Figure 6.3 Potential energy diagrams showing the Franck-Condon principle |
This results in an emission spectrum that is a mirror image of the S0 → S1 transition in terms of the shape. There are several exceptions to the mirror image rule that arise largely due to the excited state reactions of the molecule.
Quantum yield: As has been mentioned earlier, an excited molecule can come back to the ground state through non-radiative pathways.
where,
Γ is the rate of radiative process i.e. fluorescence
knr is the rate of all the non-radiative processes bringing molecule to the S0 state
Fluorescence lifetime: Lifetime of a fluorophore is defined as the average time it spends in the excited state before returning to the S0 state. It is therefore the reciprocal of the rate of processes de-exciting the molecule.
