Quantum Yield or Quantum Efficiency


Quantum Efficiency

The ratio of the number of molecules reacted or formed and the number of photons absorbed is called Quantum Yield or Efficiency. In other words, quantum yield is the number of molecules reacted or formed per absorbed photon of light. It is denoted by `\varphi`. 

In the form of the equation, it is

`\varphi=` (Number of Molecules Reacted of Formed/Number of the absorbed photon) 

Quantum Efficiency

Types of Quantum Yield:

Quantum Yield is mainly of 3 Types.

  • Standard Quantum Yield 
  • High Quantum Yield 
  • Low Quantum Yield

1. Standard Quantum Yield:

When the value of `\varphi` equals 1, means `\varphi`=1, then that is standard quantum yield. It occurs when a reaction strictly obeys the Stark-Einstein Law of Photochemical Equivalence. When only one molecule decomposes per photon. 

2. High Quantum Yield:


When the value of quantum yield is greater than 1, the reaction’s quantum efficiency is high. It is for those reactions where two or more molecules decompose per photon. 

Reasons for High Quantum Yield:

a) Secondary Reactions:

A primary reaction is proceeded by absorption of radiation and a secondary reaction occurs subsequent to the primary reaction. When a reaction has one or more secondary reaction, then more than 1 molecules decompose per absorbed photon. The excited molecules can start a subsequent secondary reaction in which a further molecule is decomposed. Just like the decomposition reaction of HI. The `\varphi` for this reaction is 2.

b) Secondary Reaction Chain:

The more secondary reaction is present, the higher the quantum yield is. A secondary reaction chain means too many subsequent reactions. Therefore, more than one molecule decomposes per photon and the value of `\varphi` crosses 1. For a chain, there should be at least 2 molecules. Here, the excited atom that is produced starts a chain of subsequent reactions. For example, the reaction of Chlorine with the presence of light (photochemical reaction), `\varphi` is extremely high. It can be between `10^4` and `10^6`.

3.  Low Quantum Yield:

Here, `\varphi` < 1. 

It means the number of molecules decomposes less than one per absorbed photon. As a result, it goes low. 

Reasons for Low Quantum Yield:

a) Reverse Reaction of Primary Reaction:

When primary reactions form a polymer, the product then undergoes a thermochemical reaction and gives back the reactant. Hereby, the value of quantum yield becomes less than 1. 

b) Deactivation of Reacting Molecules:

If the excited molecules in the primary process deactivated before they get a chance to react. 

c) Recombination of Dissociated Fragments: 

In the primary process, the reactant molecules may dissociate to give smaller fragments. These fragments can recombine to give back the reactant. The photochemical reaction of Bromine has a quantum yield of 0.01. It means 100 absorbed photon needs to decompose one molecule. 


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