Infrared Absorption and Anharmonicity of the U‐Centre Local Mode. I. Experiments

The infrared absorption due to U‐centres (substitutional H − or D − ions) is measured in NaCl, NaBr, KCl, KBr, KI, RbCl, and RbBr between 7 and 300 °K. The oscillator strength in the infrared is independent of temperature. Below 300 °K the bandwidth decreases as approximately T 2. Near helium temperatures D − bandwidths tend to saturate at a value of about 1 cm −1 , whereas H − bands in general tend to continue to decrease with T . With increasing temperatures the peaks are shifted to higher or lower frequencies depending on the substance. In the wings of the absorption band side bands appear, which are displaced by band mode frequencies [4]. In all crystals one side band peak lies in the range of the TA frequencies (near q = (100)), in KBr, KI, and NaBr additional peaks corresponding to LA and TO phonons are observed. A discussion will be given in a subsequent paper by Bilz, Fritz, and Strauch.