Terbium decay rate

The rates of internal conversion from the 5Z)3 to the 5D4 states were also measured. The backup oxide in this case was yttrium. This information was obtained by determining the rise time of the 5Z)4-state green fluorescence as a function of time, when the 5Z>3 state was excited. The rise time of the 5Z)4 state is, of course, the decay time of the 5Z>3 state. It was assumed that the decay of the 5Z)3 was predominantly due to an efficient internal conversion process to the 5D4. Measurements of the decay time of the 5Z)3 state directly were not possible, since the emission from this state is very weak if not, indeed, absent. The result of this study is shown in Fig. 23, where it can be seen that the internal-conversion time remains constant at about 17 fxsec up to a terbium oxide concentration of 1 mole per cent. At higher concentrations, the internal conversion time falls rapidly, until at 10 mole per cent terbium oxide the value is about 1.7 /xsec. This is down by a factor of 10 over samples containing 1 mole per cent or less of terbium oxide. 

The authors believe that the decreases in decay times are associated primarily with changes in quantum yield. This may be inferred from the fact that both the emission intensities and lifetimes are falling off at about the same rate with temperature. One thus concludes that the luminescence of sulfuric acid solutions of terbium sulfate is subjected to much greater temperature quenching than the luminescence in aqueous solution of the same salt. The increasing probability of radiationless transitions is undoubtedly connected in some manner with greater interaction of the radiating ion with the solvent molecules. 

In n-butanol as solvent at 293 K Tb(acac)3-3H20 undergoes intermolecular energy transfer to the complexes R(acac)3-3H20 (R = Pr, Nd, Sm, Eu, Dy, Ho, or Er) (Napier et al., 1975). Measurement of the decay time of the D4 level of the terbium(III) ion indicates that transfer takes place from that level to the excited levels of the other rare earths with bimolecular rate constants of 0.5-4.9x lO dm mol s. The fluorescence lifetime for the D4 state of terbium in gaseous Tb(DPM)3 has also been determined. These measurements have been made by Jacobs et al. (1975) as a function of temperature and pressure and the results demonstrate that intermolecular collisional deactivation is not important. Rather, the non-radiative deactivation is an intramolecular process and occurs by means of a transfer to low-lying excited states of the chelate. The fluorescence decay time is 1 s at 200°C which is very much shorter than those observed in 95% ethanol ( 600 /ts) and in the solid state (—500 fis) at room temperature. 

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