Laser excitation and x-ray spectroscopy are combined to settle a long-standing question in persistent luminescence. A reversible electron transfer is demonstrated, controlled by light and showing the same kinetics as the persistent luminescence. Exposure to violet light induces charging by oxidation of the excited ${\mathrm{Eu}}^{2+}$ while ${\mathrm{Dy}}^{3+}$ is simultaneously reduced. Oppositely, detrapping of ${\mathrm{Dy}}^{2+}$ occurs at ambient temperature or by infrared illumination, yielding afterglow or optically stimulated luminescence, respectively.

• Received 19 May 2020
• Accepted 23 June 2020

DOI:https://doi.org/10.1103/PhysRevLett.125.033001

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Published by the American Physical Society