Cluster models of O2− adsorption on regular and defect sites and Fs centers of the MgO (100) surface

We have investigated the interaction of O2− with Mg2+ ions at terraces, edges and corners and with the Fs centers of the MgO (100) surface by means of ab i3itio cluster model wavefunctions. While O2 is unbound or weakly bound at surface cations, O2− is electrostatically bound with adsorption energies from 2 to 4 eV depending on the local coordination. O2 interacts with the electrons trapped at the Fs centers to form O2− and Fs+. In a similar way, O2 becomes O2− on Fs+ centers with the formation of Fs2+O2− surface complexes. The high cost for the ionization of the Fs centers is compensated by the electrostatic attraction between O2− and the charged vacancies. The effect of the local electric field on the g-tensor is discussed.