Oxygen species adsorbed on ultraviolet-irradiated magnesium oxide

Abstract

The adsorption of oxygen, under u.v. irradiation, on the surface of MgO outgassed at 1123 K has been studied at 77 K by temperature-programmed desorption and e.s.r. spectroscopy. During adsorption ozonide and superoxide ions were formed in equal amounts. The formation of these oxygen species is explained by the reaction of oxygen molecules with a short-lived exciton produced by the absorption of a photon at a surface O^2– species in a very low coordination state. Thermal decomposition of the oxygen species adsorbed at 77 K has also been studied under a dynamic vacuum. It proceeds in four steps: (i) O^–₃ is transformed into O^– and O₂ between 77 and 300 K; (ii) O^– is changed into O^2–₂ between 77 and 473 K; (iii) O^–₂ is also transformed into O^2–₂, with the evolution of O₂ between 300 and 673 K; (iv) finally O^2–₂ is changed into O^2– and O₂ between 673 and 1123 K. This mechanism is also consistent with energy considerations. The formation and thermal decomposition of superoxide ions proceeds similarly both on surfaces subjected to u.v. irradiation and on surfaces containing thermally preadsorbed hydrogen.