Energy levels, electronic/vibrational, Jablonski

Fig. 6.1. Jablonski diagram, representing electron energy levels of fluorophores and transitions after photon excitation. S = electronic state, different lines within each state represent different vibrational levels. Blue arrows represent absorption events, green arrows depict internal conversion or heat dissipation, and orange arrows indicate fluorescence emission. Intersystem crossing into triplet states has been omitted for simplicity (see also Chaps. 1 and 12).

Fig. 2 Jablonski energy level diagram illustrating possible transitions, where solid lines represent absorption processes and dotted lines represent scattering processes. Key A, IR absorption B, near-IR absorption of an overtone C, Rayleigh scattering D, Stokes Raman transition and E, anti-Stokes Raman transition. S0 is the singlet ground state, S, the lowest singlet excited state, and v represents vibrational energy levels within each electronic state.

Fig. 7. The Jablonski diagram. Four electronic levels are depicted along with four vibrational energy levels.

The most comprehensive description and explanation of all processes that affect the fluorescence and the role of additional external factors is provided by the Jablonski diagram (Fig. 1), which represents the scheme of energy levels of electronic and vibrational states of a molecule and the possible transitions [1],... 

Figure 4.1 Simplified Jablonski diagram showing the electronic energy levels of a fluorophore, illustrating excitation (Ex), fluorescence (FI), and phosphorescence (Phos). Singlet states are labeled triplet states and virbrational energy levels Kx- Solid vertical lines illustrate radiative transitions in the direction of trhe arrows, broken lines are non-radiative transition dashed lines are inter-system crossing (ISC) and internal conversion (IC), and dotted lines are vibrational relaxations (VR).

FIG. 2. Correlation between absorption, fluorescence, and phosphorescence spectra and the electronic and vibrational energy levels in the term diagram (called Jablonski term scheme) absorption (A), fluorescence (F), and phosphorescence (P) spectra. 

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). 

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