Spectroscopic investigations of a new type of colour centres in additively coloured pure alkaline earth fluoride crystals

The spectroscopic properties, particularly absorption, fluorescence, fluorescence decay, optical bleaching, uniaxial stress, of additively coloured pure alkaline earth fluoride crystals, which exhibit the type of spectra first discovered by Görlich et al. are closely investigated. X-ray coloured CaF2 crystals with alkali metal doping showing the same type of spectra are included in the investigations. Contrary to earlier assumptions it is found that the colour centres present in these crystals are not identical with F-centres. On the other hand, it is shown that the coloration cannot be explained by impurities. On the basis of the hitherto known properties of the centres an aggregate of four F-centres in tetrahedral configuration is proposed as a new colour centre model and discussed.