Evidence That the Anomalous Emission from CaF:Yb Is Not Described by the Impurity Trapped Exciton Model.

Yb-substituted CaF exhibits an anomalous red-shifted luminescence after UV excitation, attributed to the relaxation of impurity trapped excitons (ITE). CaF:Yb is the archetype system for this model, in which the Yb ions can be excited into a long-lived (ms) exciton state. Upon de-excitation, the emission intensity should be proportional to the Yb concentration, but that could not be checked when this model was first proposed. Using the X-ray absorption near edge structure (XANES) technique, we determine the fractions of Yb and Yb for low Yb concentrations, 0.01% to 0.1%, and thus determine the net concentration of Yb. A comparison with luminescence data shows that the intensity is not proportional to the Yb concentration, and only a fraction of Yb ions contributes to the anomalous luminescence. This is inconsistent with the ITE model and illustrates the importance of checking the dependence of the emission intensity on the Yb concentration.