Molecular Geometries of Acetylene and Ethylene Chemisorbed On Cu, Ni, Pd, and Pt Surfaces

Ultraviolet photoemission measurements of the valence orbital electronic structure of acetylene and ethylene chemisorbed on Cu(100) or Cu(111), Ni(111), Pd(111), and Pt(111) are presented. We compare the measured energy levels of these chemisorbed species to those of the free molecule and use a similar comparison of the relative changes in ground state energy levels of distorted free molecules calculated with a SCF-LCAO (Self Consistent Field—Linear Combination of Atomic Orbitals) method to determine the molecular geometries of these chemisorbed species. The limitations and accuracies of such an approach are discussed. From the determined geometries we identify two trends in the structure of these chemisorbed molecules on these surfaces: first, increasingly greater molecular distortions occur with increasing atomic number of the substrate atom, and secondly, greater molecular distortions occur for ethylene than for acetylene on the same metal. These trends are consistent with a π-d bonding interaction and can be accounted for by the electronic structure of the substrate and of the molecule, respectively. With the exception of ethylene on Pd or Pt, we determine molecular geometries characteristic of small rehybridization. The molecular geometry of ethylene on Pd or Pt is generally characteristic of rehybridization to an sp3configuration.