Pulse radiolysis methods

A precursor of the studies on electron transfer reactions between short-lived radicals and colloidal particles was the development of a fast pulse radiolysis method to measure. the polarograms of radicals in the 10 s range . After considerable information had been acquired about the electron transfer reactions of a few dozen radicals at the mercury electrode, this compact electrode was replaced by metal colloids somewhat later, by semiconductor colloids These studies led to the detection of the electron-storing properties of certain colloids and of reactions of the stored electrons. 

Aldrich, J.E., Cundall, RB., Adams, G.E. and Willson, RL. (1969). Identification of essential residues in lysozyme a pulse radiolysis method. Nature 221, 1049-1051. 

Bors et al. [175] determined the rate constants and equilibrium constants for the reactions of flavonoids with ascorbate (Reaction (18)) by a pulse-radiolysis method and on their basis calculated the one-electron oxidation potentials of flavonoids (Table 29.9). 

Q = quinones) have been measured by pulse radiolysis methods. From the results a self-exchange rate constant of 1.1 x 10 s and a reduction potential of -1-0.048 V vs. NHE for Os /Os have been... 

Development of the industrial process for electrochemical conversion of acrylonitrile to adiponitrile led to extensive investigation into the mechanism of the dimerization process. Reactions of acrylonitrile radical-anion are too fast for investigation but the dimerization step, for a number of more amenable substrates, has been investigated in aprotic solvents by electrochemical techniques. Pulse-radiolysis methods have also been used to study reactions in aqueous media. 

Rate constants in excess of 10 M s are determined by pulse-radiolysis methods [4, 5]. High-energy irradiation of a solution containing the substrate and an excess of the aromatic species, generates the aromatic radical-anion. The decay of this by electron transfer to the substrate is followed using uv-spectroscopy and affords a rate constant for the second-order process. Slow rates of electron transfer are determined by adding the substrate to a solution of the aromatic radical-anion and following the reaction by conventional methods. 

Rate constants for the protonation of radical-anions in dimethylformamide by added phenol can be determined by electrochemical techniques [8], Pulse radiolysis methods have been used to measure the rate constants in an alcohol solvent. This technique generates the radical-anion on a very short time scale and uv-spectroscopy is then be used to follow the protonation of this species to give the neutral radical with different uv-absorption characteristics [9]. In the case of anthracene, the protonation rate is 5 x 10 M" s with phenol in dimethylformamide and 5 x 10 s in neat isopropanol. Protonation by hydrogen ions approaches the diflusion-controlled limit with a rate constant of 10 M s in ethanol [9]. 

Reactions of Low-Energy Electrons, Ions, Excited Atoms and Molecules, and Free Radicals in the Gas Phase as Studied by Pulse Radiolysis Methods... 

Advances in pulse radiolysis studies in the gas phase have been summarized in several review papers. In a comprehensive review by Sauer [4], a review presented by Firestone and Dorfman [5] in 1971 was referred to as the first review on gas-phase pulse radiolysis. Experimental techniques and results obtained were summarized by one of the present authors [6], with emphasis on an important contribution of pulse radiolysis to gas-phase reaction dynamics studies. Examples were chosen by Sauer [7] from the literature prior to 1981 to show the types of species that were investigated in the gas phase using pulse radiolysis technique. Armstrong [8] reviewed experimental data obtained from gas-phase pulse radiolysis together with those from ordinary steady-state radiolysis. Advances in gas-phase pulse radiolysis studies since 1981 were also briefly reviewed by Jonah et al. [9], with emphasis on an important contribution of this technique to free radical reaction studies. One of the present authors reviewed comprehensively the gas-phase collision dynamics studies of low-energy electrons, ions, excited atoms and molecules, and free radicals by means of pulse radiolysis method [1-3]. An important contribution of pulse radiolysis to electron attachment, recombination, and Penning collision studies was also reviewed in Refs. 10-15. 

In this chapter, firstly, a very brief survey is given of recent advances in such studies as classified according to the detection technique of transient species in pulse radiolysis. Secondly, examples are chosen from our recent investigations, with special emphasis on the important contributions of pulse radiolysis methods to gas-phase collision dynamics one is electron attachment, the other is Penning ionization and related processes. The detection techniques and corresponding reaction processes, together with major references, are given below ... 

The de-excitation rate constants of He(2 S) and Ne( Po, Pi, and P2) by various atoms and molecules were obtained at room temperature using a pulse radiolysis method [125,134-136]. 

A little more complicated system is the de-excitation of He(2 P) by Ne, where the deexcitation is dominated by the excitation transfer and only a minor contribution from the Penning ionization is involved. The experimental cross section obtained by the pulse radiolysis method, together with the numerical calculation for the coupled-channel radial Schrodinger equation, has clearly provided the major contribution of the following excitation transfer processes to the absolute de-excitation cross sections [151] (Fig. 15) ... 

As a brief conclusion of this section, the cross-section measurements for the deexcitation of excited rare gas atoms have been best performed using the pulse radiolysis method. The pulse radiolysis method has provided not only the most reliable cross sections... 

