Pascal Gerner, Christine Reinhard and Hans U. Güdel
Cooperative Near-IR to Visible Photon Upconversion in Yb³⁺-Doped MnCl₂ and MnBr₂: Comparison with a Series of Yb³⁺-Doped Mn²⁺ Halides
Chem. Eur. J. 10, 4735-4741 (2004)                 

Abstract: Yb³⁺-doped MnCl₂ and MnBr₂ crystals exhibit strong red upconversion luminescence under near-infrared excitation around 10 000 cm^–1 at temperatures below 100 K. The broad red luminescence band is centred around 15 200 cm^–1 for both compounds and identified as the Mn^{2+ 4}T_1g⁶A_1g transition. Excitation with 10 ns pulses indicates that the upconversion process consists of a sequence of ground-state and excited-state absorption steps. The experimental VIS/NIR photon ratio at 12 K for an excitation power of 191 mW focused on the sample with a 53 mm lens is 4.1 % for MnCl₂:Yb³⁺ and 1.2 % for MnBr₂:Yb³⁺. An upconversion mechanism based on exchange coupled Yb³⁺-Mn²⁺ ions is proposed. Similar upconversion properties have been reported for RbMnCl₃:Yb³⁺, CsMnCl₃:Yb³⁺, CsMnBr₃:Yb³⁺, RbMnBr₃:Yb³⁺, Rb₂MnCl₄:Yb³⁺. The efficiency of the upconversion process in these compounds is strongly dependent on the connectivity between the Yb³⁺ and Mn²⁺ ions. The VIS/NIR photon ratio decreases by three orders of magnitude along the series of corner-sharing Yb³⁺-Cl^–-Mn²⁺, edge-sharing Yb³⁺-(Cl^–)₂-Mn²⁺ to face-sharing Yb³⁺-(Br^–)₃-Mn²⁺ bridging geometry. This trend is discussed in terms of the dependence of the relevant super-exchange pathways on the Yb³⁺-Mn²⁺ bridging geometry.