Preamble

Although muonium is of intrinsic interest as an ‘exotic’ atom to those familiar with it, its principal contribution to chemistry will undoubtedly be as a sensitive handle on H-atom processes and in the study of isotope effects. Hydrogen is, after all, our most abundant atom, whereas muonium exists in nature with the utmost rarity – occurring fleetingly at the end of cosmic pion tracks.

Almost all of the liquid phase studies of Mu refer to solutions in water. This is where most of the reference H-atom work exists, and it reflects our aquocentred existence. Furthermore it is of great practical importance with respect to radiobiology. Free H-atoms are primary reactive species produced in the biosphere as a result of interactions with high-energy ionizing radiations, including natural cosmic and mineral sources as well as man-made nuclear devices. In order to predict, control, and alter the effects of these radiations, it is first necessary to understand the basic chemical reactions involved. But it will be necessary to reach a comprehensive understanding of kinetic isotope effects before muonium can completely usurp hydrogen atom studies for this purpose.

Isotope effects in solution are somewhat different to those outlined for gases (chapter 7, section 1). For instance, the encounter frequency of Mu with S is not given simply by the relative thermal velocities, but is controlled instead by the viscosity of the medium and may be altered by its structure. There will be ‘cage’ effects, in which many molecular collisions occur for each encounter.