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The FF′ conversion is initiated by flash‐light excitation at room temperature with the aim of obtaining data on the electron trapping and transition characteristics in KCl crystals of various purity and coloured in different ways. The purest, γ‐irradiated sample exhibits a relatively short F' lifetime and a proportionally large F centre‐to‐anion vacancy cross‐section ratio presumably resulting from an enhanced trapping ability of the F centre. In contrast, Na‐containing irradiated samples show unusually long F' lifetimes, not correlated to the corresponding cross‐section ratios. The F' yield in the pure samples is found to drop on repeated flashing which presumably reflects the drop in F centre ionization quantum efficiency as a result of clustering processes. Electrolytically coloured samples doped with divalent cations are also investigated.

The room temperature colouration process of NaCl: E u + + crystals strongly depends on both the dopant concentration, represented by the absorption coefficient a(eg), and the dopant dispersion state as well.It has been found that within the UV spectral range either some V-type absorption bands appear or only the background absorption increases. For heavily irradiated crystals the clustering of halogen-interstitial results in the formation of dislocation loops.I n dependence of the dopant concentration the colourability of as received crystals is either larger (a(eg) & 50 cm-l), or smaller (a(eg) 2 50 cni-l) of that characteristic of the solution treated ones. The difference in the effectiveness of the halogen-interstitials trapping-process has been explained in terms of the various structure and/or composition of the europium-related aggregates and/or precipitates.