Photophysics of the excited uranyl ion in aqueous solutions. Part 1.—Reversible crossing

Abstract

Upon excitation at 337 nm by a nitrogen laser the uranyl ion undergoes biexponential decay from pH 1 to pH 4, this being more evident at the higher pH values. At pH 3 the biexponential is evident for [UO²⁺₂] from 2 × 10^–3 to 0.1 mol dm^–3. The decay rates depend on [UO²⁺₂], the laser intensity and the temperature. Temperature also affects the characteristics of the decay, giving single-exponential decay temperatures < 6 °C. Under steady-state conditions comparison of the fluorescence spectra at low and high temperatures reveals another emitting state of uranyl that has a spectrum which is similar to but weaker than the normal uranyl luminescence and shows a red shift of ca. 300 cm^–1. Several possible kinetic models for analysing the experimental data are discussed and it is concluded that the data can be best interpreted in terms of a reversible crossing between two states of uranyl, U^*ca. 300 cm^–1 higher than X^*, both of which decay by unimolecular kinetic processes. The oscillator strengths of the two states determined from the fluorescence decays and spectra are in good agreement with absorption spectral data. The model allows the estimation of the rate constants of the different processes at different temperatures, uranyl concentrations and excitation intensities. At pH 3, [UO²⁺₂] affects only the rate of decay of the state U^*, whilst an increase in the excitation intensity decreases the rate of the reversible crossing. The activation energies of all the processes suggest that they are of a chemical nature. It is suggested that the reversible decays are due to a solvent exchange process and the irreversible decays are due to hydrogen abstraction from coordinated water molecules.