Evidence That the Anomalous Emission from CaF2:Yb2+ Is Not Described by the Impurity Trapped Exciton Model

Yb-substituted CaF2 exhibits an anomalous red-shifted luminescence after UV excitation, attributed to the relaxation of impurity trapped excitons (ITE). CaF2:Yb is the archetype system for this model, in which the Yb2+ ions can be excited into a long-lived (ms) exciton state. Upon de-excitation, the emission intensity should be proportional to the Yb2+ 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 Yb2+ and Yb3+ for low Yb concentrations, 0.01% to 0.1%, and thus determine the net concentration of Yb2+. A comparison with luminescence data shows that the intensity is not proportional to the Yb2+ concentration, and only a fraction of Yb2+ 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 Yb2+ concentration.