A photochemical reaction involves two processes. One is the primary process in which light is absorbed by molecules to give excited molecules. Another is a secondary process in this process exciting molecule initiates a series of thermal reactions. In such reactions, many reactant molecules may undergo chemical change by absorbing one quantum only.
In some cases, the activated molecule undergoes deactivation. Thus, less than one molecule may react per quantum. The overall result of the photochemical reaction is expressed in terms of Quantum yield.
The quantum yield or efficiency(Φ) is defined as: " It is the number of molecules which undergo chemical transformation per quantum of absorbed energy. "
Mathematically,
Φ = No. of molecules reacting in a given time/ No. of quanta absorbed in the same time
Φ = No. of moles reacting in a given time/ No. of Einsteins absorbed in the same time
Φ = Rate of chemical reaction/ No. of Einsteins absorbed
The quantum yields are dependent on the light intensity.
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