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.