Water radiation chemistry

Table 2.1 summarizes some of the events that occur in radiation chemistry through the various stages. The earliest discernible time, obtained from uncertainty principle, AE At - fi, is 1CH7 s, which accounts for the production of fast secondary electrons with energy > 100 eV Times shorter than these are just computed values. It has been suggested that, following ionization in liquid water, the dry hole H20+ can move by exact resonance until the ion-molecule reaction H20+ + H20 — H30+ + OH localizes the hole. The... 

Platzman (1962a) has emphasized the implications of superexcited states in radiation chemistry. On the whole, his conjectures have been proved correct. Figure 4.2, using the data of Haddad and Samson (1986), shows the ionization efficiency in the gas phase of water. It shows that T] starts with a value of 0.4 at the... 

Allen, A. O. (1961), The Radiation Chemistry oj Water and Aqueous Solutions, Van Nostrand, Princeton, N.J. 

G Draganic, ZD Draganic. The Radiation Chemistry of Water. New York Academic, 1971. 

Buxton, G.V. Rhodes,T. Sellers, R.M. (1983) Radiation chemistry of colloidal hematite and magnetite in water reductive dissolution by 1-mefhylefhanol radicals (EDTA) iron(ll). J. Chem. Soc. Faraday Trans. 1. 79 2961-2974 Bye, G.C. Howard, C.R. (1971) An examination by nitrogen adsorption of the thermal decomposition of pure and silica doped goefhite. J. Appl. Chem. Biotechnol. 21 324-329... 

Table 1 Approximate Time Scale of Events in Radiation Chemistry, for Example, of Liquid Water...

Allen, A.O. In The Radiation Chemistry of Water and Aqueous Solutions, D. Van Nostrand Princeton, 1961. 

The irradiation of water is immediately followed by a period of fast chemistry, whose short-time kinetics reflects the competition between the relaxation of the nonhomogeneous spatial distributions of the radiation-induced reactants and their reactions. A variety of gamma and energetic electron experiments are available in the literature. Stochastic simulation methods have been used to model the observed short-time radiation chemical kinetics of water and the radiation chemistry of aqueous solutions of scavengers for the hydrated electron and the hydroxyl radical to provide fundamental information for use in the elucidation of more complex, complicated chemical, and biological systems found in real-world scenarios. 

Radiation chemistry highlights the importance of the role of the solvent in chemical reactions. When one radiolyzes water in the gas phase, the primary products are H atoms and OH radicals, whereas in solution, the primary species are eaq , OH, and H" [1]. One can vary the temperature and pressure of water so that it is possible to go continuously from the liquid to the gas phase (with supercritical water as a bridge). In such experiments, it was found that the ratio of the yield of the H atom to the hydrated electron (H/eaq ) does indeed go from that in the liquid phase to the gas phase [2]. Similarly, when one photoionizes water, the threshold energy for the ejection of an electron is much lower in the liquid phase than it is in the gas phase. One might suspect that a major difference is that the electron can be transferred to a trap in the solution so that the full ionization energy is not required to transfer the electron from the molecule to the solvent. 

The Radiation Chemistry of Liquid Water Principles and Applications... 

Shortly after the discovery of the hydrated electron. Hart and Boag [7] developed the method of pulse radiolysis, which enabled them to make the first direct observation of this species by optical spectroscopy. In the 1960s, pulse radiolysis facilities became quite widely available and attention was focussed on the measurement of the rate constants of reactions that were expected to take place in the spurs. Armed with this information, Schwarz [8] reported in 1969 the first detailed spur-diffusion model for water to make the link between the yields of the products in reaction (7) at ca. 10 sec and those present initially in the spurs at ca. 10 sec. This time scale was then only partially accessible experimentally, down to ca. 10 ° sec, by using high concentrations of scavengers (up to ca. 1 mol dm ) to capture the radicals in the spurs. From then on, advancements were made in the time resolution of pulse radiolysis equipment from microseconds (10 sec) to picoseconds (10 sec), which permitted spur processes to be measured by direct observation. Simultaneously, the increase in computational power has enabled more sophisticated models of the radiation chemistry of water to be developed and tested against the experimental data. 

In recent years, two different approaches, deterministic [9,19] and stochastic [10,20], have been used with a good level of success to model the radiation chemistry of water. Each approach leads to reasonable agreement between calculated results and experimental data obtained for a wide range of LET from room temperature up toca. 300°C [9,10]. There are, however, fundamental differences between the two models. The deterministic model is based on the concept of an average spur [8,9,19,23] at the end of the physicochemical stage (ca. 10 sec), which contains the products of processes (I), (II), (III), (IV), and (V) in certain yields and spatial distributions, and in thermal equilibrium with the liquid. For low LET... 

As would be expected, these results indicate that the thermalization distances and spatial distribution of the hydrated electron are key parameters in modelling the radiation chemistry of water. Although the stochastic approach is the more logical one to adopt, its present status does not appear to outweigh the advantages of using the simpler deterministic model to represent the essential features of water radiolysis over a wide range of conditions. 

---