O − radical anions on polycrystalline MgO

Abstract

O − species obtained as hole centers, generated via X-ray irradiation of polycrystalline MgO, O 2− ( s) + hν→ O − ( s) +( e −) trapped are compared to the same chemical species generated via bleaching of the surface colour centers with N 2O, ( e −) trapped + N 2 O ( g) → O − ( s) + N 2( g) The electron paramagnetic resonance (EPR) spectrum of the former species presents a single, well defined feature which corresponds to one of the several species present in the complex O − spectrum, obtained by reaction with N 2O. The O − and the trapped electron, formed upon X-ray irradiation, decay simultaneously indicating that a recombination process takes place and that they are not too far apart. EPR parameters have been determined by quantum-mechanical calculations for a variety of possible electron rich sites on which an oxygen atom adsorbs. It is found that both oxygen vacancies and morphological sites on the MgO surface fulfill the spectroscopic characteristics of the electron trapping sites probed by the O − radical ion. A new model for a paramagnetic F s +(H) colour center has been proposed as a possible morphological electron trap where an oxygen atom is adsorbed. Thermochemical measurements for the reaction of (e −) trapped with N 2O give an average reaction enthalpy of 450±50 kJ mol −1, which is in reasonable agreement with the computed values.