The low-temperature behaviour of the F centre in CaO crystals has been investigated using luminescence and photoconductivity measurements. The fluorescence emission from the 3 T 1u to 1 A 1g transition of the F centre decreases in the temperature region 55 to 75K and is accompanied by an increase in the long-lived phosphorescence and in the associated photoconductivity. This behaviour is attributed to photo-induced thermal excitation of electrons from one or more excited states of the F centre to the conduction band. An analysis of the data shows that the 3 T 1u state is about 0.1 eV below the bottom of the conduction band. The radiative lifetime of the 3 T 1u level decreases by about 20% between 4 and 50K. This is not easily explained by current models of the excited states of the F centre.
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Welch et al. (1980) studied this question.
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