Abstract
A study has been made of zero-phonon lines produced by irradiation and heat treatment in irradiated and additively coloured MgO crystals. It appears that the zero-phonon lines at 361·5 and 1044·5 nm observed in neutron-irradiated crystals are due to different absorption transitions of the same defect. Numerous new zero-phonon lines are observed when neutron-irradiated crystals are annealed above 500°c and given short ionizing irradiations. Of these, the lines at 1081·6 and 1324·2 nm can also be produced in electron-irradiated, additively coloured and deformed crystals. It is suggested that these two lines are due to F-aggregate centres (aggregate of negative-ion vacancies).

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