Jablonski diagram describing photoexcitation process

The quantum yield ( 1 ) is the yield of initiating radicals produced per photon 

The 13eer-Lambert law (also often called Beer s law) relates /ghs to the total incident light intensity (/q) (eq. 7). 

If the reaction with monomer is not the rate determining. step, the rate of radical generation in photoinitiated polymerization is given by eq. 9 

The decomposition of an initiator seldom produces a quantitative yield of initiating radicals. Most thermal and photochemical initiators generate radicals in pairs. The self-reaction of these radicals is often the major pathway for the direct conversion of primary radicals to non-radical products in solution, bulk or suspension polymerization. This cage reaction is substantial even in bulk polymerization at low conversion when the medium is essentially monomer. The importance of the process depends on the rate of diffusion of these species away from one another, 

In other cases, the cage reaction may simply lead to reformation of the initiator. This process is known as cage return and is important during the decomposition of BPO (Section 3.3.2.1.1) and DTBP (Section 3.3.2.4). Cage return lowers the rate of radical generation but does not directly yield byproducts. It is one factor contributing to the solvent and viscosity dependence of and can lead to a reduced k at high conversion. 

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Figure 3.2 Jablonski diagram describing photoexcitation process.

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