The theory of multistage transitions is reviewed. A brief account of the fluorescence spectrum of CaF2:Er3+0.1 concentration of Er3+ion per mole is given. The principal regions of excitation are around 2550 and 3750 Aring;, which produce basically different spectra. The fluorescence groups excited with 2550hyphen;Aring; monochromatic radiation are tabulated. Most of the groups are found to be parts of multistage radiative relaxation processes. The stages of these mostly twohyphen;stage processes are identified, and the decay curves of twohyphen;stage transitions are analyzed. The nonexponential decay curves of the secondhyphen;stage transitions are resolved into the difference of two exponentials, and the time constants are determined. The agreement with the time constants obtained for the first and the second stages excited separately is satisfactory. The occurrences of the maxima of the decay curves for the secondhyphen;stage transitions are in agreement with calculations. The temperature and concentration dependence of the transition probabilities is investigated. The decay curves for transitions excited with several wavelengths are analyzed. All experimental results are accountable by the theory.
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