Vibronic spectra ofU4+in octahedral crystal fields

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Abstract
Excitation of Cs2ZrBr6: UBr6 2- using 647·1 nm laser radiation initially populates the (1 D 2)Γ5 vibronic sideband. Rapid radiationless decay to the (1 G 4)Γ1 and (3 H 6)Γ5 levels is followed by luminescence from these levels. In addition to the transitions to the (3 H 4)Γ1 ground state, transitions to the (3 H 4)Γ4 and Γ3 components are observed in the region above 880 nm, enabling these states to be located for the first time, at 877 cm-1 and 1192 cm-1 above the ground state. Each of the electronic transitions exhibits detailed vibronic structure which is analysed and discussed. Under intense red excitation the crystals luminesce in the blue-green spectral region and the mechanism for this upconversion is considered.