Nanosecond pulse radiolysis

Recently, by using improved nanosecond pulse radiolysis with the monitoring wavelength region from 300 to 1600 nm [44], absorption spectra due to main reactive intermediates such as the intramolecular dimer cation radical in the near-IR wavelength region were clearly observed in the pulse radiolysis of polystyrene in various solutions [47]. For example. Fig. 1 shows the absorption spectra observed in the pulse radiolysis of polystyrene solutions in CH2CI2. 

Very intense and sharp near-UV absorption bands due to radical ions of polysilanes [53a,b] and polygermanes [53c] were observed by nanosecond pulse radiolysis. Broad visible and IR absorption spectra due to the radical ions of polysilyne [54] and polygermyne [54] were also observed. Very systematic pulse radiolysis studies on many different kinds of polysilanes [55] have been made by our improved nanosecond pulse radiolysis system over a wide range of... 

Figure 5 Transient absorption spectra of irradiated additive-free solid PMMA containing MMA monomers observed by the authors improved nanosecond pulse radiolysis system over a wide range of temperatures and with the monitoring wavelength region from 300 to 1600 nm.

Ellison DH, Salmon GA, Wilkinson F (1972) Nanosecond pulse radiolysis of methanolic and aqueous solutions of readily oxidizable solutes. Proc R Soc Lond A 328 23-36 Erben-Russ M, Bors W, Saran M (1987) Reactions of linoleic acid peroxyl radicals with phenolic antioxidants a pulse radiolysis study. Int J Radiat Biol 52 393-412 Eriksen TE, Fransson G (1988) Radical-induced oxidation of glutathione in alkaline aqueous solution. Radiat Phys Chem 32 163-167... 

Nan Y, Rabani J (1976) On some fundamental reactions in radiation chemistry nanosecond pulse radiolysis. Int J Radiat Phys Chem 8 609-611... 

Fig. 1. Block diagram of the nanosecond pulse radiolysis system using the Hokkaido University 45 MeV electron linear accelerator...

The initial effects of high energy radiation on condensed systems are discussed. Evidence of short lived intermediates, i.e. radical ions and excited states is illustrated by fast pico-second and nanosecond pulse radiolysis A discusion of the nature of early events leading to excited states is discussed at length, and in particular comparisons are made to corresponding experiments at low photon energies, i.e. via laser flash photolysis. 

This excellent agreement with experimental data only lasted until sub-nanosecond pulse radiolysis experiments became common. We will return to this in Sec. 7. 

To address the questions of non-homogeneous/spur kinetics, John Hunt and his group at Toronto developed a sub-nanosecond pulse-radiolysis system.In their stroboscopic pulse radiolysis system, they could observe from about 30 to 350 ps after the pulse with a time resolution of about 10 ps. Their results showed no significant decay of the electron between 30 and 350 ps, which was not consistent with the diffusion-kinetic models of spur decay in radiation chemistry. 

The foray into sub-nanosecond pulse radiolysis was continued by Matheson and Jonah at Argonne, Tabata and co-workers in Tokyo, and Katayama and co-workers in Hokkaido.The experiments at Argonne measured the decay of the hydrated electron both from about 100 ps to 4 ns and from I ns to 40 ns. These results clearly showed that the decay measured was approximately a factor of 10 slower than that predicted by theory. The decay profiles are very similar to those determined using a linac-laser combination about 15 years later. The decay of the OH radical was also considerably slower than what theory predicted, which is, of course, no surprise that the two should decay at similar rates. 

The sub-nanosecond pulse radiolysis techniques have made it possible to study the initial yields of singlets and triplets in the radiolysis of aromatic systems, the decay of electrons in non-polar media, fast electron transfer reactions and solvation experiments. 

Buxton GV. (1972) Nanosecond pulse radiolysis of aqueous solutions containing proton and hydroxyl radical scavengers. Proc R Soc London Ser A 328 9-21. Thomas JK, Bensasson RV. (1967) Direct observation of regions of high ion and radical concentation in the radiolysis of water and ethanol. J Chem Rhys 46 4147 148. 

