Electrons in pulse radiolysis

Soon after the discovery of the absorption spectrum of the solvated electron in pulse radiolysis experiments (Chapter 2), the rates and mechanisms of its reaction with a wide variety of solutes was studied. Although it is a transient species, the solvated electron is a very important reducing agent. Indeed, its reduction potential is very negative the value of E°(H2O/e5 ) for the solvated electron in water is equal to -2.8 V with respect to the standard hydrogen electrode. The reaction rate constant and the probability of encounter with another species decide the so-called lifetime ofthe solvated electron, which therefore depends on the experimental conditions. 

In a recent paper some assignments made by Bobrowski and Grodkowski were questioned by Bhasikuttan et al [50]. These workers studied the reduction of a series of CV- and MG-type dyes by diphenylketyl radical and hydrated electrons in pulse radiolysis experiments. They showed that both MG and MGH have two absorption bands at 340 and 400 nm. Also, it was observed that the dye radicals exhibit second-order decay without bleach recovery. This suggested that the radicals probably dimerize rather than disproportionate. 

FIGURE 17.9 Temporal evolution of the G-value of aqueous electrons in pulse radiolysis measurements. 

Hayon23 studied the yields of ions and excited states in pulse radiolysis of liquid DMSO using anthracene as a solute to determine the yield of free ions and naphthalene as a solute to measure the yield of triplet excited states. Anthracene is known to react with solvated electrons to give the anthracene radical anion, A T... 

In pulse radiolysis studies, sulfonyl radicals have normally been generated by dissociative electron capture using sulfonyl chlorides as substrates, namely15. 

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 ... 

Figure 5 (a) Typical end-of-pulse absorption spectra obtained in pulse radiolysis of room temperature liquid acetonitrile (7-nsec fwhm pulse of 20 MeV electrons). The 500-nm peak is from anion-2 (dimer radical anion) the 1450-nm peak is from anion-1 (cavity electron), (b) Energy diagram and sketches of anion-1 and anion-2 (see the text). 

Figure 6 Comparison of experimental and predicted values of G(Pi) from electron scavenging as a function of the scavenger power fcio[Si]. Si = N2O ( ) CH3CI (A) C(N02)4 ( ) glycylglycine ( ) NO3" (O). The broken line is predicted from direct observation of the time dependence of G e ) in pulse radiolysis experiments. (From Ref. 49.) The solid line is the fit obtained by Pimblott and LaVerne [43] with the restriction that G°(e q) = 4.80 molecules (100 eV) ...

HO2 and 02 have characteristic absorption spectra with s ax 140 mol at 225 nm [83] and Smav = 189 mol at 245 nm [85], respectively, which are sufficiently intense to permit their reactions to be followed by direct observation in pulse radiolysis experiments. Both radicals are relatively unreactive with organic molecules [83], abstracting only weakly bonded hydrogen atoms in, for example, ascorbic acid, cysteine, and hydroquinone. Oj undergoes reversible electron transfer in its reaction with quinones (Q), which was used to establish its reduction potential [86] ... 

Absorption due to main intermediates such as polymer cation radicals and excited states, electrons, and alkyl radicals of saturated hydrocarbon polymers had not been observed for a long time by pulse radiolysis [39]. In 1989, absorption due to the main intermediates was observed clearly in pulse radiolysis of saturated hydrocarbon polymer model compounds except for electrons [39,48]. In 1989, the broad absorption bands due to polymer excited states in the visible region and the tail parts of radical cation and electrons were observed in pulse radiolysis of ethylene-propylene copolymers and the decay of the polymer radical cations were clearly observed [49]. Recently, absorption band due to electrons in saturated hydrocarbon polymer model compounds was observed clearly by pulse radiolysis [49] as shown in Fig. 2. In addition, very broad absorption bands in the infrared region were observed clearly in the pulse radiolysis of ethylene-propylene copolymers [50] as shown in Fig. 3. Radiation protection effects [51] and detailed geminate ion recombination processes [52] of model compounds were studied by nano-, pico-, and subpicosecond pulse radiolyses. 

Other transient radicals such as (SCN)2 [78], carbonate radical (COj ) [79], Ag and Ag " [80], and benzophenone ketyl and anion radicals [81] have been observed from room temperature to 400°C in supercritical water. The (SCN)2 radical formation in aqueous solution has been widely taken as a standard and useful dosimeter in pulse radiolysis study [82,83], The lifetime of the (SCN)2 radical is longer than 10 psec at room temperature and becomes shorter with increasing temperature. This dosimeter is not useful anymore at elevated temperatures. The absorption spectrum of the (SCN)2 radical again shows a red shift with increasing temperature, but the degree of the shift is not significant as compared with the case of the hydrated electron. It is known that the (SCN) radical is equilibrated with SCN , and precise dynamic equilibration as a function of temperature has been analyzed to reproduce the observation [78],... 

In principle, absorption spectroscopy techniques can be used to characterize radicals. The key issues are the sensitivity of the method, the concentrations of radicals that are produced, and the molar absorptivities of the radicals. High-energy electron beams in pulse radiolysis and ultraviolet-visible (UV-vis) light from lasers can produce relatively high radical concentrations in the 1-10 x 10 M range, and UV-vis spectroscopy is possible with sensitive photomultipliers. A compilation of absorption spectra for radicals contains many examples. Infrared (IR) spectroscopy can be used for select cases, such as carbonyl-containing radicals, but it is less useful than UV-vis spectroscopy. Time-resolved absorption spectroscopy is used for direct kinetic smdies. Dynamic ESR spectroscopy also can be employed for kinetic studies, and this was the most important kinetic method available for reactions... 

