J. Grimm, E. Beurer, P. Gerner and H. U. Güdel
Upconversion Between 4f-5d Excited States in Tm²⁺-Doped CsCaCl₃, CsCaBr₃, and CsCaI₃
Chem. Eur. J. 13, 1152-1157 (2007)                 

Abstract: Near-infrared to visible upconversion luminescence in CsCaCl₃:Tm²⁺, CsCaBr₃:Tm²⁺ and CsCaI₃:Tm²⁺ is presented and analysed. The upconversion process involves exclusively the 4f-5d excited states of Tm²⁺, which is a novelty among upconversion materials. The presence of more than one long-lived 4f-5d excited state is the prerequisite for this. Multiple emissions from Tm²⁺ are observed in the title compounds. This is made possible by the favourable energy structure within the 4f-5d states and the low phonon energies of the materials. The energy positions of the relevant 4f-5d states, and thus the photophysical and light emission properties, are affected by the chemical variation along the series. The upconversion efficiency increases from chloride to iodide and the mechanism is found to be a combination of absorption and energy-transfer steps.