Unusual Luminescence from Crystals Containing Eu2+

Luminescence at 368 nm from crystals containing Eu2+ exhibits the small spectral shifts from host to host typical of a parity-forbidden rare-earth transition, and yet has the emission oscillator strength (f≳1) of a highly allowed transition. It is improbable that this anomaly can be trivially resolved, e.g., in terms of measurement errors, stimulated emission, or impurity involvement.