Behaviour of Polarons produced by the Action of Ionizing Radiation in Aqueous Media

Abstract

IT has been suggested^1,2 that the action of, for example, γ-rays on water leads primarily to the formation of radiation-produced electrons and the corresponding positive holes and that these electrons, and possibly also the holes, become self-trapped in the polar medium, water, leading to what should be regarded as ‘polarons’, in the sense of Landau, as discussed by Mott³ and Pekar⁴. Afterwards, some definite experimental evidence was produced for the negative polaron^5,6. This was based on its chemical reactions and reactivity, which in many cases (for example, with CO₂, leading to the formation of the CO₂⁻ radical ion⁶) were found to be different from hydrogen atoms which are known to be present in irradiated aqueous solutions under certain conditions. The chemical evidence concerning negative polarons has now been corroborated by the recent work using the methods of pulse radiolysis⁷. In pure water, the majority of the positive and negative polarons, (H₂O)⁺ and (H₂O)⁻, annihilate each other according to: On the other hand, as suggested previously^1,2, the interaction of two polarons of the same species (‘bipolarons’) can lead to the ‘molecular yield’ processes. In the case of the negative polarons, the formation of molecular hydrogen, according to: has now been confirmed by the recent pulse radiolysis work of Dorfman and Taub⁸. Formation of such bipolarons, even if rather short-lived, would be energetically favoured if the repulsion between the like charges is over-compensated by the increased polarization energy of the doubly charged species.