Photoionization charge recombination

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. 

From the steady state fluorescence spectrum of indole in water a fluorescence quantum yield of about 0.09 is determined. Since the cation appears in less than 80 fs a branching of the excited state population has to occur immediately after photo excitation. We propose the model shown in Fig. 3a). A fraction of 45 % experiences photoionization, whereas the rest of the population relaxes to a fluorescing state, which can not ionize any more. A charge transfer to solvent state (CITS), that was also introduced by other authors [4,7], is created within 80 fs. The presolvated electrons, also known as wet or hot electrons, form solvated electrons with a time constant of 350 fs. Afterwards the solvated electrons show no recombination within the next 160 ps contrary to solvated electrons in pure water as is shown in Fig. 3b). 

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 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 ... 

The set of equations (3.326) was numerically solved neglecting the ion recombination to the ground state (Wr = 0) [185]. The solutions were used to calculate the total amount of charged and neutral products of photoionization that can be detected experimentally ... 

Fig. 1. Interstellar formation scheme illustrating the CH, CH, C H and higher hydrocarbon cycle. The left side of the reaction cycle pertains to tenous clouds (Uj, 100 cm ), whereas the right hand side is more appropriate to areas where is present, i.e. dense molecular clouds (n 10 -10 cm" ). The thick arrows indicate assumed preferential reaction paths leading to the higher order hydrocarbons. The following processes are involved (v, e) photoionization (v, H) photodissociation (e, v) radiative recombination (H) (Hj, v) radiative association (e, H), (e, Hj) dissociative electron recombination. (Hj, H) hydrogen abstraction reaction (C, H) charge exchange (M, M ) metal charge exchange metal = Mg, Fe, Ca, Na,...

The effect of efficient dissociative recombination is reflected in the daytime ionospheric charged particle densities shown in Fig. 2. In the FJ-region, 02 and NO+ are the most abundant ions, while in the F-region, the higher plasma densities due to increased photoionization rates dramatically reduces the abundance of all molecular ions and 0+ is the principal positively charged species. Figure 2 also depicts a typical metal ion density profile (Me" ") around 100 km. As will be outlined further, this profile can be... 

The reasons for the divergent effects of surface charge on TMB and ZnTPP photoionization yields in vesicular suspensions are unknown. Experimentally determined photoionization yields are complex quantities, which include as elementary processes primary ionization cross-section terms, dry electron escape probabilities, relatively complex electron hydration processes and recombination of various hy-... 

However, photoionization models show that such simple relations do not necessarily hold. For example, the charge transfer reaction 0++ + H° —s- 0+ + H+ being much more efficient than the Ne++ + H° —> Ne+ + H+ one, Ne++ is more recombined than 0++ in the outer parts of nebulae and in zones of low ionization parameter. 

Given that Jupiter s upper atmosphere consists mainly of molecular hydrogen, the major primary ion, which is formed by either photoionization or particle impact, is H J. H+ ions are also created by either dissociative ionization of H2 or by direct ionization of atomic hydrogen. At high altitudes, H+ can only recombine directly via radiative recombination, which is a very slow process. It was suggested some time ago that H+ could charge exchange, with H2 excited to a vibrational state v > 4. The vibrational distribution of H2 is not known, but recent calculations indicate that the vibrational temperature is elevated at Jupiter, but it is not clear how important this effect is. 

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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