The Use of Photochemistry in Disentangling Complex Mechanisms

If a reaction can be initiated photochemically, a further route to investigating the mechanism and evaluating the rate constants becomes possible. 

Radiation can be used to initiate a reaction (see Section 2.2.4). When molecules absorb radiation they are excited and often split up into radicals. Normally one molecule absorbs one quantum of radiation. This enables the rate of formation of the radicals produced as a result of absorption to be found from a measure of the radiation absorbed. These radicals may then react in a sequence of reactions, whereby more reactant is removed independently of the initial breakdown under the influence of radiation. The quantum yield is defined as the number of reactant molecules transformed per quantum absorbed, and gives a measure of how many molecules of reactant eventually react as a result of the initial first breakdown. If the quantum yield is large, this is conclusive evidence of a chain reaction, where many reactant molecules decompose per quantum absorbed. 

The identity of the radicals formed from the initially excited molecule can be studied spectroscopically. If conventional radiation sources are used, the radicals will be formed in steady state concentrations and their rates of formation and removal cannot be measured. If, however, flash or laser photolysis is used the radicals are formed in much larger concentrations and their concentration-time profiles can be determined spectroscopically see Sections 2.1.4 and 2.5.2. From this, rate constants for the overall formation and removal of these radicals can be found. 

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