J. Grimm, O. S. Wenger, K. W. Krämer and H. U. Güdel
4f–4f and 4f–5d excited states and luminescence properties of Tm²⁺- doped CaF₂, CaCl₂, SrCl₂ and BaCl₂
J. Lumin. 126, 590-596 (2007)                 

Abstract: An overview of the optical spectroscopic properties of Tm²⁺-doped CaF₂, CaCl₂, SrCl₂ and BaCl₂ single crystals reveals both similarities and differences along the series. The absorption and emission spectra are composed of two types of electronic transitions: weak and sharp-line 4f–4f transitions below 9500 cm^–1 and strong broad 4f–5d transitions at higher energies. The spectroscopic transitions are identified and characterised, including previously unobserved 5d–4f emissions in CaF₂:Tm²⁺ and SrCl₂:Tm²⁺. In addition to the chemical variation within the series, changes in the site symmetry as well as the coordination geometry occur. The influence of these variations on the spectroscopic transitions of Tm²⁺ can be followed.