Vibronic spectra of U4+ in octahedral crystal fields

Excitation of Cs2ZrBr6:UBr62- using 647.1 nm laser radiation initially populates the (1D2)GAMMA5 vibronic sideband. Rapid radiationless decay to the (1G4)GAMMA1 and (3H6)GAMMA5 levels is followed by luminescence from these levels. In addition to the transitions to the (3H4)GAMMA1 ground state, transitions to the (3H4)GAMMA4 and GAMMA3 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. (author)