Photoionisation of the F centre in calcium oxide

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Abstract
The low-temperature behaviour of the F centre in CaO crystals has been investigated using luminescence and photoconductivity measurements. The fluorescence emission from the 3T1u to 1A1g 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 3T1u state is about 0.1 eV below the bottom of the conduction band. The radiative lifetime of the 3T1u 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.