Fluorescence lifetimes energy level diagram

Figure 27-1 Energy-level diagram shows some of the processes that occur during (a) absorption of incident radiation, (b) nonradiative relaxation, and (c) fluorescence emission by a molecular species. Absorption typically occurs in 10 s, while vibrational relaxation occurs in the 10 " to 10 " s time scale. Internal conversion between different electronic states is also very rapid (10 - s), while fluorescence lifetimes are typically lO- to I0 s.

From the fluorescence lifetime of 1(X) ps for 1 and 8 ns forto. k2 is found to be 9.9 xlO s in methylene chloride solutioa The quantum yield of the charge separated state is calculated from the expression x-k2 and is 0.99. The energy level diagram shown in Hg. 3 illustrates these steps note that the charge separated state preserves about 1.4 eV of the original 1.9 eV porphyrin anglet state. The energies of the transient species are estimated feom spectroscopic or electrochemical measurements. 

Jablonski (48-49) developed a theory in 1935 in which he presented the now standard Jablonski diagram" of singlet and triplet state energy levels that is used to explain excitation and emission processes in luminescence. He also related the fluorescence lifetimes of the perpendicular and parallel polarization components of emission to the fluorophore emission lifetime and rate of rotation. In the same year, Szymanowski (50) measured apparent lifetimes for the perpendicular and parallel polarization components of fluorescein in viscous solutions with a phase fluorometer. It was shown later by Spencer and Weber (51) that phase shift methods do not give correct values for polarized lifetimes because the theory does not include the dependence on modulation frequency. 

Just as above, we can derive expressions for any fluorescence lifetime for any number of pathways. In this chapter we limit our discussion to cases where the excited molecules have relaxed to their lowest excited-state vibrational level by internal conversion (ic) before pursuing any other de-excitation pathway (see the Perrin-Jablonski diagram in Fig. 1.4). This means we do not consider coherent effects whereby the molecule decays, or transfers energy, from a higher excited state, or from a non-Boltzmann distribution of vibrational levels, before coming to steady-state equilibrium in its ground electronic state (see Section 1.2.2). Internal conversion only takes a few picoseconds, or less [82-84, 106]. In the case of incoherent decay, the method of excitation does not play a role in the decay by any of the pathways from the excited state the excitation scheme is only peculiar to the method we choose to measure the fluorescence (Sections 1.7-1.11). 

Fig. 13.1 Relaxation in the X X (ground electronic state) and A n (excite electronic state) vibrational manifolds of the CN radical in Ne host matrix at T = 4 K, following excitation into the third vibrational level ofthe If state. Populations in individual vibrational levels in both electronic states are monitored independently by fluorescence (for the If state) and by laser induced fluorescence (for the X state). The preferred relaxation pathway for energies above the origin of the Tt state is found to be medium assisted internal conversion as indicated by arrows in the left panel. The right panel shows the dynamics of population and subsequent decays of the vibrational levels 6, 5, 4, and 3 of the ground X state. Levels 6 and 5 relax much faster (lifetimes in the order of 1-3 /xs) than levels 4 and 3 (lifetimes in the ms range). For the latter the internal conversion-assisted pathway is closed as seen in the state diagram on the left, so these long lifetimes correspond to pure vibrational transitions. (From V. E. Bondybey and A. Nitzan, Phys. Rev. Lett. 38, 889 (1977).)...

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