Are the changes in the lifetime of the excited uranyl ion of chemical or physical nature? Original Research Article

By use of time-resolved laser-induced fluorescence and of basic spectrophotometric methods, we have studied the effects of water/perchloric acid mixtures (up to 12 M), NaClO4 in water/perchloric acid mixtures and triflic acid (diluted or pure) on the absorption and emission spectra and on the lifetime of the uranyl ion in liquid solutions. Models proposed in order to explain similar results in the case of various elements are discussed. It is shown that the main effect remains largely unexplained, as none of the previously proposed models is able to recover the data without severe contradictions. The two best models, chemical equilibrium or energy transfer, were tentatively modified in order to unravel the inconsistencies pointed out; however, it was not possible to reconcile our data with models based on chemical reactions including ionic strength effects. A model of internal conversion leads to an approximative mathematical treatment, and the quality of the fit is not perfect. Finally, the data may indicate that a complexation of uranyl with perchlorate ion occurs above 4.5 M. The association of HClO4 and triflic acid are observed at very high concentrations.