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Generation of O Radical Anions on MgO Surface: Long-Distance Charge Separation or Homolytic Dissociation of Chemisorbed Water?

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journal contribution
posted on 18.06.2009, 00:00 by Sergei E. Malykhin, Alexander M. Volodin, Alexander F. Bedilo, George M. Zhidomirov
O radical anions were observed over a partially hydroxylated MgO surface after illumination by monochromatic light with λ = 280 and 303 nm. As all corner oxygen atoms are covered with adsorbed hydroxyl groups after activation at 450 °C, this process is initiated by selective excitation of 3-coordinated complexes [Mg2+−O2−]3C containing chemisorbed water. A new mechanism for generation of the O radical anions is suggested. It is based on homolytic dissociation of chemisorbed water followed by migration of a hydrogen atom, probably, to a different nanoparticle rather than on long-distance separation of charges. The surface structure remaining after its detachment consists of an OH radical stabilized near the corner oxygen atom. The imminent charge transfer generates a hole at the corner oxygen atom stabilized by a hydroxyl group [OH···Mg2+−O]3C. DFT simulation of this structure showed that it reproduces the main characteristics of the radical anions O3C. The overall structure is electrically neutral. The charge of the hole is compensated by the nearby hydroxyl group. So, no long-distance charge separation is required.