THEORETICAL-STUDIES OF ADSORBATE BINDING AND IONIZATION OF METHOXY ON THE CU(111) SURFACE

Model studies on small CunOCH3, n = 1, 2, 3 clusters are used to determine the detailed chemisorptive interaction of the methoxy (OCH3) radical with the Cu(111) surface as well as surface-induced shifts of the adsorbate valence ionization potentials. The calculations are based on the ab initio Hartree-Fock-LCAO method using all-electron Gaussian basis sets. The model results from energy optimizations for various lateral adsorption sites are consistent with experimental findings which suggest that OCH3 stabilizes at a 3-fold central site on the Cu(111) surface. The OCH3-metal interaction is characterized by considerable Cu to OCH, charge transfer filling the OCH3 2e lone pair orbitals while OCH3 to Cu charge transfer involving the OCH3 5a1 orbital is small. As a result, the adsorbate becomes negatively charged in the presence of the surface. The calculated adsorbate binding energies (E(B) = 1.7 eV for Cu3OCH3 with OCH3 at the central and bridge site, E(B) = 1.3 eV at the on top site) are found to be rather large containing ionic and covalent contributions of about the same magnitude. Calculations on valence hole states of Cu3OCH3 corresponding to ionization of OCH3-type cluster orbitals are performed. The results show differential shifts of the OCH3 5a1 and 2e derived ionization potentials (IP) compared to the free radical values which are explained by the electrostatic adsorbate-substrate interaction and the charge distributions of the orbitals. The reduction of the 5a1-1e IP separation due to the chemisorptive interaction is consistent with experimental data from photoemission on the OCH3/Cu(111) adsorbate system.