The absorption of light by a molecule leads to its excited state. If the excited state is unstable, the absorbed light is emitted back through fluorescence. If the excited state is long lived (nano-seconds, microseconds), the molecule may decay into other excited states and finally decay to the ground state by transferring energy to the medium during collisions. The excitation energy could be either a laser light (which can be tuned to specific excitation wavelengths) or could be sunlight, which is the main source of energy for all natural photochemical reactions such as photosynthesis. The range of frequencies in the sunlight is from UV to the far infrared. The photochemical efficiency of a reaction induced by light is called the quantum efficiency and it is defined by
quantum efficiency or quantum yield =
= number of molecules reacted / number of photons absorbed
(30.23)
Figure 30.2 Common physical features associated with absorption of light. The molecule, on excitation goes from the ground state S to the excited state S*. During this process, the spin of the excited electron is unchanged and it is a siglet. This excitation can be given out as fluorescence. This excitation can cross over to an excited triplet state, T*. Usually T* is long lived and decays slowly to the ground state through phosphorescence.
If we know the wattage (W) and the wavelength (
) of a source, we can calculate the number of photons generated per second, ns, by
ns = W /(h c / )
(30.24)
If we know the duration of time (t) over which the light was incident, the number of photons incident is given by
ns t. One mole of photons is one einstein.
The excited state resulting from light absorption may be denoted by A*. A* may be a singlet (all spins paired) or a triplet (two unpaired spins). This excited state may be involved in several chemical changes such as ionizations, dissociations, isomerizations, induced dissociations and so on.
= number of molecules reacted / number of photons absorbed 
) of a source, we can calculate the number of photons generated per second, ns, by 
