Pulse Radiolysis Experimental Techniques

The optical absorption spectra of sulfonyl radicals have been measured by using modulation spectroscopy s, flash photolysis and pulse radiolysis s techniques. These spectra show broad absorption bands in the 280-600 nm region, with well-defined maxima at ca. 340 nm. All the available data are summarized in Table 3. Multiple Scattering X, calculations s successfully reproduce the experimental UV-visible spectra of MeSO 2 and PhSO 2 radicals, indicating that the most important transition observed in this region is due to transfer of electrons from the lone pair orbitals of the oxygen atoms to... 

The first experimental measurements of the time dependence of the hydrated electron yield were due to Wolff et al. (1973) and Hunt et al. (1973). They used the stroboscopic pulse radiolysis (SPR) technique, which allowed them to interpret the yield during the interval (30-350 ps) between fine structures of the microwave pulse envelope (1-10 ns). These observations were quickly supported by the work of Jonah et al. (1973), who used the subharmonic pre-buncher technique to generate very short pulses of 50-ps duration. Allowing... 

The systems that we investigated in collaboration with others involved intermolecular and intramolecular electron-transfer reactions between ruthenium complexes and cytochrome c. We also studied a series of intermolecular reactions between chelated cobalt complexes and cytochrome c. A variety of high-pressure experimental techniques, including stopped-flow, flash-photolysis, pulse-radiolysis, and voltammetry, were employed in these investigations. As the following presentation shows, a remarkably good agreement was found between the volume data obtained with the aid of these different techniques, which clearly demonstrates the complementarity of these methods for the study of electron-transfer processes. 

The kinetics of myoglobin oxidation and reduction have been studied by a variety of experimental techniques that include stopped-flow kinetics, pulse radiolysis, and flash photolysis. In considering this work, attention is directed first at studies of the wild-type protein and then at experiments involving variants of Mb. 

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. 

Electron attachment to solutes in nonpolar liquids has been studied by such techniques as pulse radiolysis, pulse conductivity, microwave absorption, and flash (laser) photolysis. A considerable amount of data is now available on how rates depend on temperature, pressure, and other factors. Although further work is needed, some recent experimental and theoretical studies have provided new insight into the mechanism of these reactions. To begin, we consider those reactions that show reversible attachment-detachment equilibria and therefore provide both free energy and volume change information. 

Separation of bulk and surface properties in macroscopic semiconductors is less than straight forward and requires highly sensitive experimental techniques. In contrast, the large surface-to-volume ratios in nanosized semiconductor particles render the examination of surface processes in and/or on these colloids to be experimentally feasible. Advantage has been taken of pulse radiolysis to inject electrons (in aqueous, N20-saturated solutions which contained 2-propanol see Eqs. 22,23, and 25) or holes (in aqueous, N20-saturated solutions which did not contain 2-propanol see Eqs. 22 and 23) into nanosized semiconductor particles [601, 602], Electron injection into CdS particles, for example, decreased the extinction coefficient at 470 nm (the absorption onset) by — 5 x 104 M-1cm-1 (Fig. 98) [576]. Hole injection resulted in the appearance of a transient absorption band in the long-wavelength region and in much less... 

The experimental methods used in the investigation of the hydrated electron include competition kinetics and product analysis, as well as pulse-radiolysis and flash-photolysis techniques. All these methods have... 

Relative rate constants for reactions of OH have been measured by competitive methods in 7-irradiated solutions where product formation or reactant destruction have been monitored. These methods have generally been of low accuracy and can sometimes be misleading because of the possible complications in the processes between the initial reaction and the final products. Several competitors that allow the competition at the initial step to be followed became available for use with the pulse radiolysis technique in 1965 (Adams et al., 1965). Most of the rate constants for OH reported in the literature have been determined by this method. Numerous rates have also been determined by pulse radiolysis in an absolute way, i.e. by directly observing the kinetics of the formation of transient absorption or of the decay of the parent compound absorption. Direct observation of OH (or of H) by pulse radiolysis cannot help in obtaining reaction rates, because the absorption is in the far ultraviolet and one can observe only the tail of this absorption which at 200-250nm has a very low extinction coefficient ( 500 M -1 cm-1) (Pagsberg et al., 1969). A review of the experimental methods and summary of the rate constants of OH reactions has recently been published (Dorfman and Adams, 1973) and another compilation of rate constants is currently being prepared (Farhataziz and Ross, 1975). 

It is from these perspectives that we have reviewed the pulse radiolysis experiments on polymers and polymerization in this article. The examples chosen for discussion have wide spread interest not only in polymer science but also in chemistry in general. This review is presented in six sections. Section 2 interprets the experimental techniques as well as the principle of pulse radiolysis the description is confined to the systems using optical detection methods. However, the purpose of this section is not to survey detail techniques of pulse radiolysis but to outline them concisely. In Sect. 3, the pulse radiolysis studies of radiation-induced polymerizations are discussed with special reference to the initiation mechanisms. Section 4 deals with applications of pulse radiolysis to the polymer reactions in solution including the systems related to biology. In Sect. 5 reaction intermediates produced in irradiated solid and molten polymers are discussed. Most studies are aimed at elucidating the mechanism of radiation-induced degradation, but, in some cases, polymers are used just as a medium for short-lived species of chemical interest We conclude, in Sect. 6, by summarizing the contribution of pulse radiolysis experiments to the field of polymer science. 

