Quantum Yield of Photochemical Reactions Differ from One
Quantum Yield of Photochemical Reactions Frequently Differ from One, Explain.
According to second law of photochemistry, quantum yield (φ) is equal to one for all photochemical reaction but such reactions are rare. All the activated atoms or molecules may not convert to product. In secondary processes, quantum yield is more or less than one, since one activated atom or molecule may undergo many thermal reactions (φ > 1) or may get deactivated before going to the secondary process (φ < 1).The value of quantum yield can be as high as 106 or as low as 10-2 in reactions.
For example- In the following reaction, the overall quantum yield is 2 since one absorbed photon activates two reactants.
HI + h𝜈 → H* + I*
HI H* → H2 + I*
I* + I* + M → I2 + M*
If the activated atom or molecule produced in the primary process involves in many thermal reactions in the secondary process or one activated molecule activates other deactivated molecules, the quantum yield of the reaction is higher than unity. On the contrary, if the activated reactant undergoes fluorescence or phosphorescence, Internal conversion, Inter System Conversion and get deactivated before converting to product, the quantum yield is lower than unity.
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