BINDING OF RADICAL SPECIES TO SURFACES - CLUSTER-MODELS FOR OH ON CU(111)

The chemisorptive interaction of the hydroxy radical (OH) with the Cu (111) surface and its consequences for the surface electronic structure are studied by Cu(n)OH (n ranging from 1 to 25) cluster models using ab initio Hartree-Fock and correlated wave functions. A comparison of the adsorbate binding at the Cu (111) on top, bridge, and the two threefold hollow [face-centered-cubic and hexagonal-close-packed (fcc and hcp)] sites based on extended geometry optimizations yields the fcc site energetically favored with a binding energy E(B) = 3.1 eV at the configuration interaction (CI) level. Correlation contributions account for about 0.9 eV of the binding and are almost independent of the binding site and cluster size. The OH-metal interaction is characterized by considerable Cu to OH charge transfer filling the partially occupied 0 2p lone pair orbitals (OH 1pi), while OH to Cu charge transfer involving the OH 3sigma orbital is small. As a result, the adsorbate is negatively charged in the presence of the surface and ionic binding contributions become important. This makes the OH-metal binding very similar to that of other radicals such as, e.g., OCH3. The adsorbate binding character is also reflected in the results of cluster core and valence hole states corresponding to OH adsorbate ionization. These results may be compared with experimental photoionization data as they become available.