Configuration interaction study of the excited states of CO adsorbed on a Pt97 cluster

Abstract

A procedure is developed to obtain pseudo-canonical localized molecular orbitals for use in ab initio multireference CI calculations of the electronic spectra of molecules adsorbed on the surfaces of large clusters of metal atoms. The Pt97CO system is employed as a test and potential curves are computed for a large number of its electronic states. The present method is able to accurately describe internally excited states of the adsorbed CO molecule without computing most of the charge-transfer states of lower energy which occur in the same MRD-CI secular equations. It is found that the potentials of the σ–π* states are repulsive along the Pt–C coordinate and hence that their transition energies from the ground state are much higher (11.4–11.6 eV) than for the isolated CO molecule. The corresponding π–π* states are all relatively strongly bound, however, similarly as for the ground state, with transitions to them occurring as much as 1 eV lower than when the Pt97 cluster is absent. This behavior can be traced to the fact that the Pt–C bonding orbital in the ground state has a large amount of CO sigma character, consistent with earlier calculations reported for the Pt7CO model system, whereas neither the π nor the π* MO is strongly affected by the approach of the metal cluster.