Low‐Temperature Optical Properties of Impurity Aggregates in Pb2+‐Doped KCl Crystals

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Abstract
Low‐temperature absorption, emission, and excitation spectra of substitutional Pb2+ ions (2 to 100 ppm) in KCl crystals subjected to various thermal pretreatments are examined. The existence of two quite different aggregate types composed of Pb2+‐cation vacancy dipoles is ascertained. The aggregates of the first type, thermally stable below 80 °C, appear in samples stored e. g. at room temperature. Their optical properties (small changes in the original absorption spectrum, characteristic emission bands partially overlap that ascribed to the isolated dipoles) suggest a rather weak interaction between component impurity dipoles. The aggregates of the second type appear in samples annealed at higher temperatures up to 250 °C. Their optical properties (new distinct absorption bands, characteristic emission separated from that ascribed to isolated dipoles) suggest a strong interaction between component impurity dipoles. The optical properties (15 K) related with the above aggregates as well as with isolated dipoles and some unidentified aggregation products are compiled.