Kinetic features of photochemistry

If the photochemical reaction were a single elementary step, then the quantum yield in terms of the reactants would also be unity. However, the quantum yield may differ from unity for several reasons. 

In the primary process one quantum is absorbed by one molecule of HBr, as a result of which a second molecule of HBr can react. For one quantum absorbed, effectively two molecules of HBr react, giving a quantum yield of two for the reactant. 

The Cl produced in the primary process is involved in the propagation steps which result in many molecules of Cl2 and CO being transformed, giving a very large quantum yield. 

The quantum yield is found by measuring the amount of reactant removed, or product formed, as a result of absorption of a given amount of radiation. The stoichiometry of the reactions involved may be such that the quantum yields for reactants and products differ. For instance, in the decomposition of HBr quoted the quantum yield in terms of HBr is two, and in terms of H2 and Br2 is unity. 

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