In pulse radiolysis experiments these radicals are formed by a short pulse, 10-12-10-6 s depending on the experimental set up, in concentrations enabling their physical observation. The linear electron accelerator of the Hebrew University of Jerusalem, which is used, forms up to... 

More recently the same reactions have been monitored for Ru(III)-Os(II) analogs. These complexes are generated by the reduction of the Ru(III)—Os(III) dimers by e (aq) or C02 (aq) radicals in pulse radiolysis experiments 429). Because of the much lower inner-sphere reor-ganizational energy terms for the [Ru(NH3)5L]3+/2+ couples compared with Codll/II) analogs, the rates of intramolecular electron transfer in the Ru(III)—Os(II) dimers are much larger than those of Co(III)-Os(II) dimers 429). 

In pulse radiolysis studies of Urd and its derivatives (but not with dUrd), spectral changes are observed after the completion of the S04, reaction [k = 3 x 10s s 1 Bothe et al. 1990] that are not typical for S04 reactions with pyrimidines. On the basis of EPR experiments (Hildenbrand 1990 Catterall et al. 1992), these observations can be interpreted by an (overall) intramolecular H-transfer giving rise to a radical at the sugar moiety. This requires that considerable amounts of Ura are released which is indeed observed (Fujita et al. 1988 Aravindakumar et al. 2003 Table 10.4). Chain reactions occur as with the other pyrimidine/peroxodisulfate systems. This increases the Ura yield beyond that expected for a non-chain process, but when corrections are made for this by carrying out experiments at the very high dose rates of electron-beam irradiation, a... 

In deaerated methanolic solution, iron(II) hydrido complexes of the type [FeH(L)(dppe)2]+ (L = MeCN, MeCH2CN, CH,=CHCN, PhCN and p-MeC6H4CN) are reduced to [FeH(L)(dppe)2] by solvated electrons produced by y-radiolysis of the solvent.228 In pulsed radiolysis studies [FeH(L)dppe2] decays to five-coordinate [FeH(dppe)2] (half-life < 1 s). In continuous radiolysis however, [FeH(dppe)2] is not the final product and here as in its reaction with acids it is suspected that oxidative addition of a proton yields an unstable iron(III) dihydride which decom-... 

Fig. 3.9. Method used to study the diffuse reflectance of opaque samples in pulse radiolysis (Adams et al., 1991). The insert shows the method used to mount the sample. The electrons pass through the thin aluminium plate and the probe light through the quartz window. The scattered beam is analysed in the normal way with a monochromator, care being taken to exclude normal reflected light.

Hole capture by a solute (S) is probably not sufficient to explain the Ps yield enhancement as recombination might well occur as easily between the electron and either M+ (the hole) or S+ (the trapped hole). An explanation to the phenomenon is probably that e+ cannot react with the electron once recombined with M+ whereas it can pick up an electron shallowly trapped by S+. The weakness of recombined S+/e pairs has been shown in some instances in pulse radiolysis experiments, where a delayed formation of e s has been observed from such a state [16]. The concentration range necessary for efficient enhancement of Ps formation is similar to that related to total inhibition, indicating that the processes involved occur on a very short time-... 

Ab initio and Monte-Carlo calculations. Attempts have appeared in pulse radiolysis to describe the dynamics of free electron production, recombination and solvation on a microscopic scale [31-34]. This requires the knowledge of a number of physical parameters solvated electron and free ion yields, electron and hole mobilities, slowing-down cross-sections, localization and solvation times, etc. The movement and fate of each reactant is examined step by step in a probabilistic way and final results are obtained by averaging a number of calculated individual scenarios. 

Electron Beam Resist Reactions of CMS. The lifetime of the excimer fluorescence of CMS observed in pulse radiolysis of CMS solutions in cyclohexane and tetrahydrofuran (THF) is almost independent of chloromethylation ratio from 0% to 24%. The intensity of the excimer fluorescence decreases with increasing degree chloromethylation indicating that the precursor of the excimer is scavenged by the chloromethylated part of CMS. In this case, an electron (quasi-free electron in cyclohexane and solvated electron in tetrahydrofuran, which are the precursors of the excimer), is scavenged by the chloromethyl group. The excited singlet state... 

The absorption due to the substituted benzyl type polymer radical, Pi is observed in pulse radiolysis of CMS solutions in cyclohexane and THF. An electron in cyclohexane or THF reacts with CMS resulting in formation of Pi and Cl by dissociative electron capture (reaction (10)). 

Electron Beam Resist Reactions of SNR. The transient absorption spectrum observed in pulse radiolysis of SNR (partly chloromethylated diphenyl siloxane) solutions in benzene as shown in Fig. 3 is very similar to that of CMS (of Fig. 2). 

A recent study of Cgo in pulse radiolysis reported a spectral feature at 650 nm, assigned to the radical cation [82], This study apparently did not use a near-IR sensitive detector, so that the strong 980 nm absorption was not observed, and it is possible that this 650 nm absorption is caused by products of radical addition to Ceo- Reaction of triplet Ceo with strong electron acceptors produces an exciplex and the free C o radical cation in benzonitrile [24]. 

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