Effect of mass on particle diffusion in liquids studied by electron spin exchange and chemical reaction of muonium with oxygen in aqueous solution

Abstract

The dephasing rate constant of the muon precession signal of the light hydrogen isotope muonium (Mu) in the presence of oxygen in liquid water is (1.8 ± 0.1)× 10¹⁰ l mol^–1 s^–1 at 297 K. (90 ± 17)% of this is ascribed to electron spin exchange interaction, leaving no measurable amount for chemical reaction. Based on a spin statistical analysis, the diffusion-limited rate constant of Mu encounters with O₂ is determined to be (5.7 ± 0.5)× 10¹⁰ l mol^–1 s^–1 for Mu at 297 K, which is a factor of 1.78 higher than the value deduced from literature data for H. From this we conclude that the ratio of diffusion coefficients, D_Mu/D_H, is 2.0 ± 0.2 and D_Mu=(14 ± 4)× 10^–9 m² s^–1. This reveals an unprecedented mass effect which by comparison with literature values appears, in clear contradiction to Stokes–Einstein behaviour, to scale approximately with the inverse cube root of the mass of the diffusing particle. The behaviour is discussed by comparison with literature values for the diffusion of noble gases.