The temperature dependence of the absorption spectrum of the solvated electron has been recorded not only in water but also in alcohols (Fig. 3). Measurements are performed using nanosecond pulse radiolysis with a specific cell for high temperature and high pressure in a temperature range up to around 600 K depending on the solvent. Indeed, by increasing the temperature, the decay of solvated electrons becomes faster for example, this decay is much faster in alcohols than in water, so, the data obtained with nanosecond set-up are limited at lower temperatures for alcohols compared with water. 

Baldacchino G, Vigneron G, Renault JP, Pin S, Abedinzadeh Z, Deycard S, Balanzat E, Bouffard S, Gardes-Albert M, Hickel B, Mialocq JC. (2004) A nanosecond pulse radiolysis study of the hydrated electron with high energy ions with a narrow velocity distribution. Chem Phys Lett 385 66-71. 

By now most of the studies were carried out using nanosecond pulse radiolysis techniques coupled with spectroscopic detection method, only a few were reported to use muonium reactions, ° steady state (7 )radiolysis, laser phtolysis, and picosecond pulse radiolysis. Since conventional pulse radiolysis techniques are well known, here we just briefly introduce the high temperature high pressure (HTHP)... 

As is known, the estimation of G values of water decomposition products can be done by pulse radiolysis techniques or steady state radiolysis with product analysis methods. For pulse radiolysis, although a direct measurement of these transient species is desirable, it is difficult to be effectuated by nanosecond pulse radiolysis because of the acceleration of spur reactions and/or the limitation of detection techniques (e.g. the absorption of OH radical is in deep UV with a rather small absorption coefficient). One is forced to adopt the scavenging method, that is, to use a chemical additive to react with the transient species and form another easy-to-detect and relatively stable product. In this section, we mainly introduce the estimation of G values by pulse radiolysis, with the support by y-radiolysis of some aromatic compounds. 

When people consider confinement effects, they consider mainly an increase in the encounter probability inside a single pore and therefore, expect an acceleration of the reaction. Such in-pore acceleration has been quantified by Tachiya and co-workers for diffusion-limited reactions through the so-called confinement factor [see Eq. (11.58) in Ref. 40]. From this treatment, confinement effects are expected to disappear when the reaction radius is less than one tenth of the confinement radius. Considering the reaction radii of radiolytic species, no acceleration by confinement should be expected for pore diameter larger than 10 nm. For smaller pore size, acceleration of the recombination reactions within spurs would be critical in the determination of radiolytic yields in the nanosecond time range. However, the existence of such an acceleration of radiolytic reactions has not been suggested in the nanosecond pulse radiolysis of zeolites and has still to be assessed using picosecond pulse radiolysis. 

Aromatic sulfides analogous to thiophenols constitute a group of molecules that fulfils the structural conditions necessary for the observation of FET (Sec. 2.4), i.e. they exhibit a low barrier to rotation about the Qp2 S bond. Thus, the torsion motions of the substituents can be accompanied by considerable fluctuation of the electrons in the highest molecular orbitals with two extreme examples of conformers, planar and vertical. The presence of two radical cation conformers was deduced as primary products of the bimolecular free electron transfer (FET) from aromatic sulfides PhSCH2Ph, PhSCHPhj, and PhSCPhg to w-butyl chloride radical based on the nanosecond pulse radiolysis experiments. ... 

Neighboring group participation was investigated in the OH-induced oxidation of S-methylglutathione (y-Glu-S-Me-Cys-Gly) in aqueous solutions by means of nanosecond pulse radiolysis. The following sulfur-centered radicals and radical cations were identified an intramolecularly bonded [>S.. NH2] intermediate, an intermolec-ularly S.. S-bonded radical cation, and an intramolecularly (S.. O/-bonded intermediate (Chart 9). 

Two distinct XeBr formation processes in electron-beam-excited Xe-Br2 mixtures have been identified in nanosecond pulse radiolysis experiments. One formation process, involving ion recombination, could be slowed relative to the second process, involving excitation energy transfer from Xe to Brz, by using very low electron-beam intensities, thereby allowing a kinetic analysis of the second process to be made. XeBr (B) emission at 280 nm has been observed following 193 nm excitation of IBr in Xe-IBr mixtures. ... 

