Formation of colour centres in irradiated alkaline ice

Abstract

The different paramagnetic species, viz. OH radicals, O− ions and trapped electrons, formed in the radiolysis (60Co γ-rays) of frozen aqueous solutions of alkali hydroxide at 77°K have been studied using electron spin resonance (E.S.R.) techniques. Trapping of electrons in irradiated alkaline ice has been interpreted as a consequence of the reaction of the radiation produced hole (designated by (H2O)+) according to: thus inhibiting to a greater or lesser extent the recombination between the primarily produced electrons and holes. The OH radicals, formed by the above reaction as well as those from the direct action of the radiation on water, are converted to O− radical ions by the proton transfer reaction: The experimental results lead to the conclusion that electrons are trapped in a vacant orbit around the O− radical ions. Such a mechanism also gives the correct dependence of the yields of the different species on the alkali concentration. The widths of the E.S.R. line of the trapped electron in the H2O and D2O systems agree with the values calculated on the basis of a model in which the electron is delocalized over about four water molecules, giving rise to an inhomogeneously broadened envelope of the individual unresolved nuclear hyperfine lines. The observed negative g-factor shifts suggest that excited levels of the alkali cations which bound the vacancy also contribute to the ground-state wave function of the trapped electron somewhat similar to the case of the F-centres in alkali halide crystals.