A Combined EPR and Quantum Chemical Approach to the Structure of Surface Fs+(H) Centers on MgO
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Abstract
Fs+(H) color centers at the surface of MgO have been studied using a combined EPR and quantum chemical approach. Fs+(H) are paramagnetic 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 simulation of the experimental profile. Fs+(H) centers are characterized by an axial g tensor and display a hyperfine interaction with a hydrogen nucleus (belonging to an hydroxyl group stabilized nearby the vacancy) and two distinct families of 25Mg nuclei characterized by a large (10.5 G) and a small (0.7 G) hyperfine coupling constant, respectively. Both EPR and ab initio calculations on clusters of ions converge in indicating that the features of the centers are due to the polarization of the electron density by the positive charge of the hydrogen in the OH group toward two (or possibly three) equivalent magnesium ions of the vacancy close to the OH group itself. The formation mechanism of the centers is strictly analogous to that occurring upon contact of low ionization energy metals with the surface of MgO leading to the formation of another type of color centers. Partially hydrated MgO samples 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.
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