# 168. LAWS OF PHOTOCHEMISTRY (14)

In the last post we started talking about quantum yield. This quantity tells us how many molecules react by absorbing how many quanta of light.

As seen in the earlier post quantum yield is defined as “the number of molecules of the substance undergoing the photochemical change per quantum of radiation absorbed.”

Thus, the quantum yield tells us the number of molecules undergoing a photochemical change per absorbed photon. We also learnt that for a primary process, that obeys Stark-Einstein’s law, QUANTUM YIELD IS ONE. This means that one molecule undergoes the desired photochemical change by absorbing one photon.

However, there are photochemical reactions which show exception to this behaviour i.e they do not obey the Stark- Einstein’s law. For such reactions, the quantum yield is NOT one. Let us study these exceptions –

EXCEPTION 1Quantum Yield is more than 1 (Φ>1).

In some photochemical reactions, involving intense lasers and for some radiation induced chain reactions, a single photon might trigger a series of chain reactions. Thus, absorption of a single photon leads to decomposition of two or more molecules. In this case , Φ>1

Let us assume that 2 molecules decompose by absorbing one photon , then the quantum yield will be –

Φ = # of molecules reacting / # of quanta absorbed = 2/1 = 2. Thus, Φ >1.

Thus, when the quantum yield is more than one, the number of molecules reacting is much larger than the number of photons absorbed.

EXCEPTION 2Quantum Yield is less than 1 (Φ<1).

This exception is opposite to the earlier one. Here the quantum yield is less than one because the number of molecules reacting are much less than the number of photons absorbed.

The reason for low quantum yields are –

1) the activated molecule gets deactivated before forming products as shown below-

M* → M + hν where,

M* – Excited molecule
M – Deactivated molecule
– emitted light

2)excited molecules collide with non-excited ones and transfer their energy to them.

M*+ A→ M + A* where,

M* – Excited molecule
A – Another molecule
M – Deactivated molecule
A* – Other molecule in excited state.

3)The decomposed fragments may recombine to form the reactant again. Thus, no new products are formed.

A → a + b
a + b → A
where,

A – Reactant molecule
a , b – Decomposed fragments.

We have already stated that, the Stark Einstein law holds true only for primary process of a photochemical reaction. But what is a primary process? We shall learn more about the primary and secondary photochemical process, in the next post. Till then,

Be a perpetual student of life and keep learning…..

Good day !