Energy transfer in oxides. Part 1.—Adsorption of oxygen on magnesium oxide during γ-irradiation

Abstract

The γ-induced adsorption of oxygen on MgO has been shown to involve an energy transfer mechanism between the bulk and surface of each crystallite. In pure MgO, ionization events which occur within a subsurface region of effective depth ∼90 Å release electrons for the adsorption of oxygen as O^–₂ at the surface. In manganese-doped MgO this effective depth is less and the positive holes are trapped at Mn²⁺ ions which lie within 40 Å of the surface. The initial G_T values for oxygen adsorption thus depend on particle size and impurity content and are in the range 1–10. The adsorbed O^–₂ ion has been identified by electron spin resonance and this species has been shown to possess an optical absorption in the reflectance spectrum at 2.9 eV, which is possibly due to the ²Π_g–²Δ_u transition. A combination of e.s.r., reflectance and desorption studies during annealing indicates the decrease in the stability of the O^–₂ ion with adsorbed coverage.