Flash photolysis and reaction dynamics of diradicals

If the reaction is photoinitiated by a single UV laser flash at high temperatures (T 180 K) the entire time-dependent reaction series, DR2 DR3 DR4 DR5 DRg, can be observed by monitoring the transient optical absorption of each of the products DR2 to DRe. By this experiment we were able to analyze the reaction kinetics of these thermally activated steps separately [37, 44], As an example Fig. 9.19 shows the transient absorption 

Assuming that DR2 is produced hy the flash promptly, DR3 hy a dark reaction from DR2, DR4 by a subsequent dark reaction from DR3, and so on, we used a simple kinetic 

As an example, the fit of this model to the transient DR4 at 200 K is shown in Fig. 9.21. The fit does not show any significant deviation from the experimental curve. 

A second result of these experiments is an estimation of the polymer yield for the photoreaction as defined by the number of polymerized monomer molecules per absorbed UV photon. Q is the quantum yield for the initiation process. Thus, the polymer yield P is the product of Q and the kinetic chain length L, i.e. the number of monomer molecules which are added to the chain after one initiation process  

The polymer yield was determined from the increase of the polymer absorption AOD due to UV irradiation [53]. 

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