Schüth, Joachim

Zero frequency resonance: Another way to measure muon-electron hyperfine constants

Digital Document

At a magnetic field determined by the zero crossing of the ω12 frequency of a paramagnetic species like a muonium-substituted radical some muons experience a total magnetic field of zero, simply because the hyperfine field exactly cancels the externally applied field. Since the muon polarization does not rotate under this condition, an integral positron asymmetry is seen even in transverse field if the resonance condition is met. Essentially, the same data acquisition set-up and high beam currents as used in other integral μSR techniques, such as ALCR, can be used to scan for the resonant field, which scales linearly with the hyperfine constant, Aμ. A theoretical treatment of the resonances in the case of isotropic Aμ is presented along with measured spectra demonstrating the technique.

Hyperfine coupling constants of muonium in sub and supercritical water

Digital Document

Muonium, like the hydrogen atom, is a hydrophobic solute in water under standard conditions. Molecular dynamics simulations suggest that the free atom exists in a transient clathrate-like cage of hydrogen-bonded water molecules. The hyperfine constants of Mu and H are very close to their vacuum values, supporting the picture of an atom “rattling” around in a hole in the liquid. Muonium has now been studied in water over a wide range of temperatures and pressures, from standard conditions to over 400°C and 400 bar (the critical point is at 374°C, 221 bar). Drastic changes occur in the properties of water over this range of conditions, so large changes in the muonium hyperfine constant might well be expected. Surprisingly, the changes are small. The hyperfine coupling constant goes through a minimum in the subcritical region, and then increases toward the vacuum value under supercritical conditions. [ABSTRACT FROM PUBLISHER]

Digital Document

Muonium has been studied in muon-irradiated water over a wide range of conditions, from standard temperature and pressure (STP) up to 350 bar and up to 420[degree]C, corresponding to water densities from 1.0 down to 0.1 g cm. This is the first report of muonium in supercritical water. Muonium was unambiguously identified from its spin precession frequencies in small transverse magnetic fields. The hyperfine constant was determined and found to be similar to the published values for muonium in water at STP and in vacuum. Muonium was found to be long-lived over the whole range of conditions studied. The fraction of muons which form muonium was found to vary markedly over the density range studied. Correlation of the muonium fraction with the ionic product of water suggests a common cause, such as the rate of proton transfer between molecules involved in the radiolysis of water and the formation of MuOH, which competes with muonium formation.

Free radicals formed by H(Mu) addition to fluoranthene

Digital Document

Muonium has been used as an H atom analogue to investigate the free radicals formed by H addition to the polyaromatic hydrocarbon fluoranthene. There are nine unique carbons in the molecule, but only five radicals were detected. Muon and proton hyperfine constants were determined by transverse field µSR and µLCR, respectively, and compared with calculated values. All signals were assigned to radicals formed by Mu addition to C-H sites. There isno evidence for addition to the tertiary carbons at ring junctions.

Digital Document

Rate constants are reported for near-diffusion-controlled reactions of muonium in sub- and supercritical water. Specifically, the spin-exchange interaction of muonium with Ni2 + and the addition of muonium to hydroquinone were studied as a function of temperature and pressure over a wide range of conditions, from standard to over 400 °C and 400 bar (the critical point of water is at 374 °C, 220 bar). At elevated temperatures the rate constants were found to have values far below those predicted by Stokes–Einstein–Smoluchowski theory. Furthermore, the temperature variation of the isobaric rate constants has a maximum in the subcritical region. The pressure dependence of the rate constants increases with temperature, consistent with the increase in compressibility of the solvent; the effective activation volumes are negative. Various models are explored to interpret the temperature and density dependence of the kinetic data. It is concluded that a key factor in the drop of rate constants at high temperature is the cage effect, in particular the number of collisions between a pair of reactants over the duration of their encounter.

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