Presolvated electron

It can be noticed that the solvation time of the trapped electron (presolvated) is surprisingly in excellent agreement with an extrapolation of measurements done in 3 M CaCl2 cooled to 236 °K (Garett, 1980). In this work, where an activation energy was assumed, a 0.2 ps time was anticipated for the solvated electron time in water at ambient temperature. We may wonder... 

Hentz and Kenney-Wallace (1974) obtained the evolution of es yield in some common alcohols by comparison with the corresponding yield of ehand extrapolated the results to 30 ps. The picosecond data for the alcohols were obtained from the work of Wolff et al (1973) and Wallace and Walker (1972) the nanosecond work was in substantial agreement with Baxendale and Wardman (1971). The evolution of the es yields in the common alcohols shows considerable decay from the picosecond to nanosecond regime and a comparable decay from the nanosecond to microsecond time scales. However, the microsecond yields are also probably somewhat larger than previously reported, especially for methanol and ethanol (see Dorfman, 1965). In agreement with this, Lam and Hunt (1974) report es yields in aliphatic alcohols at -100 ps to be greater than 3. Nevertheless, there is room for neutralization of the dry electron in the presolvated state. 

Anion solvation has been studied by observing the shift in the absorption spectrum of the benzophenone anion in various solvents and as a function of temperature. The benzo-phenone anion was formed from the reaction of the benzophenone molecule and a precursor to the solvated electron. Approximately 0.25 M benzophenone is put into the solution so that all the presolvated electrons will react with the benzophenone and virtually none will form the solvated electron. This process occurs much more quickly than the solvation processes that are observed [14,20]. 

The photodynamics of electronically excited indole in water is investigated by UV-visible pump-probe spectroscopy with 80 fs time resolution and compared to the behavior in other solvents. In cyclohexane population transfer from the optically excited La to the Lb state happens within 7 ps. In ethanol ultrafast state reversal is observed, followed by population transfer from the Lb to the La state within 6 ps. In water ultrafast branching occurs between the fluorescing state and the charge-transfer-to-solvent state. Presolvated electrons, formed together with indole radicals within our time resolution, solvate on a timescale of 350 fs. 

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). 

The population transfer between the excited La and Lb states of 6.5 2 ps is determined from indole dissolved in ethanol and cyclohexane. In water the appearance of presolvated elections within the time resolution of our experiment and the fluorescence quantum yield of 0.09 indicate an ultrafast branching between the fluorescing state and the CTTS state immediately after photoexcitation. The solvation of the generated electrons shows the same initial dynamics of 350 fs for solvated indole and for pure water but differs on longer timescales. 

The water radical cation, produced in reaction (1), is a very strong acid and immediately loses a proton to neighboring water molecules thereby forming OH [reaction (3)]. The electron becomes hydrated by water [reaction (4), for the scavenging of presolvated (Laenen et al. 2000) electrons see, e.g., Pimblott and LaVerne (1998) Pastina et al. (1999) Ballarini et al. (2000) for typical reactions of eaq, see Chap. 4], Electronically excited water can decompose into -OH and 11- [reaction (5)]. As a consequence, three kinds of free radicals are formed side by side in the spurs, OH, eaq , and H . To match the charge of the electrons, an equivalent amount of ED are also present. 

Tran-Thi, T.H., Koulkes-Pujo, A.M., Sutton, J., Anitoff, O. 1984, Pulse radiolysis of amides and their aqueous organic mixtures. Effect of the environment on the reactivity of the presolvated electrons. Radiat. Phys. Chem. 23(1-2) 77-87. 

Fig. 5.5 Temperature dependencies for Ps formation probability, electron solvation time and positron lifetime in n-propanol. With changing temperature from 150 K to 300 K the electron solvation time in n-propanol varies within 5 orders of magnitude. At the highest studied temperature r approaches 10 ps. At low temperatures it exceeds e+ lifetime T2 more than 1000 times. If e had really contributed to Ps formation, Ps yield would have to decrease. However, it even slightly increases. This fact favors the presolvated electron as the main precursor of Ps formation.

Correlation between Ps inhibition constants and l/cf7 stressed earlier in [20] is now proved by a large number of data [45, 46, 47, 48]. This presents strong experimental evidence that just a presolvated electron is the main precursor of the Ps. 

Long FH, Lu H, Eisenthal KB (1990) Femtosecond studies of the presolvated electron An excited state of the solvated electron Phys Rev Lett 64 1469-1472. 

The Reactions of the Presolvated Electron with the 2-Deoxyribose Phosphate Moiety... 

While e does not react with the sugar-phosphate moiety, presolvated electrons that retain some excess energy may also undergo dissociative attachment to the phosphate group of DNA [reactions... 

The primary cathodic step of hot electron-induced electrochemiluminescence (HECL) was postulated to be an injection of hot electrons into aqueous electrolyte solution by tunnel emission through an insulating barrier, followed by reduction reactions induced either by presolvated hot electrons or by fully hydrated electrons [37, 38]. The introduction and details of this kind of hot electron electrochemistry and HECL has been reviewed very recently [33] and only short descriptions of the basic features are given here. 

It is likely that the cathodic reductions at these electrodes can be produced simultaneously by different types of electrons (1) presolvated hot electrons, (2) hydrated electrons, (3) heterogeneously transferred electrons from the conduction band of the insulating film at insulating film/electrolyte interface, and (4) probably also from the surface states of the insulating film [35, 36, 38,50,54,55]. Normally, only presolvated hot electrons or hydrated electrons are sufficiently energetic to participate in reaction pathways leading to ECL in aqueous solutions. 

In these expressions, 1 is the interaction length, A the final solvated electron concentration, 6 ( p) the molar extinction coefficient of the solvated (presolvated) species at wavelengthA.C (T ) which is a third order correlation between the probe at and the pump pulse is normalized. The total absorbance of the test pulse is therefore Aa t) = AAp jj + A ( T ), expression which takes into account all the population evolution occuring during the excitation and probe. The instantaneous kinetics in n-heptane determines strictly c t ) in shape and position along the time axis. 

The infrared data obtained in pure liquid water and anionic aqueous solutions are in agreement with an initial electron-medium interaction inducing the formation of a negatively charged cluster i.e. a presolvated state... 

R. Muller,/. Chem. Phys., 129, 027101 1-2 (2008). Comment on Resonant Dissociative Electron Transfer of the Presolvated Electron to CCl in Liquid Direct Observation and Lifetime of the CC1 Transition State [J. Chem. Phys. 128, 041102 (2008)]. 

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