Ethylene adsorbed on Ni(110): An experimental and theoretical determination of the two-dimensional band structure

We have investigated the saturated ethylene layer on Ni(110) by low-energy electron diffraction (LEED), angle-resolved ultraviolet photoemission spectroscopy (ARUPS), and near-edge x-ray-absorption fine structure (NEXAFS). This layer exhibits a c(2×4) LEED pattern that corresponds to a structure containing two adsorbates per primitive unit cell. The ethylene molecules are adsorbed with the molecular plane parallel to the surface and the C-C axis preferentially aligned along the [11¯0] direction of the substrate, as is independently determined from the ARUPS and NEXAFS experiments. The two-dimensional (2D) adsorbate band structure is determined from the ARUPS spectra at various photon energies. Except for the π orbital, all ethylene-derived bands show significant dispersion (up to 2 eV), but no splitting as would be expected for a structure with two molecules per unit cell. The experimentally determined band structure is reproduced in all details by extended-Hückel-theory calculations for an unsupported ethylene layer. The structural model derived from LEED, ARUPS, and NEXAFS is confirmed both by force field and by the 2D band-structure calculations. This indicates that the adsorbate-adsorbate interactions are essentially decoupled from the adsorbate-substrate interaction, that is responsible for the chemisorption bond.