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

We have investigated the interaction of O~- with Mg 2+ ions at terraces, edges and comers and with the Fs centers of the MgO (100) surface by means of ab initio cluster model wavefunctions. While O 2 is unbound or weakly bound at surface cations, O~- is electrostatically bound with adsorption energies from 2 to 4 eV depending on the local coordination. 02 interacts with the electrons trapped at the F s centers to form 02 and Fs +. In a similar way, 02 becomes 02 on F + centers with the formation of F~ ÷ -02 surface complexes. The high cost for the ionization of the F~ centers is compensated by the electrostatic attraction between O~- and the charged vacancies. The effect of the local electric field on the g-tensor is discussed.