Daniel R. Gamelin and Hans U. Güdel
Excited-State Dynamics and Sequential Two-Photon Upconversion Excitation of Mo³⁺-Doped Chloro- and Bromo-elpasolites
J. Phys. Chem. B 104, 10222-10234 (2000)                 

Abstract: The photophysical properties of Mo³⁺-doped chloro- and bromo-elpasolites at 10 K and as a function of temperature are presented. Visible upconverted luminescence is observed with near-infrared laser excitation, and the properties of this two-photon excitation process are experimentally characterized by a variety of luminescence and time-dependent measurements. Three upconversion mechanisms are active in these Mo³⁺-doped materials. Two involve the sequential absorption of two near-infrared photons of the same color, and a third involves the sequential absorption of two near-infrared photons of different colors. The two one-color upconversion mechanisms are distinguishable on the basis of time dependence measurements and simulations. Of the three mechanisms, the two-color sequential two-photon absorption process is found to be more efficient than the corresponding one-color mechanism by a factor of >10², and this is related to the relatively small effective spin-orbit coupling magnitude in this ion. These excited-state processes are assisted by an extremely long 10 K ²E lifetime of 67.5 ms in 2.5% Mo³⁺:Cs₂NaYCl₆, and 48.5 ms in 2.5% Mo³⁺:Cs₂NaYBr₆. The upconversion properties of Mo³⁺ presented here reflect the rich photophysics of this ion, and represent a significant new addition to the emerging area of transition-metal upconversion.