Photoionization thresholds of rare-earth impurity ions. Eu2+:CaF2, Ce3+:YAG, and Sm2+:CaF2

The spectral dependence of the photoionization energy of Eu2+:CaF2, Ce3+:YAG, and Sm2+:CaF2 systems have been measured and thresholds experimentally determined and compared with theoretical values calculated from electrostatic models. It is shown that the excited state absorption transitions or the persistent hole burning observed by other authors occur above the threshold energy of photoionization of the impurities and that the states of the crystal which form the bottom of the conduction band may play an important role in the strong probability of these processes. A review of thresholds now known is also given.