Radiation chemistry : From basics to applications in material and life sciences
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Since cumulative radiation damage of the sample can be a problem, sample solutions are often flowed once-through or recirculated during experiments. Many interesting studies are thus not practical to perform because samples are not available in the necessary quantities or they cannot be flowed. To address this problem, techniques to measure complete time profiles or spectra, in one shot or just a few shots, have recently been developed.
The Osaka group has obtained spectra with high signal-to-noise in a single-shot, using a CCD to detect broadband absorption by reducing the time interval between sample and reference shots to 1 ms .
Figure 7 : Schematic of a general detection system for picosecond pulse radiolysis courtesy of Dr. Saeki, Osaka University.
Heavy ion sources Radiolysis experiments using heavy ion beams protons and heavier atomic nuclei stripped of all electrons occupy an important role, despite the relatively small number of investigators in this area. The high local density of ionizations produced by heavy ions creates localized areas of damage that can be useful, as in the ion beam modification of polymers . Ion beams have been used to fragment polymers to form membranes, and conversely, to polymerize precursor substrates to form forests of polymer rods on surfaces.
The localized damage caused by heavy ions has important implications for radiation biology and radiation medicine. Proton radiotherapy is a precise way of delivering large radiation doses to kill tumors. Heavy ion radiolysis of water produces the hydroperoxyl radical HO2t, which is not formed by low-LET radiolysis in the absence of molecular oxygen.
It is an important means of causing oxidative damage to hypoxic tumors. Generally, heavy ion radiolysis experiments are performed at large, multipurpose accelerator facilities using cyclotrons or tandem Van de Graaff accelerators. Heavy ion sources in the U. Because heavy ions are stopped in very short distances within samples, special techniques and equipment configurations are used to optimize transient signals.
Future trends The past decade has been an encouraging period in the development of radiolysis capabilities that has reversed an earlier trend of decline in number and accessibility. Two new technologies, photocathode electron guns and laser wake-field accelerators, have emerged and spawned a large new generation of ultrafast accelerator facilities. These installations are developing advanced experimental techniques and making sophisticated experiments available to a larger community of researchers than ever before. Earliergeneration picosecond accelerators have been upgraded to high levels of performance.
New nanosecond linacs were installed at Notre Dame, Saclay and Pune, and the University of Manchester has founded a program and Chair in Radiation Chemistry that will reinforce a field that was in danger of disappearing from the U. It is not taking a risk to predict that performance and capabilities of the new radiolysis installations will markedly improve as these young facilities mature. But what other developments can we look forward to?
Certainly there is strong interest in bringing additional spectroscopic tools into the pulse radiolysis laboratory. Efforts are underway to adapt transient mid-infrared detection techniques to pulse radiolysis, to take advantage of the specificity of vibrational spectroscopy . Strong interest in nanoscience and the mechanisms of reactions in heterogeneous systems will push the development of interfacespecific spectroscopies in radiation chemistry, for example surface-enhanced Raman spectroscopy and second-harmonic or sum-frequency generation.
The author wishes to thank R. Crowell and A. Saeki for Figures 6 and 7.
Materials Science Division
References  Bronskill M. A, , , — A, , , B, , , An apparatus for a. Its description, the analysis of its true nature and of its main properties thus appears as essential in chemistry. The peculiarity comes from the fact that the charge is delocalised but without a nucleus, and that solvent molecules rally around it Fig.
The solvated electron is a thermodynamically stable radical but like most free radicals it has a short lifetime due to its great chemical reactivity. As early as the nineteenth century, the solvated electron was observed but not identified Tab. In , Sir H. David and later, in , W. Weyl reported the intense blue colouring obtained by dissolving alkali metals in ammonia NH3.
Weyl also found that the resulting solutions had reducing properties when used in organic synthesis, but he did not discover the nature of these blue solutions.
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He suggested that this species was an electron surrounded by ammonia molecules behaving like an anion. In , to explain the bleaching of aqueous solutions containing methyl blue upon irradiation in the presence of carbon dioxide CO2 , G.
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Stein also proposed the transient formation of a hydrated electron, similar to the solvated electron in ammonia. The direct spectroscopic observations Figure 1 : Schematic representation of the hydrated electron obtained by molecular simulations . The delocalised negain by E.
Hart and J.
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Boag tive charge is surrounded by water molecules creating a cavity of transient solvated electrons with a radius of about 2. After these first observations, the solvated electron was soon detected in various solvents through its intense optical absorption band in the visible or near infrared domain . Owing to this property, the reactivity of the solvated electron has been studied by transient absorption measurements in many solvents using pulse techniques.
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In addition to pulse radiolysis, other methods can be used to produce a solvated electron and allow its study in different environments Inset. Due to the development of ultrashort laser pulses, great strides have been made towards the understanding of the solvation and short-time reactivity of the electron, mainly in water but also in other polar solvents. So, despite its short life-time, the solvated electron is a unique chemical moiety whose properties may be compared in many solvents. In this chapter, we consider the main properties of the solvated electron, its reactivity and recent results concerning the solvation dynamics of the electron.
Some physical properties of the solvated electron The identification and the understanding of the chemical properties of the solvated electron can be made through the knowledge of its physical properties. The properties of the solvated electron depend on several factors such as solvent, temperature and pressure. Main dates for the discovery and study of the solvated electron.
Years Strides Authors Report on blue colouring of ammonia at the touch of alkali metals.
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David a Blue colouring observed for solutions of alkali metals in liquid ammonia, methylamine and ethylamine ; reducing properties of these solutions. Weyl a,b Identification of a species with a negative elementary charge in ammonia solutions of alkali metals, by conductivity measurements. Kraus a Suggestion of transient formation of solvated electrons to explain the bleaching of aqueous solutions containing methyl blue under irradiation in the presence of CO2. Stein, R. Platzman b First pulse-radiolysis experiments of aqueous solutions revealing the formation of solvated electrons. Hart, J.
Baxendale  Solvation dynamics of the electron in water at room temperature observed by ultrashort pulse photolysis. Migus et al. Nordlund et al.
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Volume In , by dissolving an alkali metal in liquid ammonia, C. Kraus and W. Lucasse observed a volume expansion of the solution greater than that obtained for the dissolution of ordinary salts . They attributed this volume expansion to the formation of the solvated electron with a cavity, regarded as a particle since the electron itself has a negligible volume.
For example, the dissolution of 3 moles of sodium in one litre of liquid ammonia induces an increase in volume of 43 cm3 compared to the pure liquid. Assuming that all the metal is dissociated, it may be deduced that in ammonia the electron occupies a spherical volume with a radius of 0. In fact, the cavity radius of the solvated electron in ammonia is greater than that value and is about 0.
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Historically, the first method was the dissolution of alkali metals in amine solvents ; nevertheless, this is useful only in media in which the lifetime of the solvated electron is long enough at least a few hours in the pure solvent. Under high-energy radiations G- or X-rays, beams of accelerated electrons or positive ions , electrons may be ejected from the most abundant solvent molecules in the medium. These ejected electrons have excess kinetic energy that is lost in collision with solvent molecules, which may be electronically excited, or ionised to produce more electrons in a cascade scheme.
Absorption of UV or visible light from a flash lamp or a laser is used to produce the solvated electron.