New colour centres in KCl:(Tl+ + Ca2+) and KCl:(Tl+ + Sr2+) crystals

Electrolytic colouring under unusual conditions (low temperature and high voltage) gives rise to the appearance of three new absorption bands peaking at 3.2, 2.5, and 1.7 eV and at 2.8, 2.1, and 1.5 eV in KCl:(Tl+ + Ca2+) and in KCl:(Tl+ + Sr2+) single crystals, respectively. The modifications of the absorption spectra of the coloured crystals induced by the application of a reversed electric field at the colouring temperature or by heat treatment are investigated. It is likely that the colour centre responsible for the new absorption bands is an aggregate centre which, besides an Tl--complex, contains also at least an Ca(Sr) ion, a trapped electron, and an anionic vacancy. (author)