Embedded-cluster study of hydrogen interaction with an oxygen vacancy at the magnesium oxide surface

An embedded-cluster Hartree-Fock approximation is adopted for simulating th e formation of F-s(H) color centers at the (001) surface of magnesium oxide . This process is assumed to take place in two steps at an isolated surface anion vacancy: first, a hydrogen molecule is adsorbed dissociatively at th e defect; second, following UV irradiation, a neutral hydrogen atom is remo ved and an electron remains trapped at the vacancy with a hydroxyl group ne arby. According to the present calculations, the activation energy for the dissociation is appreciable (about 25 kcal/mol) and the products (a proton bound to a low-coordinated oxygen and a hydride ion above the vacancy) are considerably less stable than the reactants. The excitation of the adsorbed species owing to the UV irradiation is simulated by considering a singlet- triplet transition of the hydride-vacancy complex, which then dissociates i nto an H atom and a trapped lone electron. The electronic structure and the EPR parameters of the resulting paramagnetic state are explored. The theor etical results agree in many respects with the experimental data as concern s one of the forms of heterolitically dissociated hydrogen which are found at the defective MgO surface. However, from the viewpoint of the energetics , this model is untenable because that species is known to form irreversibl y at room temperature with low activation energy.