Geminate recombination spin conversion

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],... 

As has been shown, the fitting of the linear viscosity dependence of chemiluminescence is completely different if the geminate recombination is considered alone or accompanied by the bulk reaction. In the former case the faster the spin conversion, the better, while in the latter case it can be set to zero provided the rate of electron transfer through the triplet channel is high enough. A similar alternative will be presented in the next section. There the combination of geminate and bulk reaction appears more preferable, especially because the spin conversion carried out by the hyperfine interaction is usually weak. 

The shape of the kernels (3.651) is rather obvious from the physical point of view. The kernels R and R are exactly the same as in Eq. (3.369) of the spinless theory. When spin conversion during encounter is negligible, the backward electron transfer to the ground state remains the single channel of geminate ion recombination. Therefore, the kernel R is 4 times smaller than... 

The first two represent the reversible ionization of the triplet excitations and accumulation of triplet RIPs. In the absence of the spin conversion, since there is no geminate recombination of triplet RIPs to the ground state these kernels are equal. R describes the recombination of triplet RIPs to the triplet excited states. The last kernel represents the recombination of ions to either the triplet or ground state, in proportion to the equilibrium weights of competing channels. 

The conversion of [D+ A-] from the singlet to triplet state allows the geminate pair to recombine not only in the ground state but also into the excited triplet product, 3D (Fig. 3.80). However, the quantum yield of the latter inspected in Section XI.B is never as large as necessary to make the ionization fully irreversible. This can be possible only if one takes into account the free ion recombination in the bulk. Since these ions meet with uncorrelated spins, 75% of the newly born RIPs appear in the triplet state from where they recombine fast and irreversibly into the same triplet product, 3D. This mechanism allows extending the Rehm-Weller diffusional plateau up to the border between exergonic and endergonic electron transfer at AG, = 0. 

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