Contact approximation charge separation

The theory of geminate recombination experienced a similar evolution from primitive exponential model and contact approximation [19,20], to distant recombination carried out by backward electron transfer [21], However, all these theories have an arbitrary parameter initial separation of reactants in a pair, / o. This uncertainty was eliminated by unified theory (UT) proposed in two articles published almost simultaneously [22,23], UT considers jointly the forward bimolecular electron transfer and subsequent geminate recombination of charged products carried out by backward electron or proton transfer. The forward transfer creates the initial condition for the backward one. This is the distribution of initial separations in the geminate ion pair/(ro), closely analyzed theoretically [24,25] and inspected experimentally [26,27], It was used to specify the geminate recombination kinetics accompanied by spin conversion and exciplex formation [28-31], These and other applications of UT have been covered in a review published in 2000 [32],... 

This maximum separates the branch where Z increases with diffusion from that where Z decreases with it. As we have seen in Figure 3.4067 ), the latter can never be reached if the forward and backward transfer rates are the same. In such a case the initial charge distribution is always more remote than the reaction zone, so that ions enter it mainly from outside, only in this case the popular contact approximation for recombination is a reasonable alternative provided... 

The exchange term represents the electrostatic interaction between the charge clouds. The transfer in fact occurs via overlap of the electron clouds and requires physical contact between D and A. The interaction is short range because the electron density falls off approximately exponentially outside the boundaries of the molecules. For two electrons separated by a distance in the pair D-A, the space part of the exchange interaction can be written as... 

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