Oliver S. Wenger and Hans U. Güdel
Chemical Tuning of the Photon Upconversion Properties in Ti²⁺-Doped Chloride Host Lattices
Inorg. Chem. 40, 5747-5753 (2001)                 

Abstract: The photophysical properties of Ti²⁺-doped NaCl and MgCl₂ at 15 K are compared. At this temperature both materials emit luminescence from their respective lowest excited states and from the ³T_1g(t_2ge_g) higher excited state. In Ti²⁺:MgCl₂ the ligand field is significantly stronger than in Ti²⁺:NaCl, leading to ¹T_2g(t_2g²) and ³T_2g(t_2ge_g) lowest excited states, respectively, in these materials. The near-infrared to visible upconversion mechanisms of these two materials are identified. The upconversion efficiency calculated for Ti²⁺:MgCl₂ is approximately 2 orders of magnitude higher than for Ti²⁺:NaCl. This is mainly due to the more efficient energy storage in the intermediate upconversion level in Ti²⁺:MgCl₂, and its higher ³T_1g(t_2ge_g) -> ³T_1g(t_2g²) luminescence quantum yield relative to Ti²⁺:NaCl.