Rafael Valiente, Oliver S. Wenger and Hans U. Güdel
Upconversion luminescence in Yb³⁺ doped CsMnCl₃: Spectroscopy, dynamics, and mechanisms
J. Chem. Phys. 116, 5196-5204 (2002)                 

Abstract: Single crystals of CsMnCl₃ doped with 0.9% Yb³⁺ were grown from the melt by the Bridgman technique and studied by means of variable temperature optical spectroscopy. At cryogenic temperatures, near-infrared Yb³⁺-excitation around 1 mm leads to intense Mn²⁺ upconversion luminescence in the red spectral region. This very efficient upconversion process is possible because of magnetic Yb³⁺-Mn²⁺ exchange interactions, and a new type of upconversion mechanism is found to be active in this system. The upconversion properties of Yb³⁺:CsMnCl₃ are compared to those of Yb³⁺:RbMnCl₃ and Yb³⁺:CsMnBr₃. The upconversion efficiencies at cryogenic temperatures differ by many orders of magnitude. The bridging geometry between Yb³⁺ and Mn²⁺ is found to be a key factor for the efficiency of the process. The highest efficiency is observed for the title compound, and this is correlated with the most likely linear Yb³⁺-Cl^–-Mn²⁺ arrangement in this crystal. At 15 K the dominant upconversion mechanism in the title compound involves an energy transfer step. By increasing the temperature to 100 K a new and very efficient mechanism involving a sequence of ground state and excited state absorption steps becomes dominant.