Heavy Element Photophysics and Photochemistry

Comparison of Observed and Calculated Luminescence Decay Rates for Uranyl Fluoride Complexes in Aqueous Solution

In the figure at the right, our observed luminescence decay rate data for uranyl fluorides are shown as solid circles. The solution contained 3x10^-6 mole/liter (M) uranyl in 0.1 M HClO₄ at 299 K. Sodium fluoride was added to the solution as a solid to vary fluoride ion concentration. The solid line through the decay rate data is a fit of the observed data to our speciation model. This model assumes that equilibrium is achieved among electronically excited uranyl fluorides on the time scale of the observed luminescence decays. The species considered include uranyl complexes containing from one to four fluoride ions. Nonlinear least-squares modeling (solid line curve) of the observed data shown in the figure has provided the first stability quotients for the formation of electronically excited uranyl fluoride complexes as well as values for their luminescence lifetimes. Also shown (dashed line) is the calculated free fluoride ion concentration as a function of added NaF based on literature data for hydrofluoric acid species.

Models, such as those used to interpret the data shown here, provide the best current basis for speciation of electronially excited uranyl fluorides.

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