Annina Aebischer, G. Mackay Salley and Hans U. Güdel
Near infrared to visible photon upconversion in Re⁴⁺ doped Cs₂ZrBr₆
J. Chem. Phys. 117, 8515-8522 (2002)                 

Abstract: Two visible luminescence bands are observed in Re⁴⁺:Cs₂ZrBr₆ under NIR excitation at temperatures between 10 and 300 K. We perform absorption, luminescence, excitation, and time resolved spectroscopic studies at various temperatures to characterize this upconversion luminescence and its driving mechanism. From the time dependent measurements we assign a ground state absorption/energy transfer upconversion type mechanism to the upconversion process. The visible to near-infrared photon ratio is 0.008 at T = 50 K with a laser power density of 415 W/cm². Most of the visible luminescence is quenched by a strong cross-relaxation process out of the highest emitting level as well as intense radiative interexcited state transitions. The ReBr₆^2- unit is one of very few transition metal complexes with three emitting electronic levels in the near-infrared and visible range of the electromagnetic spectrum.