Rate constants and lifetimes of reactive energy states

2 RATE CONSTANTS AND LIFETIMES OF REACTIVE ENERGY STATES 

For a photoexcited molecule, the time allowed for a reaction to occur is of the order of the lifetime of the particular excited state, or less when the reaction step must compete with other photophysical processes. The photoreaction can be unimolecular such as photodissociation and photo isomerization or may need another molecule, usually unexcited, of the same or different kind and hence bimolectdar. If the primary processes generate free radicals, they may lead to secondary processes in the dark. 

The efficiency of a photochemical reaction is defined (Section 1.2) in terms of quantum yield of reaction r, 

The quantum yield of a reaction can be related to reactivity of a given state only after the rates of other competing processes are identified and measured. Consider the reaction 

Since the lifetimes of excited states are small, by applying the steady state approximation, respective concentrations are obtained, 

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Rate constants and lifetimes of reactive energy states 212... 

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