Electronic Structure and Reactivity of the FS(H)+ Defect Center at the MgO (001) Surface

F S (H) + centers (oxygen vacancies with a trapped electron and a nearby hydroxyl group) at the (001) surface of MgO have been characterized in terms of electronic structure, magnetic properties, and chemical reactivity toward adsorption of O 2 and N 2 molecules by means of cluster model density functional calculations. Two different arrangements of the OH group on the surface have been considered: the computed properties are in excellent agreement with observed EPR and IR data and show the existence of important analogies between the two arrangements (e.g., a similar polarization of the spin density), but also of some significant differences (e.g., for one kind of F S (H) +, a special interaction between the OH group and the trapped electron has been detected). At short distances from the surface, a transfer of the trapped electron from the surface to the admolecules occurs, with formation of charge-transfer complexes F S (H) 2+ /N 2 - or F S (H) 2+ /O 2 - . While F S (H) 2+ /O 2 - is stable even at room temperature with respect to dissociation into F S (H) + + O 2 fragments, and F S (H) 2+ /N 2 - is a metastable surface complex.