Abstract
We present here a further study of unusual features observed in photoluminescence (PL) of Eu 2+ in CaF 2 layers of CaF 2–CdF 2 SLs (SLs) [N.S. Sokolov, S.V.Gastev, A.Yu. Khilko, S.M. Suturin, I.N. Yassievich, J.M. Langer, A. Kozanezcki, Phys.Rev B 59 (1999) R2525]. PL of Eu 2+ in SLs, in contrast to bulk CaF 2:Eu crystals, fades out considerably under continuous UV excitation. This is mainly due to tunneling-assisted photoionization of Eu 2+. We have measured the PL time dependence in uniformly and selectively doped SLs with different periods and compared the results with the calculations in the tunneling model. A careful analysis of PL in selectively doped SLs revealed an additional ionization process suggested to occur in all SLs independent of the Eu ion position. With the account of this process we find that the experimental results are in good agreement with theoretical calculations. We have estimated the values of the main parameters in the tunneling model, which show the best fit with the experiment.
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