Optically reversible Tm3+ → Tm2+ radiophotoluminescence in NaMgF3:Tm

Infrared radiophotoluminescence was observed in NaMgF3:Tm from the X-ray-induced reduction of Tm3+ -> Tm2+. The 4f(13)(2F(5/2)) -> 4f(13)(2F(7/2)) Tm2+ emission only appeared after irradiation and peaked at 1130 nm where the emission energy was compressed by a factor of 1.15 times relative to isoelectronic Yb3+ in NaMgF3:Yb. The Tm2+ emission intensity increased monotonically with absorbed dose up to at least 360 Gy and the integrated intensity increased linearly up to at least 60 Gy. The Tm2+ photoluminescence could be probed non-destructively by exciting into the peak of the lowest energy 4f(13) -> 4f(12)5d(1) band at 450 nm, and destructively probed using stimulation at 280 nm, whereupon the photo-induced oxidation Tm2+ -> Tm3+ occurred. Additionally, Tm3+ optically stimulated luminescence was observed after irradiation when stimulating into the F-centre absorption band at 340 nm. The optically reversible radiophotoluminescence and optically stimulated luminescence phenomena have potential applications in radiation dose monitoring and optical information storage.