Geminate recombination contact approximation

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],... 

The geminate recombination is actually controlled by diffusion, if the initial separation of ions is so large that their transport from there to the contact takes more time than the reaction itself. The exponential model excludes such a situation from the very beginning, assuming that ions are bom in the same place where they recombine. Thus, EM confines itself to the kinetic limit only and fixes Z = z = const. The kinetic recombination in the contact approximation does not imply that the starts are taken from the very contact. If they are removed a bit and diffusion is fast, the recombination is also controlled by the reaction and its efficiency Z = qz is constant although smaller than in EM. 

In this limit geminate recombination becomes quasistatic and can be accomplished before the particles leave their initial positions. This is the static limit missed in the contact approximation ... 

In Figure 3.57 we show almost the full identity of the results obtained with IET and UT. They demonstrate the well pronounced maximum appearing when i > is shorter than the characteristic time of the subsequent geminate recombination [22]. In the contact approximation the results are qualitatively the same but the ionization quantum yield / is half as much as in distant theories (see Table III). This was expected because at such a short To a significant fraction of ions are produced during the initial static ionization that is missed in the contact approximation. 

The difference between the two screened potentials is illustrated by Figure 13. Both predict a remarkably similar value at the contact distance, a. The main difference between the potentials is in their width rather than their depth. The narrower Naive Approximation potential restricts the proton to be closer to the parent anion thus leading to more geminate recombination and larger probabilities for the bound (acidic) state. 

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