Kenneth E. Newman

Spin depolarization in muonium by hydrated electrons

Abstract The missing fraction of muon polarization in water is shown to originate from encounters between muonium and hydrated electrons. This takes place at ≈1 ns after the primary events in which muonium and hydrated electrons are born.

Partial spin depolarization of muonium in ice

Abstract Muon spin precession signals arising from both muonium and a diamagnetic muon species have been studied in single crystal of ice over the temperature range 90–263 K. Conversion of initial signal amplitudes to fractions of muon polarization reveals that for temperatures above 200 K part of the original muon polarization is unaccounted for. Such a missing fraction is well known for liquid water. but was not found in earlier work on polycrystalline ice. Muonium signals in Polycrystalline ice were reanalyzed using a non-exponential decay function appropriate to the powder pattern spectrum. Smaller muonium fractions were found. consistent with those determined for single crystall, thus confirming the existence of the missing fraction. The origin of the missing fraction is discussed, and it is proposed that non-reactive spin exchange encounters between muonium and hydrogen atoms may be the cause.

Muonium diffusion in ice

Abstract Muonium (Mu, μ+e−) is generally regarded as a light isotope of hydrogen. The procession signals of muonium in single crystals of H2O and D2O ice have been studied from 8 to 263 K using the muon spin rotation (μSR) technique. Transverse spin relaxation rates have been extracted and interpreted in terms of modulation of the dipolar interaction between muonium and the protons/deuterons in the lattice by translational diffusion of muonium. In contrast to the situation for H and a previous claim for Mu, muonium is found to be diffusing at temperatures as low as 8 K. An activation energy of 40 meV is obtained by fitting the highest temperature data to an Arrhenius expression. At low temperature muonium is thought to diffuse by quantum tunnelling.

Observation of two distinct diamagnetic muon signals in the liquid phase using selective paramagnetic relaxation

Abstract Using selective paramagnetic relaxation, two distinct diamagnetic muon signals have been detected in solutions of manganous nitrate hexahydrate in acetone and in methanol. This is the first time that separate signals have been observed for chemically diamagnetic muon species in the liquid phase.

Variable temperature proton (8 to 360 MHz) and carbon-13 (50 MHz) N.M.R. study of the dimethylsulphoxide exchange on iron(III)

Variable temperature N.M.R. data are reported for solutions of [Fe(DMSO)6](ClO4)3 in dimethylsulphoxide (DMSO). Proton measurements were the following: (i) free solvent T 1 and T 2 at 8 MHz; (ii) bound solvent T 2 and bound to free solvent chemical shifts at 60, 200 and 360 MHz; (iii) free solvent T 2 values at 360 MHz. Carbon-13 free solvent T 2 values were obtained at 50 MHz. Combined with the earlier proton N.M.R. free solvent T 1 and T 2 values at 8 and 60 MHz, the data are analysed in terms of the Swift and Connick equations with bound solvent relaxation characterized by both dipolar and scalar processes and outer sphere relaxation by dipolar processes. Electron relaxation is treated in terms of the modulation of the zero-field splitting. The thirteen data sets are analysed using a simultaneous non-linear least squares procedure with twelve unknown parameters. The following kinetic parameters for solvent exchange are obtained: k(298) = 9·3 ± 1·6 s-1; ΔH * = 62·5 ± 1·9 kJ mol-1 and ΔS * = - 16·7 ± 5·2...

A reply to arguments against a spur model for muonium formation

A clear distinction is made between the spur model for muonium formation in condensed matter and the alternative hot model. Arguments against the spur model are considered and found lacking. Some new data is presented for aqueous systems: the initial diamagnetic fraction increases with OH− concentration, and unequivocal muonium inhibition is demonstrated for HClO4 solutions.

Muonium in ice

Muonium has been studied in single crystals of H2O and D2O. Two-frequency precession in low transverse fields and a single zero-field oscillation indicate a small anisotropy of axial symmetry in the muonium hyperfine interaction. The anisotropy is shown to be the cause of the hitherto unexplained temperature independent contribution to muonium spin relaxation in polycrystalline samples. Relaxation rates for 99 K–263 K are reported for muonium in a single crystal of H2O. Relaxation is attributed to electron-nuclear dipolar coupling of muonium to lattice protons, modulated by translational diffusion of muonium alongc-axis channels of the ice lattice. A simple model for H and Mu diffusion in ice is investigated.

Muonium as a probe of hydrogen-atom reactions

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, kM=(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 HSSO2–3 which decomposes in 1 ns or less to give HS–+ SO–3, or possibly H++·S–+ SO2–3. The consequences of time-dependent rate constants on the residual-polarization analysis are discussed in an appendix.

Variable-field muon spin-lattice relaxation studies of aqueous solutions of manganese(ii) nitrate: separation of scalar and dipolar relaxation

Abstract Variable-field muon spin-lattice relaxation measurements of a solution of composition Mn(NO 3 ) 2 ·48.9H 2 O are reported. They reveal two field-dependent regions (dispersions), where the product of the electron precession frequency and the correlation time characterizing the relaxation is unity. The lower-field dispersion is due to scalar relaxation, the other is due to dipolar relaxation.

Pressure-dependent muonium kinetics in aqueous solution

Abstract Muonium decay rates have been measured as a function of pressure for the first time in condensed matter. The reactions of muonium with NO - 3 and MnO - 4 were found to have activation volumes of -7.1 ± 1.5 cm 3 mol -1 and + 3.1 ± 1.6 cm 3 mol -1 , respectively. These volumes are compared with -5 cm 3 mol -1 and +2cm 3 mol -1 for activated and diffusion-controlled reactions of hydrogen atoms. The results support the concept of local order of the water molecules around each H or Mu atom in solution.