CLUSTER MODEL OF O-2(-) ADSORPTION ON REGULAR AND DEFECT SITES AND F-S CENTERS OF THE MGO(100) SURFACE

We have investigated the interaction of O-2(-) with Mg2+ ions at terra ces, edges and corners and with the F-s centers of the MgO (100) surfa ce by means of ab initio cluster model wave functions. While O-2 is un bound or weakly bound at surface cations, O-2(-) is electrostatically bound with adsorption energies from 2 to 4 eV depending on the local c oordination. O-2 interacts with the electrons trapped at the F-s cente rs to form O-2(-) and F-s(+). In a similar way, O-2 becomes O-2(-) on F-s(+) centers with the formation of F-s(2+)-O-2(-) surface complexes. The high cost for the ionization of the F-s centers is compensated by the electrostatic attraction between O-2(-) and the charged vacancies . The effect of the local electric iield on the g-tensor is discussed.