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The results of an analysis of the energy transfer and two-step excitation in LaF3:Yb,Er are presented. The physical model assumes that the Yb3+ and Er3+ ions are tightly coupled and transfer occurs both ways. The simple model used is able to explain the experimentally observed time dependence and radiant efficiency provided one assumes the following decay times: Yb3+(5F5/2), 2 msec; Er3+(4F7/2), ∼1 μsec; Er3+(4S3/2), 0.125 msec.

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