Concentration-dependent excited-state annihilation in aqueous uranyl complexes under pulsed laser excitation

At high excitation density, the luminescence decay of aqueous uranyl complexes can deviate from monoexponential kinetics due to an additional excitation-dependent deactivation pathway, complicating the interpretation of TRLFS lifetimes. Here we investigate nonlinear excited-state deactivation of aqueous uranyl complexes under pulsed laser excitation. A two-particle annihilation model was quantitatively tested using a concentration series of uranyl sulfate solutions, with a phosphate-containing U(VI) solution used for comparison with a different ligand environment. For sulfate complexes, the extracted two-particle deactivation parameter increased linearly with total U(VI) concentration, as expected for bimolecular excited-state annihilation. The corresponding effective interaction radius was estimated as 5.1 ± 0.5 Å, consistent with short-range collisional deactivation. The phosphate-containing solution showed qualitatively similar excitation-dependent nonlinear decay, with an effective interaction radius of 2.4 ± 0.1 Å. These results provide quantitative evidence for a concentration-dependent bimolecular contribution to uranyl luminescence decay at high excitation density and show that excitation conditions should be controlled when TRLFS lifetimes are used for speciation analysis or compared across different experimental conditions.