VCenters in Potassium Bromide Crystals

The nature of $V$ centers stable at room temperature in KBr crystals has been investigated by means of preferential bleaching with polarized light. $V$ centers were produced by ionizing radiation, by electrolysis with a pointed anode, or by additive coloration with bromine at high pressure. In pure KBr crystals, the properties of $V$ centers depended on the method of production. Two absorption bands of $〈100〉$-type symmetry were formed at 253 and 278 mμ both by electrolysis and by additive coloration, but no corresponding bands could be found in irradiated crystals. The main product of additive coloration was a broad absorption band extending from the fundamental absorption edge to 320 mμ, appearing in all crystals, pure or doped, which is attributed to bromine molecules. It is suggested that this band forms a large part of the broad absorption band produced by prolonged irradiation of pure crystals, usually referred to as the $V_2$ and $V_3$ bands. Furthermore it was shown that the $V_3$ band is not caused by hydride ions. In KBr doped with alkaline-earth impurities an absorption band which does not depend on the method used to introduce $V$ centers appeared at 262 mμ in KBr(Ca) and KBr(Sr) and at 267 mμ in KBr(Ba). This band has $〈100〉$-type dichroism at 78°K. Optical bleaching of this band at room temperature produced two overlapping bands arising from a common center with $〈111〉$-type dichroism at 78°K. Possible models of the centers responsible for these bands are discussed.