Evidence That the Anomalous Emission from CaF 2 :Yb 2+ Is Not Described by the Impurity Trapped Exciton Model C. MacKeen, † F. Bridges,* ,† M. Kozina, ‡ A. Mehta, ‡ M. F. Reid, § J.-P. R. Wells, § and Z. Barandiara ́ n ∥ † Physics Department, University of California, Santa Cruz, California 95064, United States ‡ Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States § Dodd-Walls Centre for Photonic and Quantum Technologies and Department of Physics and Astronomy, University of Canterbury, PB 4800, Christchurch 8140, New Zealand ∥ Departamento de Química, Instituto Universitario de Ciencia de Materiales Nicola ́ s Cabrera, and Condensed Matter Physics Center (IFIMAC), Universidad Autó noma de Madrid, 28049 Madrid, Spain ABSTRACT: Yb-substituted CaF 2 exhibits an anomalous red-shifted luminescence after UV excitation, attributed to the relaxation of impurity trapped excitons (ITE). CaF 2 :Yb is the archetype system for this model, in which the Yb 2+ ions can be excited into a long- lived (ms) exciton state. Upon de-excitation, the emission intensity should be proportional to the Yb 2+ 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 2+ and Yb 3+ for low Yb concentrations, 0.01% to 0.1%, and thus determine the net concentration of Yb 2+ . A comparison with luminescence data shows that the intensity is not proportional to the Yb 2+ concentration, and only a fraction of Yb 2+ 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 2+ concentration. I n a large number of lanthanide-doped crystals there is a large Stokes shift of the luminescence emission upon UV excitation, particularly for Yb 2+ and Eu 2+ ions. 1 The optical properties of CaF 2 :Yb 2+ were first reported about 50 years ago, 2,3 and the unusually large red-shift and bandwidth of the luminescence emission were considered anomalous. A model to describe such systems was developed in the 1980s by McClure and others, 1,4,5 in which the optical center is called an impurity trapped exciton (ITE). Dorenbos 1 provides a long list of potential ITE systems with anomalous emissions, mostly with Yb and Eu dopants. Upon UV excitation (within this model), one of the 4f 14 electrons of Yb 2+ is excited to the 4f 13 5d state; this state decays to the exciton state in which the excited electron forms a delocalized extended state on neighboring metal atoms. 5 The resulting Yb 3+ ion plus the delocalized electron state forms the transient ITE state−a bound electron−hole pair. While in the ITE state, the cube of surrounding F − atoms should collapse slightly (again a transient effect), leading to a decreased Yb−F bond distance, by ∼0.2 Å, 4 which explains the bandwidth of the emission. The ITE model has been accepted in the literature for over 30 years 1,4−8 and continues to be used in the current literature. 9,10 Recently, however, Barandiara ́ n and Seijo 11 have examined the ITE hypothesis by means of ab initio relativistic quantum chemical calculations. The results for Yb 2+ in CaF 2 allow the authors to conclude that none of the electronic states of the Yb 2+ active centers can be considered responsible for the anomalous emission; hence the need of direct experimental scrutiny of the ITE model. Yb substitutes for Ca in CaF 2 :Yb and forms several defects. Clusters form at higher concentrations, 12, 13 while the anomalous emission centers form at much lower concen- trations. In the ITE model, the number of excitons excited by UV should be proportional to the number of Yb 2+ ions present. At the time the ITE model was first developed, there was no way to measure the Yb 2+ concentration in order to check for a linear dependence of anomalous luminescence intensity. In fact only recently has this been possible for very low defect concentrations down to 0.01% Yb. Here we provide direct experimental evidence that the ITE model cannot explain anomalous luminescence in CaF 2 :Yb; first the anomalous luminescence intensity is not proportional to the Yb 2+ concentration, and second, only a small fraction of Yb 2+ ions are involved in anomalous emission. Normalized Yb L III absorption edges are plotted in Figure 1 for three concentrations: 0.01, 0.05, and 0.1% Yb; the plots are normalized well above the edge. The L III edge has two peaks separated by about 7 eV: the lower one (8942 eV) is associated with Yb 2+ , while the upper one (8949 eV) is for Yb 3+ . For the Received: May 5, 2017 Accepted: July 5, 2017 Published: July 5, 2017 Letter pubs.acs.org/JPCL © 2017 American Chemical Society 3313 DOI: 10.1021/acs.jpclett.7b01103 J. Phys. Chem. Lett. 2017, 8, 3313−3316