H atom kinetics in superheated water studied by muon spin spectroscopy

It is possible to study H atom chemistry in aqueous systems over a wide range of conditions, from standard to supercritical, using the exotic atom muonium (Mu) as an effective light isotope of hydrogen. The Mu rate constants exhibit marked non-Arrhenius behaviour, going through a maximum and fall-off as the density is reduced in the neighbourhood of the critical point, and subsequent recovery as the medium becomes more gas-like. This is illustrated with new kinetic data for the reaction of Mu with methanol.