Photochemical combination of H2 and Cl2 : A mixture of H2 and Cl2 on exposure to light give rise to the formation of HCl, showing a chain reaction and thereby producing 106 to 108 molecules of HCl per photon absorbed.
The mechanism leading to very high yield of HCl as a result of chemical change can be as follows. Molecule of the chlorine absorbs radiant energy to form the excited molecule which decomposes to chlorine free radicals (Cl) to give chain initiation step.
Light absorption step: 
........(i)
Chain initiation step: 
........(ii)
The chlorine free radical then combines with H2 molecule to form HCl and 
free radical. The free radical so formed again combines with another Cl2 molecule to give HCl and 
free radical back resulting into chain propagation step.
Chain propagation step : 
........(iii)
The combination of two free radicals leads to chain terminating step.
Chain terminating step : 
........(iv)
(ii) Photochemical combination of H2 and Br2 : The combination of H2 H2 and Br2 to form HBr in presence of light is also an example of chain reaction like photochemical combination of H2 and Cl2. Here two Br2 molecules absorb photon, though, insteed of the chain reaction one molecule of HBr is formed for each 100 photon absorbed by 100 molecules of Br2.
Mechanism
Light absorption step : 
........(a)
Chain initiation step : 
........(b)
Chain propagation step : 
........(c)
........(d)
Chain termination step : 
........(e)
The lower values of HBr formation per photon of light absorbed has been attributed to the fact that step (III) is highly endothermic and thus before step (III) can take place most of the bromine free radicals recombine as per step (V) to give Br2 molecule and thus providing less feasibility for step (IV) i.e. steps regenerating free radicals. The decomposition of HBr also increases with increase in temperature.
(3) Quantum yield (or quantum efficiency) : The quantum efficiency or yield 
of a photochemical reaction may be expressed as,
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