The electronic structure of ethylene on Ni(110): an experimental and theoretical study

The electronic structure of ethylene adsorbed on Ni(110) has been studied by ARUPS using linearly polarized synchrotron radiation, and by LCGTO-LDF model cluster calculations. The adsorption system has also been characterized by TPD and LEED. The ARUPS measurement were performed for a dilute ethylene layer (θ ≈ 1 2θ_SAT), where lateral interactions are not important. From the detailed analysis of the polarization, polar angle and azimuthal dependence of the ARUPS spectra we deduce an orientation of the ethylene molecules with the molecular plane coplanar to the surface and the C-C axis preferentially aligned along the [110] azimuth. The symmetry of the adsorption complex is determined as C₁. The ethylene π-orbital 1b_2u exhibits a differential shift of 1.1 eV to higher binding energy as compared to the free molecule. The corresponding theoretical value is found to be ∼ 1.7 eV. In the cluster calculations a partial optimization of the adsorbate geometry has been carried out assuming C₂ symmetry. The adsorption bonding is found to be quite similar in π and di-σ coordination, for both cases the π donation to the substrate being stronger than the π^* backdonation. The π donation is mediated mainly by Nis and p contributions, gp^* backdonation mostly by Ni d orbitals. The optimized geometry parameters for the π-bonded species are: CC: 1.42 Å: NiC: 2.01 Å tilting of a CH₂ group relative to the (110) crystal plane: 23°.