Rafael Valiente, Oliver Wenger and Hans U. Güdel
New photon upconversion processes in Yb³⁺ doped CsMnCl₃ and RbMnCl₃
Chem. Phys. Lett. 320, 639-644 (2000)                 

Abstract: Excitation around 980 nm and 935 nm leads to visible luminescence in the title compounds below 100 K. The broad luminescence band centered around 690 nm and 630 nm in Yb³⁺ doped CsMnCl₃ and RbMnCl₃, respectively, is identified as a ⁴T₁ ⁶A₁ transition on Mn²⁺. The upconversion excitation spectrum follows the ²F_7/2 ²F_5/2 absorptions around 980 nm and 935 nm. Excitation with 10 ns pulses induces instantaneous upconversion luminescence, indicating that no energy transfer is involved in the process. The most likely mechanism for this new type of upconversion process is based on exchange interactions between neighboring Yb³⁺ and Mn²⁺ ions. In a dimer picture the intermediate excitation is mainly, but not exclusively localized on Yb³⁺, whereas the upper emitting state is mainly but not exclusively localized on Mn²⁺.