Chlorophyll Fluorescence
Mechanism of fluorescence
As was mentioned in the previous section, the absorption of a light quantum (photon) by a chlorophyll molecule leads to its excitation or transition from the lower-energy ground state into the higher-energy excited state of the molecule. The excited state is unstable and is rapidly converted back to the stable ground state via one of the several possible de-excitation pathways:
Fig. 4. A schematic diagram of the mechanism of fluorescence emission 
- Excitation energy transfer to another pigment
(which is the normal de-excitation pathway of the antenna chlorophylls) - Photochemical reaction
(characteristic to the reaction centre pigments) - Thermal dissipation
(the energy of the excited state is dissipated as heat) - Fluorescence
(the energy of the excited state is emitted as light — Fig. 4)
So, fluorescence is the light emitted by an excited pigment molecule upon its return to the ground state.
Fluorescence and photosynthesis
The biological purpose of chlorophyll fluorescence is to serve as a safety valve that dissipates the excessive light energy that is collected by the light-harvesting antenna, but cannot be utilized because the capacity of the photosynthetic machinery is exceeded. The close connection between fluorescence and the photosynthetic process makes it a valuable intrinsic probe for the functioning of this process.
The link between photosynthesis and fluorescence is obvious if we look at the mechanism of fluorescence and consider the fluorescence as one of the possible pathways for de-excitation. If one of these pathways is disabled (blocked), then more electrons will return to the ground state via the remaining pathways. Normally, more than 90% of the excited electrons are used for photosynthesis, and the fluorescence is low. However, if photosynthesis is blocked, no more electrons will be involved in a photochemical reaction, and more electrons will be deactivated via the fluorescence pathway, i.e. the fluorescence will be high.