Chlorophyll Fluorescence in Plant Biology

Several molecules absorb light energy which they emit after a time difference (lifetime) as radiation energy. Molecules remain at a low energy level or the ground electronic singlet state (So) or the lowest vibrational level at room temperature (Noomnarm and Clegg, 2009). On absorption of a photon, the molecule is excited from So to the first electronic excited singlet state S1 within < 10-15 s-1 (Figure 1). These molecules can also be transferred to higher energy levels (S2 to Sn) also. These excited state molecules can relax to the S1 electronic state via vibrational relaxation within 10-12 s-1. The molecule will ultimately relax to the So state through photon emission, which is called fluorescence emission. Also here, the energy of the emitted photon must equal the changes in the energy levels.