Ab initio study of hydrogen dissociation at a surface divacancy on the (001) MgO surface

An embedded-cluster Hartree-Fock approximation is adopted for simulating th e heterolytic dissociation of hydrogen at a "tub" divacancy of MgO correspo nding to the removal of two nearest neighbor ions at the (001) surface. Two stable dissociated configurations are identified, where the hydroxyl is fo rmed either at a surface four-coordinated oxygen (T alpha) or at the five-c oordinated oxygen at the bottom of the tub (T beta). In both configurations , the other atom of the molecule forms a hydride-vacancy complex (HV) near the site of the missing oxygen. The results are discussed with reference to a previous investigation concerning the isolated anion vacancy, where the dissociated configuration (F alpha) was similar to T alpha. The dissociatio n process in the three cases is shown to critically depend on the electrost atic field at the defect and on steric hindrance constraints. T alpha and T beta are stable with respect to the undissociated molecule by a few kcal/m ol, but only the latter can be reached with low activation energy; F alpha is thermodynamically unstable. The dissociation of the HV complex in the pr esence of ultraviolet radiation is next studied, resulting in the removal o f a neutral H atom, while a lone electron remains trapped at the vacancy. T his paramagnetic state is characterized, and its features compared with tho se obtained from recent experiments concerning the F-s(H) color center. On the whole, only the T beta model is in fair agreement with experimental evi dence concerning that center, although definite discrepancies are left. (C) 1999 American Institute of Physics. [S0021-9606(99)30445-1].