CO bonding and vibrational modes on a perfect MgO(001) surface: LCGTO-LDF model cluster investigation

Abstract

The adsorption of isolated CO molecules on a perfect MgO(001) surface is investigated theoretically by the LCGTO-LDF cluster method. Internuclear distances, adsorption energies as well as frequencies and absolute intensities of the MgOCO and CO vibrational modes are calculated for a number of MgO/CO and MgO/OC model clusters containing 2–18 substrate atoms and a large array of surrounding point charges. The convergence of the computed observables with cluster size is discussed. The LDF results are compared to those of previous HF studies. MgOCO adsorption is found to be mainly due to the electrostatic interaction of the CO molecule with the Madelung field on the surface. A small charge transfer of about 0.1 au from the CO 5σ orbital to the substrate also takes place and, besides Pauli repulsion, contributes to the overall blue shift of the CO vibrational frequency. The cluster models predict an approximate doubling of the CO vibrational intensity upon adsorption. This intensity enhancement derives to a large extent from a change of the π component of the CO dynamical dipole moment due to Pauli repulsion between the adsorbate at the cation atop position and the nearest neighbour surface anions. The calculated change of the CO vibrational intensity is at variance with that obtained in a previous analysis of the observed coverage dependence of the CO frequency. The MgOCO vibrational mode is calculated to have a frequency lower than 200 cm −1 and an absolute intensity of about 0.5 km/mol.