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 snorter 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^-, SO₄^2-) affected the structure of the emission spectrum to some extent.