Photoionization in Micellar Solutions

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. 

In a study of the biphotonic ionization of pyrene with a tunable laser, Wallace et al. [102] found that the ionization potential in SDS is lower than in the gas phase and in methyl alcohol. They applied the following expression for the ionization potential of a molecule in a medium  

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. 

The use of micellar solutions enables one to study in aqueous solutions the reactivity of substances which are insoluble in water in the absence of additives, in particular the photoionization and electrochemical oxidation of the photoxynthetic dyes - chlorophyll and bacteriochlorophyll [110-113]. 

Photopprotolytic dissociation occurs from a relaxed state and the Frank-Condon states are of no importance [114,115]. So the photoprotolytic dissociation is not affected by the energy of the incident photons but is determined by the pK value of the subsance in question in the excited state (pK ). 

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