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Subnanosecond pulse radiolysis studies Wallace, Stephen Charles

Abstract

This thesis reports pulse radiolysis investigations on the very early physicochemical and chemical events in the radiation chemistry of polar liquids, in particular liquid water, where transient absorptions due to the hydrated electron have been observed with lifetimes as short as ∼30 picoseconds. These studies were carried out using the novel technique of Cerenkov reabsorption spectroscopy, which in conjunction with an extremely high dose-rate electron accelerator (peak dose-rate = 1.8 x 10²⁸ eV cm⁻² sec⁻¹) yields information pertaining to subnanosecond radiation chemical events from measurements made with existing nanosecond technology. A critical evaluation of all experimental aspects of this newly developed technique, as well as a partial theoretical analysis of the relationships between the physical parameters and experimental observables, have both been performed in order to establish the validity of such pulse radiolysis studies. Kinetic effects in molar concentrations of hydrated electron scavengers and relative solvated electron yields in water and the alcohols, have been interpreted to suggest that the time-scale of charge neutralization in radiation chemistry is very much shorter than previously proposed, thus necessitating a modification of the existing description of subnanosecond events as contained in the spur diffusion model for radiation chemical yields. The primary reducing species in the radiolysis of formamide, a liquid of very high dielectric constant, was postulated as a negative ion, rather than a solvated electron; this negative ion being formed directly by solvent scavenging of thermalized electrons. With the availability of new spectral data for solvated electrons from this and other work, a revival of the earlier suggested spectral correlation with iodide ion C.T.T.S. spectra has been proposed, with many of the earlier discrepancies now removed. Finally, because absorption spectra are deduced from light emission measurements in the Cerenkov reabsorption technique, an added feature of this work was the possible observation of radiation-induced molecular luminescence from liquid water; however over the spectral range 200 nm - 850 nm the limiting 100 eV yield of such luminescence was ≤10⁻³ with a lifetime ≤ 2 nanoseconds.

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