A COMBINED EPR AND QUANTUM-CHEMICAL APPROACH TO THE STRUCTURE OF SURFACE F-S(+)(H) CENTERS ON MGO

F-s(+)(H) color centers at the surface of MgO have been studied using a combined EPR and quantum chemical approach. F-s(+)(H) are paramagnet ic excess electrons centers where the unpaired electron is trapped in a surface anion vacancy. They are formed at the surface of thoroughly dehydrated MgO (1073K) upon UV irradiation under hydrogen in parallel with the formation of minor fractions of different color centers. The whole EPR spectrum resulting from irradiation has been analyzed by sim ulation of the experimental profile. F-s(+)(H) centers are characteriz ed by an axial g tensor and display a hyperfine interaction with a hyd rogen nucleus (belonging to an hydroxyl group stabilized nearby the va cancy) and two distinct families of Mg-25 nuclei characterized by a la rge (10.5 G) and a small (0.7 G) hyperfine coupling constant, respecti vely. Both EPR and ab initio calculations on clusters of ions converge in indicating that the features of the centers are due to the polariz ation of the electron density by the positive charge of the hydrogen i n the OH group toward two (Or possibly three) equivalent magnesium ion s of the vacancy close to the OH group itself. The formation mechanism of the centers is strictly analogous to that occurring upon contact o f low ionization energy metals with the surface of MgO leading to the formation of another type of color centers. Partially hydrated MgO sam ples also give rise to another family of paramagnetic center based on electrons trapped in anion vacancies. This finding indicates that the structure of the partially hydroxylated oxide and the mechanism of its dehydration are still open questions.