Radiative decay rate enhancement mechanisms

Herein, F is the radiative decay rate and km is the nonradiative decay rate, which comes from quenching. It has been demonstrated that silica nanomatrixes can change the fluorescence quantum yield and lifetime of fluorophores. Several groups have reported that both quantum yield and lifetime of fluorophores increased in DDSNs [27, 28, 52, 65-67]. However, the mechanisms regarding this enhancement were reported differently. 

Merkle et al. (1981) discussed three possible mechanisms of the pressure-enhanced decay rate an increase of the radiative decay rate of each Nd3+ ion, an increase of the nonradiative decay rate of each Nd3+ ion, or an increase in the interaction between Nd3+ ions leading to luminescence quenching. The nonradiative decay rates for the 4F3/2 multiplet were estimated to contribute less than 20% to the total decay rate (Powell et al., 1980) at ambient pressure. 

Figure 19.4C presents data on the phosphorescence decay dynamics on two of the samples in Figure 19.4B, the one without silver and the one having optimum enhancement. These exhibit the characteristic signature of plasmon-enhanced emission, namely that increases in luminescence are accompanied by decreases in excited state lifetime. Ordinarily, radiative rates are fixed by quantum mechanical matrix elements and variation in excited state lifetime is due to changes in nonradiative rates so that increases in lifetime correspond to lower non-radiative rates and increases in luminescence yield. Here, the lifetime is reduced by about a factor of 3 due to increased emissive rate even as the luminescence increases 215 times. These large enhancements cannot be accounted for simply by increase in phosphorescence yield since the yield is greater than 10 % in the absence of silver. It is evident that a substantial fraction of the increase must be accounted for by absorption enhancement. 

---