In science, one builds models based on experimental data and one then attempts to verify these models. Experiments using isotope sources provided data that were explained with microscopic models. However, these models could only be indirectly tested because entities that took part in these reactions were too short-lived to be directly observed. Photochemistry had the same problems and to solve it, the techniques of sector photolysis and flash photolysis were developed. The attempts to create sector radiolysis were only marginally successful. The analog of flash photolysis, pulse radiolysis, was developed in three laboratories almost simultaneously and the first publications appeared within a month of each other. ° ... 

The experimental methodology in radiation-induced oxidation of benzene systems involved the measurement of rate constants and the transient absorption spectra by pulse radiolysis and the determination of yields of hydroxylated products on oxidation of the hydroxycyclo-hexadienyl radicals under steady-state conditions. The two commonly used oxidants — K3Fe(CN) "and IrCl " — convert quantitatively the OH adducts to the corresponding phenolic products. Thus, the pulse radiolysis technique in combination with product analysis using analytical techniques such as UV-VIS spectroscopy, HPLC, GC-MS, etc. under steady state conditions has provided valuable information in the understanding of the oxidation reaction mechanism of aromatics in... 

This review covers the interaction of radicals generated from low LET radiation in water with protein components, proteins and, finally, the metallocenters themselves. It commences with a discussion of the experimental techniques that have been the most amenable to these studies, those of gamma and pulse radiolysis. It then addresses, very generally, the radiolysis of the building blocks of proteins, amino acids, and follows with the radiolysis of the proteins themselves. In both cases, the discussion is limited to radiolysis of dilute solute in water, where the initial radiation is absorbed totally by water and does not directly interact with the amino acids/proteins. 

Generation of peroxynitrite by pulse radiolysis involves very tricky adjustments of concentrations of additives and many other experimental conditions. Radiolysis of aerated aqueous solutions containing sodium formate and potassium nitrite in the pH range 3 to 10 produced peroxynitrite according to Eqs. (21) to (23), (40), (41) and (11). For this, initial concentrations of nitrite and formate have to be adjusted in such a way that the radiolytically produced concentration of the radicals of [0 ] -l- [HOj] > NO. However, reactions of peroxynitrite with antioxidants are not generally studied by pulse radiolysis technique. Chemical methods such as ozonolysis of alkaline sodium azide solutions are commonly used to produce peroxynitrite in large quantities and its reactions studied by mixing techniques. 

Saveant and coworkers [87-89, 123-125] introduced this approach for determining kc in linear sweep voltammetry. The experimental voltammograms were compared with simulations to determine cleavage rate constants for radical anions ranging from 10 to 5 x 10 s . The indirect approach is therefore a very useful supplement to direct techniques such as cyclic voltammetry [126-130], pulse radiolysis [131-135], and flash photolysis [136], which have proven to be convenient and effective tools when the cleavage rate constant of RX is lower than 10 s . ... 

Pulse Radiolysis . W.L. Waltz in Photoinduced Electron Transfer, Part B. Experimental Techniques and Medium Effects (Eds. M.A. Fox, M. Chanon), Elsevier, Amsterdam, 1988, Chap. 2.2, pp. 57-109. 

The role of FNO in atmospheric chemistry is related to the destruction of stratospheric ozone.244 246 The reaction F + NO has been extensively studied using different experimental techniques, including mass spectrometry, chemiluminescence, ESR, IR, and UV spectroscopy.241,247 251 The pressure dependence of the kinetics of FNO formation was recently studied by Pagsberg et al.25i in order to obtain the fall-off curve and the high- and low-pressure limiting rate constants. The reaction was initiated by pulse radiolysis of a SF6/NO gas mixture. In the presence of NO, the decay of the formed... 

A simplified view of the early processes in electron solvation is given in Figure 7. Initially, electron pulse radiolysis was the main tool for the experimental study of the formation and dynamics of electrons in liquids (Chapter 2), first in the nanosecond time range in viscous alcohols [23], later in the picosecond time range [24,25]. Subsequently, laser techniques have achieved better time resolution than pulse radiolysis and femtosecond pump-probe laser experiments have led to observations of the electron solvation on the sub-picosecond to picosecond time scales. The pioneering studies of Migus et al. [26] in water showed that the solvation process is complete in a few hundreds of femtoseconds and hinted at the existence of short-lived precursors of the solvated electron, absorbing in the infrared spectral domain (Fig. 8). The electron solvation process could thus be depicted by sequential stepwise relaxation cascades, each of the successive considered species or... 

Application of pulse-radiolysis techniques (see Sect. 1.4 for more details on the experimental set-up) revealed that the following intramolecular (Eqs 1.6 and 1.7) and intermolecular (Eq. 1.8) electron-transfer reactions, where cyt c represents cytochrome c, all exhibit a significant acceleration with increasing pressure. The reported volumes of activation are —17.7 0.9, -18.3 0.7, and —15.6 0.6 cm mol respectively, and clearly demonstrate a significant volume collapse upon reaching the transition state [63]. 

This chapter will begin by delineating the capabilities of the two techniques, radiolysis and laser photoexcitation, for examination of factors that regulate ET rates. We will begin with a list summarizing the capabilities of the two techniques. Descriptions will be supplemented by reference to published work and examples from pulse radiolysis experiments in our laboratory, along with a brief description of the experimental setup. In this discussion we shall see... 

This section discusses experimental techniques used in our laboratory for pulse radiolysis, with emphasis on the study of ET reactions and with some general comments on techniques. 

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