NaCl2 molecules.6 The existence of these molecules indicates an attractive exit channel for the reaction M + X2—MX + X. Optical and e.s.r. spectroscopy of y-irradiated KZnF3 crystals has shown7 that three types of perturbed F2 species are produced at 77 K. The mechanisms of the reactions of the radical ions Cl2, Br2, and I2, produced by nanosecond pulse radiolysis of the aqueous halides, have been established by y-radiolysis and flash photolysis experiments.8... 

The very rapid oxidation of phenols by solvent radical cations can be expected to yield phenol radical cations as the first products. These species are short-lived, except in highly acidic solutions, and were not observed in the microsecond pnlse radiolysis experiments described above. They were detected, however, in frozen matrices and with nanosecond pulse radiolysis Gamma irradiation of phenols in w-butyl chloride or in l,l,2-trichloro-l,2,2-trifluoroethane (Freon 113) at 77 K produced phenol radical cations, which were detected by their optical absorption and ESR spectra . Annealing to 133 K resulted in deprotonation of the radical cations to yield phenoxyl radicals. Pulse radiolysis of p-methoxyphenol and its 2,6-di-fert-butyl derivative in w-butyl chloride at room temperature produced both the phenol radical cations and the phenoxyl radicals. The phenol radical cations were formed very rapidly k = 1.5 x 10 ° M s ) and decayed in a first-order process k = 2.2 x 10 s ) to yield the phenoxyl radicals. The phenoxyl radicals were partially formed in this slower process and partially in a fast process. The fast process of phenoxyl formation probably involves proton transfer to the solvent along with the electron transfer. When the p-methoxy group was replaced with alkyl or H, the stability of the phenol radical cation was lower and the species observed at short times were more predominantly phenoxyl radicals. 

A positive ion of CCl,f is observed in the nanosecond pulse radiolysis of pure CCZV The ion which has an absorption maximum at 4750 A. where t is 2.34 X 104, shows an initial rapid first order decay, t1/2 = 15 it 2 nsec, followed by a much slower decay over several y sec. The slow decay is 4% of the fast decay. Many solutes remove the positive ion forming solute positive ions with spectra in the visible which also exhibit a rapid and slow decay. At the same time, long-lived species are observed with absorptions towards th ultraviolet and these may be caused by Cl atom/solute complexes or free radicals. 

It is possible that the lifetimes of the ions are too short to be observed by conventional fisec. pulse radiolysis (14), but may be observed by nanosecond pulse radiolysis techniques. 

The nanosecond pulse radiolysis technique has been described (8, 14). Carbon tetrachloride was purified as follows Matheson Research grade CC14 was dried over anhydrous potassium carbonate for several days, and subsequently distilled, discarding initial and final fractions. However, untreated research grade CCLj gave identical results to that treated as above. Zone refined naphthalene, anthracene, biphenyl, and N,N,N, N -tetramethyl-paraphenylenediamine (TMPD) were used pyrene, 1 2 benzanthracene were recrystallized from absolute alcohol, and aniline was purified as described in an earlier paper (6). Normal hexane, cyclohexane, 3-methylpentane, benzene, and toluene were Matheson research grade methanol and ethyl alcohol were analytical grade. 

The absorption spectra of a wide range of carotenoid radical cations and anions were first established by nanosecond pulse radiolysis under conditions of mono-electronic processes (Dawe and Land, 1975 Laffertyetal., 1977). This workreported the spectra in hexane for the radical cations and in hexane and methanol for the radical anions. Subsequently, such studies for the radical cations have been extended to other solvents (Hill et al., 1995 Edge et al., 1998). Table 1 gives a selection of A, values for carotenoid radical cations in four... 

Table 37 contains a summary of the gas phase data on Arrhenius parameters for the above class of reactions. Studies have recently been made of the H-abstraction reactions of cyano radicals [381] from nanosecond pulse radiolysis of (CN)2 at low pressures in an excess of... 

---