TRLFS with fs-Lasers – a tool to study interactions of actinides with organic ligands

Due to the sensitive detection of light fluorescence properties often are used to study the interaction of metal ions with various ligands. Several actinides as UO2++, Am3+ and Cm3+ show intense fluorescence properties and can be detected at very low concentrations. However, for neptunium and plutonium fluorescence properties are not known up to now. Nevertheless many organic ligands present in the environment as humic substances and wood degradation products fluorescence after excitation. To study the interaction of non-fluorescent actinides with these organic ligands fs-laser pulses were used for excitation. The light emitted from the ligand is observed by ICCD-cameras in a picosecond to nanosecond time scale. Using this TRLFS system the interaction of Np(V) with several small organic ligands was studied. As examples results will be presented for the interaction of Np(V) with 2,3-dihydroxybenzoic acid and 4-hydroxy-3-methoxybenzoic acid. For the interaction of uranyl with 2,3-dihydroxybenzoic acid a complex formation with the phenolic OH groups was found. In the case of Np(V) the interaction is very different. At pH values below 5.0 only the interaction with the carboxylic group can be observed and the interaction with the phenolic group starts at higher pH. This results in a 2:1 complex. Studies of the complex formation of Np(V) with 4-hydroxy-3-methoxybenzoic acid at pH 6 showed also a 2:1 complex, demonstrating the Np(V) interaction with the carboxylic group and the phenolic OH group.
Nevertheless the fluorescence spectra did not show in any case a single component spectrum. This may be connected to other reactions of the non-complexed ligand. Therefore the influence of excited state reactions of the ligand will be discussed.