In the gas phase at one to several atmospheric pressures, electron-ion recombination processes have been studied with a pulse-radiolysis method, and observed recombination rate constants kj are expressed by,... 

It is known that more than 30 reactions are needed to reproduce the radiation-induced reactions occurring in pure water. Intensive measurements with a pulse radiolysis method have been done at elevated temperature up to 300°C [25 2], and the temperature dependence of some reactions does not exhibit a straight line but a curved one in Arrhenius plot. These examples are the reactions of the hydrated electron with N2O, NOJ, NO2, phenol, Se04, 8203 , and Mn [33,35], and two examples, egq + NOJ and ejq -i- NOJ, are shown in Fig. 2. The rate constant for the reaction of hydrated electron with NOJ is near diffusion-controlled reaction at room temperature and is increasing with increasing temperature. Above 100°C, the rate does not increase and reaches the maximum at 150°C, and then decreases. Therefore the curve is concave upward in Arrhenius plot. 

It seems to the present authors that the above-mentioned scheme of the initiation process in the glass matrices can be extended, at least, to the radiation-induced ionic polymerizations in liquid solutions at higher temperatures. This will be verified by rapid techniques of measurement, such as the pulse radiolysis method. 

The opticol absorption spectra of the solvated electron have now been reported for a number of organic liquids. The chemical reactivity in the aliphatic alcohols has been studied by the pulse radiolysis method. The absorption maxima for a series of five aliphatic alcohols are in the visible to near infra-red. These maxima show a red shift with decrease in the static dielectric constant. The solvated electron undergoes reactions of electron-ion combination, electron attachment, and dissociative electron attachment. Absolute rate constants have been determined for these reactions. 

The pulse radiolysis method has been described in detail in some of the early papers (22, 22), in a brief review of the subject (23), and in a current comprehensive review (14). It is, in brief, a fast reaction method in which the external perturbation applied to the system is a microsecond pulse of electrons. The current is sufficiently high to produce an instantaneous concentration of transient species high enough to be observed by fast measurement of the optical absorption. Spectra may be recorded either photographically or spectrophotometrically. The kinetics are studied by fast spectrophotometry. Since a perturbing pulse as short as 0.4 /xsec. has been used, the time resolution has approached 10-7 sec. The flash photolysis method used in some of the other studies (27, 15) has been reviewed in detail (24). 

In a pulse radiolysis study of ethylenediamine soon to be published, Anbar and Hart (2) find a conflicting result. These authors report a single broad absorption band with a maximum at 9200 A. The absorption curve continues to fall off with increasing wavelength toward 11,500 A., the sensitivity limit of their detector. In this system the counter-ion is the molecular ion of the solvent, and no alkali metal atom is present. The result raises a serious question about the assignment of the 13,000 A. band to the solvated electron and calls for further study of these systems by the pulse radiolysis method. 

It would appear at this stage that a good deal of useful information has yet to be obtained by the pulse radiolysis method concerning the absorption spectra of the solvated electron in various organic liquids. Such data would help to remove uncertainties regarding the assignment of bands and would serve as criteria for the validity of developing models. 

Radiation-induced reactions include photo-induced reactions or those initiated by radicals generated by an electron beam—the pulse radiolysis method. Reactions initiated by a light signal clearly can be of conventional time range or rapid. The latter... 

Some new trends can be recognized in the points such as the interaction of short-lived active species in some spatial distributions measured by spin echo and pulse radiolysis methods. The application of polymers for drug-delivery systems is here discussed with reference to low temperature radiation polymerization techniques. Ion beam irradiation of polymers is also reviewed for which further research is becoming important and attractive for so-called LET effects and high density excitation problems. In the applied fields the durable polymers used in strong and dense irradiation environments at extremely low temperature are here surveyed in connection with their use in nuclear fusion facilities. 

Pulse radiolysis is a powerful tool for the creation and kinetic investigation of highly reactive species. It was introduced to the field of radiation chemistry at the end of the 1950s and became popular in the early 1960s. Although the objects of this modern technique were, at first, limited to solvated electrons and related intermediates, it was soon applied to a variety of organic and inorganic substances. As early as 1964, ionic intermediates produced by electron pulses in vinyl monomers were reported for the first time. Since then, the pulse radiolysis method has achieved considerable success in the field of polymer science. 

The pulse radiolysis method is a powerful means of studying the kinetics in radiation chemistry. We investigated the ion beam interaction with polystyrene using this method. It is a unique system, because pulse radiolysis is usually an electron pulse radiolysis. 

Very recently. LET effects on fluorescence lifetimes of low molecular polyethylene model compounds (n-alkane) have been studied by many kinds of pulse radiolysis - methods such as electron beam, ion beam and synchrotron radiation (SR) [40] pulse radiolysis techniques [41]. Figure 10 shows time profiles of the fluorescence from neat n-dodecane liquids irradiated many kinds of radiation with different LET. The fluorescence lifetimes from irradiated neat... 

Misonidazole and its azo- and azoxy derivatives have been investigated in detail by polarography, cyclic voltammetry, and pulse radiolysis methods [947],... 

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