Bi-exponential decay of Eu(III) complexed by Suwannee River humic substances: spectroscopic evidence of two different excited species.

The bi-exponential luminescence decay of europium (III) complexed by Suwannee River fulvic acid (SRFA) and humic acid (SRHA), is studied in time-resolved luminescence spectroscopy using two different gratings at varying delay after the laser pulse, increasing accumulation time in order to obtain comparable signals. The two hypotheses found in the literature to interpret this bi-exponential decay are (i) a back transfer from the metal to the triplet state of the organic ligand and (ii) the radiative decay of two different excited species. It is shown that evolutions of the (5)D(0)-->(7)F(0) and (5)D(0)-->(7)F(2) luminescent transitions are occurring between 10 and 300 micros delay. First, the (5)D(0)-->(7)F(0) transition is decreasing relative to the (5)D(0)-->(7)F(1) showing a slightly greater symmetry of the 'slow' component, and is also slightly red shifted. Second, a slight modification of the (5)D(0)-->(7)F(2) transition is also evidencing a slightly different ligand field splitting. No significant modification of the (5)D(0)-->(7)F(1) magnetic dipole, which is less susceptible to symmetry changes, is noted in line with expectations. The (5)D(0)-->(7)F(0) transitions are adjusted with either one or two components. The use of a simple component fit seems to be well adapted for representing an average comportment of these heterogeneous compounds, and a two-component fit constrained by the bi-exponential decay parameters and accumulation times yields in the proposition of the spectra for the fast and slow components.