Quantum Yield

Quantum Yield

Quantum Yield

Quantum yield the ratio of the number of molecules reacted to the number of quanta of light absorbed in a photochemical reaction. It is denoted by Φ.
Quantum Yield

The quantum yield value should always be unity provided that Einstein-Stark law is strictly obeyed. This law is applicable to only primary process of activation of the reactant molecule ia a photochemical reaction. The quatum yield depends or the light intensity.
Many photochemical reactions do not obey Einstein-Stark law as their quantum yields are widely off from unity. In many such reactions, a molecule activated photochemically initiates a series of secondary processes such as free radical and recombination reactions, i.e. chain reactions. Hence many reactant molecules undergo reactions by absorbing one quantum only. Therefore these reactiors have high quantum yield.
Some reactions have low quantum yields because some excited molecules undergo quick deactivation thereby decreasing the effective number of molecules capable of undergoing reacticns while some excited molecules may not get enough energy to react and dissociation fragments obtained from the excitation may also recombine. In such cases, the number of molecules reacted is too less than one per quantum absorbed.

Reasons of High Quantum Yield

There are some reasons which may be responsible for the high quantum efficiency.
1. The primary process of absorption of radiation produce excited atoms, molecules or free radicals which initiates a series of chain reaction called secondary processes. Thus, by absorbing only one quantum of radiation, several reactant molecules undergo chemical reaction. Hence Φ will be greater than unity.
2. Formation of an intermediate product acts as a catalyst and readily propagate the reaction.
The secondary reaction may be exothermic which activates other secondary process as a result more reactant molecules undergo chemical change without absorption of radiation.

Reasons for Low Quantum Yield

Some photochemical reactions are reported to have very low quantum efficiency and the reason for such phenomenon are given below-
1. If excited molecules formed in primary process are such that they cant react due to their deactivation by collisions or by internal arrangement, the quantum yield will be extremely low.
2. Collision of excited molecules with non-excited molecules may cause to loss their energy. This is another cause of low quantum yield.
3. The excited molecules produced in the primary process may recombine to form the reactant so as to give low quantum yield.
4.If a reacting molecules Is initially present at such a low energy level, so that it does not aquire an optimum energy level to take part in photochemical process by photoexcitation.
5. Some of the photochemically excited molecules in primary process donot undergo secondary reaction. Thus, there is some time interval between primary and secondary process. And they lose some energy. This will give low quantum yield.