Muonium as a probe of hydrogen-atom reactions

Abstract

Muonium is a light isotope of hydrogen and can be used as a tracer substitute for hydrogen to investigate liquid-phase hydrogen-atom reactions not amenable to study by more conventional means. The residual polarization method of muon spin rotation is illustrated by an investigation of the reaction of muonium with sodium thiosulphate in aqueous solution. The rate constant has been determined directly from measurements of muonium decay rates in very dilute solutions, k_M=(1.5 ± 0.4)× 10¹⁰ dm³ mol^–1 s^–1. Possible reaction mechanisms have been explored by analysis of the field and concentration dependence of the diamagnetic signal amplitude in concentrated solutions (0.01–3.0 mol dm^–3). The conclusion is that hydrogen atoms react with thiosulphate, probably first forming a radical adduct HSSO^2–₃ which decomposes in 1 ns or less to give HS^–+ SO^–₃, or possibly H⁺+·S^–+ SO^2–₃. The consequences of time-dependent rate constants on the residual-polarization analysis are discussed in an appendix.