A fast irreversible adsorption of oxygen at 20°C on γ- or neutron-irradiated MgO or CaO is correlated with observable surface paramagnetic centres (S centres) and colour centres. The S centre in MgO with g_av= 2.0007 is believed to be an intrinsic defect consisting of an electron trapped at a surface anion vacancy. An additional oxygen-sensitive signal S_H is related to the presence of some hydrogen contamination on the surface. The S centre in CaO, with g∥= 1.9992 and g⊥= 1.9969 after γ-irradiation to low doses, is similar in behaviour to that on MgO; however, at high γ-doses or after neutron irradiation a complex signal is produced. A surface centre with g_av= 1.9798 and of low intensity has been detected on γ- and neutron-irradiated SrO, but this accounts for only a small fraction of the total oxygen adsorption on SrO.

The formation of anion vacancies at the surfaces of unirradiated and irradiated oxides is discussed. These defects play a dual role in the irreversible adsorption of oxygen by providing a trap for an electron at the surface and a site capable of stabilizing the adsorbed oxygen species; other sites for oxygen adsorption are increasingly important in CaO and SrO and account for adsorption on the unirradiated oxides.