Charge separation quantum yield geminate ionization

When the ionization is irreversible, tpa = 1, the charge separation quantum yield is equal to cpp, which becomes the share of RIPs that escaped geminate recombination into the ground state. In more general case, when the reverse electron transfer into the excited state can not be neglected, this is the fraction of ions that escaped any recombination, in either the ground or excited states. To clarify this point, let us illustrate it by an example of contact electron transfer. 

Here an important definition for the rate constant of free carrier production, k, is given. The latter differs from the ionization rate constant by a multiplier equal to the charge separation quantum yield tpm, obtained in the Markovian approximation. This difference indicates that the number of photogenerated ions that avoid geminate recombination and become free is less than their total amount, cpm < 1. 

Neither the maximum nor the descending branches of the upper curves, representing geminate recombination, are reproduced in the Markovian theory. It predicts the monotonous ion accumulation and still further decrease in the ionization quantum yield /. This is because the Markovian theory does not account for either static or subsequent nonstationary electron transfer. When ionization is under diffusional control, both these are faster than the final (Markovian) transfer. EM is a bit better in this respect. As a non-Markovian theory, it accounts at least for static ionization and qualitatively reproduces the maximum in the charge accumulation kinetics. However, the subsequent geminate recombination develops exponentially in EM because the kinematics of ion separation is oversimplified in this model. It roughly contradicts an actual diffusional separation of ions, characterized by numerous recontacts and the power dependence of long-time separation kinetics studied in a number of works [20,21,187],... 

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. 

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