Room temperature FF′ conversion in KCl crystals of various purity and method of coloration

Abstract

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.

Kinetics of Flash‐Stimulated Changes of F‐Centre Absorption in γ‐Irradiated KCl

“…The decay of Fand the rebuilding of F proceed in the same nonlinear fashion. This is in agreement with previous observation of F--centre in ms periods [9], which ensures that our study concerns the same phenomenon. Furtherniore, the nornialization of kinetic curves of Fand F results in a very close overlap as is seen in Fig.…”

“…The flash of the photolysis lamp results in the decrease of absorption in the I?-band and the increase of absorption in the region above 600 nm. After the flash these changes diminish gradually in a period of the order of s as can be seen in Fig 9 x lo4, 17 x lo4 Gy ma1 decay of F--centres characterized by the braad absorption with maxiinuin near 950 nm [9]. This decay results in F, formation.…”

“…As indicated in [9], reaction (3) may proceed via tunnelling. It leads to the formation of correlated, closely located pairs F--a. Consequently, there is a possibility of electron tunnelling from the F--centre back to the anion vacancy after the flash.…”

“…The crucial point in respect to the first question is the estimation of the probability of reaction (3) between close F* and F. The number of close lying F-centres before the flash depends on the F-centre concentration and IS rather low. The laser photolysis experiments show, however, that in times shorter than microseconds after the light absorption the main process is the depopulation and repopulation of the ground state (see [2], Chap. The tunnel decomposition of Fa t rooin temperature has been discussed in [9] and was suggested to be unimportant. We shall try to show that both the height of the energy barrier and the tunnelling distance make this process relatively fast.…”