title: Single electron traps at the surface of polycrystalline MgO: assignment of the main trapping sites.
abstract: 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 (25)Mg, (17)O, and (1)H 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.
authors.affiliation: Dipartimento di Chimica IFM, Università di Torino, via P. Giuria 7, 10125 Torino, Italy.
authors.forename: Mario
authors.initials: M
authors.lastname: Chiesa
authors.forename: Maria Cristina
authors.initials: MC
authors.lastname: Paganini
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Spoto
authors.forename: Elio
authors.initials: E
authors.lastname: Giamello
authors.forename: Cristiana
authors.initials: C
authors.lastname: Di Valentin
authors.forename: Annalisa
authors.initials: A
authors.lastname: Del Vitto
authors.forename: Gianfranco
authors.initials: G
authors.lastname: Pacchioni