Oliver S. Wenger and H. U. Güdel
Broadband near-Infrared Sensitization of Visible Upconversion Luminescence in V³⁺ and Mo³⁺ Co-Doped Cs₂NaYCl₆
J. Phys. Chem. B 106, 10011-10019 (2002)                 

Abstract: Crystals of Cs₂NaYCl₆ singly doped with Mo³⁺ or V³⁺ and a series of V³⁺:Mo³⁺ codoped Cs₂NaYCl₆ crystals were studied by low-temperature optical absorption and luminescence spectroscopy with particular focus on their near-infrared (NIR) to visible (vis) photon upconversion properties. In the codoped crystals 15 K Mo^{3+ 2}T_2g ⁴A_2g upconversion luminescence around 14300 cm^–1 is not only observed after direct excitation into the weak and narrow Mo^{3+ 2}E_g and ²T_1g ground-state absorptions around 9700 cm^–1, but also after excitation into the broad V^{3+ 3}T_1g ³T_2g absorption band between 10000 and 12500 cm^–1. This is because the excitation energy is transferred from V^{3+ 1}T_2g to Mo^{3+ 2}E_g, and this process is studied as a function of the relative V³⁺ and Mo³⁺ dopant concentrations. The Mo^{3+ 2}T_2g higher excited-state luminescence is not affected by the V³⁺ codopants, and this is because neither of the Mo^{3+ 2}T_2g emissions spectrally overlaps with V³⁺ absorption. The upconversion mechanisms which are active in the V³⁺:Mo³⁺ codoped Cs₂NaYCl₆ crystals at 15 K are identified on the basis of one- and two-color upconversion luminescence excitation spectroscopy. V³⁺ is found not only to act as a sensitizer for Mo^{3+ 2}E_g, but also to participate in a nonradiative energy transfer upconversion (ETU) process. At 15 K, there is a strong spectral overlap of V^{3 + 3}T_1g ³T_2g ground-state absorption (GSA) with Mo^{3+ 2}E_g ⁴T_2g excited-state absorption (ESA), leading to efficient GSA/ESA upconversion in 0.8% V³⁺:0.3% Mo³⁺:Cs₂NaYCl₆ at 15 K. Excitation with the broadband NIR output of a tungsten lamp induces more than an order of magnitude more 15 K vis upconversion luminescence in 0.8% V³⁺:0.3% Mo³⁺:Cs₂NaYCl₆ than in purely 0.3% Mo³⁺ doped Cs₂NaYCl₆.