The temperature dependence of the dichroic properties of the M center in KCl crystal was investigated. At the temperatures above 60°C, the thermal reorientation of the M center in the ground state occurs. The measured value of the activation energy for it was 1.0 eV. It was suggested that vacancy clusters may trap free electrons to form new M centers in the course of the bleaching of the M band and this process may be responsible for the temperature dependence of dichroism of the M band.
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Ishii et al. (1959) studied this question.