Photoionization charge separation

Finally, solute radical ions can be generated by light-induced, one-photon or multiphoton ionization of their parent compounds (Chaps. 5 and 16). This approach is particularly useful in the ultrafast studies of short-lived, unstable radical ions that aim to unravel their solvation, recombination, reaction, and vibrational relaxation dynamics of the primary charges (see, e.g., Chap. 10). Whereas the time scale of radiolytic production of secondary ions is always limited by the rate with which the primary species reacts with the dispersed parent molecules, light-induced charge separation can occur in <100 fsec. There are many studies on photoionization of solute molecules in liquid solutions we do not intend to review these works. 

Suppose that we now fill the space between our planar electrodes with a solution. First let us choose a pure solvent of low dielectric constant (e.g., hexane) with no charge carriers present. How does this compare to the previous situation First, we are limited in the field we can achieve before breakdown in the dielectric occurs. It is virtually impossible to field ionize a molecule in such a medium. On the other hand, photoionization can be accomplished with the field providing an impetus to charge separation. As in a vacuum, the photoionized molecule and the electron are accelerated in opposite directions, but now a terminal velocity is readily achieved depending on the viscous drag of each charged particle. The solvated photoelectron will, of course, move far more rapidly than the ion. 

The surprising occurrence of photoionization in aryl carboxylic acids may be explained by the transfer of excitation energy from the ring system to the carboxyl group to give an intermediate with charge separation at the side-chain terminus—e.g.,... 

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. 

The ability of micelles to enhance photoionization yields of hydrophobic molecules was demonstrated in the early 1970s. Thus, the photoionization yields of pyrene [59], phenothiazine [60] and tetramethylbenzidine [61] cations increased when these molecules were encapsulated in anionic micelles. The effect was attributed to efficient escape of electrons from the geminate charge-separated species formed within the micelle, which is accelerated by the anionic interface. The negative micellar surface imposes an electrostatic barrier between the cations, which remain with the micelle, and the aqueous electron in the bulk water phase, thus increasing the lifetimes of the photoredox products. 

Photoinduced Electron Transfer and Photoionization. Photoinduced electron transfer reactions have attracted much attention in the last few years because of their relevance to energy conversion and storage. Cyclodex-trins were used mainly to preorganize the donor-acceptor pair and to affect the reciprocal orientation and the distance of the radical products in an effort to improve the overall efficiency of the charge separation. 

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. 

A less important charge separation process in the cluster model is photoionization and escape of electrons, leading to a net positive charge on the sample. This can only take place in a thin layer at the top of the sample. From the electron capture cross sections, it can be shown that only a thin (few nanometers) layer emits electrons. Below this they are captured to form matrix anions (leading to further secondary products). A large net positive charge excess caimot build up in the bulk of the sample. 

The total process of photoionization can be divided into two sequential stages accumulation of charges and their recombination/separation. The latter stage is represented by two first terms on the RHS of Eq. (3.290). Setting WR = D = 0, one stops the recombination and conserves the ions at the very same place where they were produced, by forward electron transfer. In such a special case only the last term remains on the RHS of Eq. (3.290) and its integral represents merely the accumulation of ions over time at any given distance ... 

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