Single electron traps at the surface of polycrystalline MgO: Assignment of the main trapping sites

Paramagnetic centers at the surface of ionic oxides in the form of trapped electrons can be generated by exposure of the material to alkali metal or hydrogen atoms or of molecular hydrogen under UV irradiation. For many years, it has been assumed that the resulting paramagnetic centers consist of oxygen vacancies filled by one electron. High-resolution electron spin resonance spectra and ab initio quantum chemical calculations show that the paramagnetic centers consist of (H+)(e(-)) electron pairs formed at morphological irregularities of the surface. At least three different kinds of (H+)(e(-)) centers, [A], [B], and [C], have been identified with abundances of 80%, 10%, and 8%, respectively. In this work, we compare a wide set of measured and computed g-factors and hyperfine coupling constants of the unpaired electron with the surrounding Mg-25, O-17, and H-1 nuclei and we propose a general assignment of the centers. (H+)(e(-)) pairs formed at Mg-4c, ions at steps and edges account for species [A], centers formed at Mg-4c ions at reverse corners correspond to species [B], and species [C] originates from (H+)(e-) pairs formed at Mg-3c ions at corners and kinks.