Photoionization geminate recombination

If one studies only the fluorescence quenching by irreversible bimolecular ionization (3.52), there is seldom any need to trace the fate of the charged products. On the contrary, those who are interested in photoinduced geminate recombination (3.188) rarely care about the kinetics of ionization, its quenching radius, and all the rest studied in Section III. All that they need to obtain the charge separation yield is the initial ion distribution mo(r), prepared by photoionization. However, the latter is scarcely so simple as in Eq. (3.201), which is usually favored. Even so, the initial separation ro is not a fitting parameter but the characteristic interion distance, which is dependent on the precursor reaction of photoionization. 

In dealing with the reversible photoionization, it is very important to differentiate between the steady-state excitation accompanied by stationary fluorescence and free-ion production and the 8-pulse excitation followed by geminate recombination/separation of ions whose yields and subsequent time evolution are studied. They should be considered separately, step by step. 

The photoionization of pure water would be a valuable source of e aq in that, for those electrons which escape immediate geminate recombination, it would provide a uniformly dispersed distribution over a small volume. For most of the other methods described such a goal is virtually impossible. 

Photoionization generates the initial condition for geminate recombination. Now the difference of a few angstroms between Rq and R can be of crucial importance, increasing the survival chances of the newborn ions attracted by the Coulomb potential. The distribution of ions provided by Eq. (9.125) can be used as the initial condition only if ionization is so fast that it is completed before the recombination actually starts. In general, one should consider the backward and forward ET simultaneously, by including a nonlocal source term into the evolution equation for the ion pair probability distribution (r(r, t), as shown by Burshtein et al. [330] and by Dorfman and Payer [331]. As a result, one obtains for the evolution of the total population I t) of ion pairs. 

The value of the quantum yield of the primary charge separation products depends upon the ratio of the carriers separation and geminate recombination rate constants. The threshold (minimal energy) of the photoionization is determined by the energy necessary for an electron to leave any given molecule. As the photoionization takes place at the instant of the photon absorption the medium has no time to solvate the photoionization products (the electronic polarization of the medium, not the orientational, is important), so the effect of the medium upon the ionization threshold is relatively weak. For studies of the photoionization processes, the electron traps located in the bulk phase are usually used. 

The photoionization of phenothiazine and its derivatives in SDS micelles, which is not observed in homogeneous solutions and the premicellar region was studied by Gratzel et al. [106-109]. They suppose that the anionic micelles decrease the ionization potential of the phenothiazine derivatives and the photoionization includes an electron tunnelling from the micelle into the solution through the Stem layer. However, there are no direct data for these systems to give a value of the photoionization threshold. A drastic increase of the photoionization quantum yield at the transition to anionic micelles could also be caused by a decrease of the role of the geminate recombination. 

This reaction is usually considered in its own right, separately from the precursor photoionization. The latter creates the initial conditions for the pair distribution m(r, t) which describes the time and space evolution of the geminate pair [D+ A-]. All theories of this reaction differ from one another by choice of initial conditions and the model of the reaction layer where recombination takes place. 

Ionization was also achieved in cationic micelles in the presence of the electron scavenger naphthoquinone sulphonate adsorbed at the periphery of the micelle. In the absence of scavenger the probability of geminate ion recombination is high and therefore the photoionization efficiency is low. 

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