Elementary Processes in the Radiation Chemistry of Water

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Abstract
Schematic potential-energy diagrams are constructed for ground and excited states of H2O, H2O+ and H2O-, making use of thermochemical, spectroscopic, and electron-impact data. On electron impact a gaseous water molecule is raised to various states of H2O, H2O+ and H2O-, which may subsequently decompose to give H, O, H2, and OH and the ions H+, H-, O+, O-, OH+, and H2+. The exact mechanisms by which these atoms, radicals, and ions are produced are discussed in detail with reference to the potential-energy curves, due attention being paid to the appropriate correlation and selection rules. It is concluded that the production of O- at 173 kcal electron energy probably proceeds via a 2A1, 2B1, 2A2, or 2B2 state of H2O- which decomposes into 2H+O- without the prior formation of OH-. The production of O+ at 437 kcal must necessarily proceed via a 4A2 or 4B2 state of H2O+, while H2+ at 530 kcal must involve the formation of a 2A2 or 2B2 state; both these latter states are stable with respect to the simple breaking of an O—H bond. The nonappearance of OH- in electron-impact experiments is shown to be related to the high stability of OH-, states of H2O- which correlate with H+OH- being of very high energy. The bearing of the treatment on the radiation chemistry of liquid water is briefly considered.