The minimum energy for the onset of photoconductivity has been measured for the ions Ho2+, Dy2+, and Tm2+ in CaF2, SrF2, and BaF2. This energy is identified with the difference between the impurity level and the bottom of the conduction band. The value of this energy difference is given to within 0.5 eV by a simple electrostatic model including corrections for polarization and for the change of the positions of the ions surrounding the impurity in the crystal.
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Pédrini et al. (1979) studied this question.
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