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 0,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) 0; is transformed into 0.-and 0, between 77 and 300 K; (ii) 0is changed into 0;-between 77 and 473 K; (iii) 0; is also transformed into Og-, with the evolution of 0, between 300 and 673 K; (iv) finally 0;is changed into 02and 0, 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.
Während der O2‐Adsorption unter UV‐Bestrahlung an einer bei 1123 Kentgasten MgO‐ Oberfläche bei 77 Kbilden sich die Spezies O5′ und O; zu gleichen Teilen (TPD‐ und ESR‐Untersuchungen).
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