Luminescence properties of tetravalent uranium in aqueous solution

The luminescence spectra of U4+ in aqueous solutions were observed in the UV-VIS region at ambient and liquid nitrogen temperatures. The excitation spectrum indicates that the luminescence is arising from the deexcitation of a 5f electron at the 1S0 level and no other emissions of U4+ in aqueous solutions were detected for other f-f transitions. All the luminescence peaks were assigned to the transitions from 1S0 to lower 5f levels. To estimate the luminescence lifetime, luminescence decay curves were measured using time-resolved laser-induced fluorescence spectroscopy. At room temperature, the decay curve indicated that the lifetime was shorter than 20 ns. On the other hand, the frozen sample of U4+ in aqueous solution at liquid nitrogen temperature showed the same emission spectrum as at room temperature and its lifetime was 149 ns in H2O system and 198 ns in D2O system. The longer lifetime at liquid nitrogen temperature made it possible to measure the spectrum of U4+ at the concentration as low as 10-6 M. The difference in the anion species (ClO4-, Cl-, SO42-) affected the structure of the emission spectrum to some extent. (orig.)