formed by nitrogen monoxide on Rh(100) at 123 K. Preliminary results confirm the bridge adsorption geometry model. The last example is a chemical shift PED study of the c(4 x 2) phase of carbon monoxide on Pt(111). In this system, CO molecules are coadsorbed at two different adsorption sites, the energy separation of the respective C1s components being 0.7 eV. Structural determination has been achieved by an independent analysis of the diffraction yield originated by the two chemically shifted C1s components. The structure of Pt(111) + c(4 x 2)-2CO has been refined with an automated search of the best parameters using a modified version of MSCD (multiple scattering calculation of diffraction) code.