The fraction of singlet spin-correlated pairs

The fraction of singlet-correlated pairs is a very important characteristic of primary radiation chemical events. The scale of magnetic field effect as well as the product composition are the functions of this quantity. 

In irradiated solutions this fraction is substantially below unity because of the complex structure of the radiation track. While isolated pairs are believed to be singlet correlated, for a double pair, the fraction might be as low as 1/2 due to cross recombination. For multi-pair spurs, the spin correlation might be completely lost. 

There are several approaches [4, 25] that try to determine the fraction of spin-correlated radical ion pairs in radiolysis. The transient emission and absorption [25] suffer, however, from the lack of exact data on the extinction coefficients and luminescence quantum yields of intermediate products. The magnetic field effect technique [4] is more straightforward. However, it requires a detailed knowledge of spin evolution in zero field which is a problem in most cases. 

When applying the quantum beats technique, it is essential that only the singlet-correlated pairs contribute to the oscillating component of spin evolution. Therefore, for systems with a single oscillation frequency, the singlet state population Ps t) is determined by 

The results for different alkanes irradiated by fast electrons are compared in Table 2 with those obtained by other techniques. To this end, the fraction of singlet recombination /s is given in the Table for all cases. This value contains an extra singlet contribution from uncorrelated pairs. 

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