Judith Grimm, Eva Beurer and Hans U. Güdel
Crystal Absorption Spectra in the Region of 4f-4f and 4f-5d Excitations in Tm²⁺-Doped CsCaCl₃, CsCaBr₃, and CsCaI₃
Inorg. Chem. 45, 10905-10908 (2006)                 

Abstract: Low-temperature absorption spectra of single crystals of Tm²⁺-doped CsCaCl₃, CsCaBr₃, and CsCaI₃ in the spectral range from 8700 to 47000 cm^–1 are presented. Weak sharp-line 4f-4f absorptions around 8800 cm^–1 are essentially independent of the nature of the halide. More-intense broad absorptions cover the region between 12000 and 47000 cm^–1. They are assigned to 4f-5d excitations and interpreted in terms of a simple qualitative picture taking into account the most important interactions. As a result of two counterbalancing effects, the onsets of the 4f-5d spectra are almost coincident in the three materials: The blue-shift of about 3000 cm^–1 between chloride and iodide resulting from the decreasing crystal field splitting of 5d is roughly balanced by the red-shift resulting from the reduced energy gap between the average energy of the 4f¹³ and the 4f¹²5d¹ electron configurations. The absorption helps the understanding of the most unusual light emission properties of these materials.