Siu-Keung Leung

Hyperfine constants for the ethyl radical in the gas phase

Abstract Muon spin rotation and level-crossing spectroscopy have been used to measure the muon, proton, deuteron and 13C hyperfine coupling constants for the isotopically substituted ethyl radicals CH2CH2Mu, CD2CD2Mu and 13CH213CH2Mu in the gas phase.

Structure and intramolecular motion of muonium-substituted cyclohexadienyl radicals

Abstract Hyperfine coupling constants of isotopically substituted cyclohexadienyl radicals have been measured as a function of temperature by muon spin rotation and level-crossing spectroscopy. Data are presented for the muon, proton and deuteron hyperfine couplings of the methylene groups in C6H6Mu and C6D6Mu, and also for all the 13C hyperfine couplings of 13C6H6Mu. Comparison of the results with semi-empirical calculations supports a planar ring configuration with complex motion of the methylene substituents.

Intramolecular motion in the tert-butyl radical as studied by muon spin rotation and level-crossing spectroscopy

Abstract Muon spin rotation and muon level-crossing spectroscopy have been used to determine muon ( A μ ) and proton ( A p ) hyperfine coupling constants for the muon-substituted tert-butyl radical (CH 3 ) 2 CCH 2 Mu over a wide range of temperature in isobutene. A p (CH 3 ) is almost constant, but A μ (CH 2 Mu) falls and A p (CH 2 Mu) rises with increasing temperature, consistent with a preferred conformation of the methyl group in which the CMu bond is coplanar with the symmetry axis of the radical orbital. The A μ data cover the temperature range from 297 K down to 43 K, where the solution is frozen. There is a discontinuity in A μ at the melting point, as well as a change in temperature dependence. It is suggested that the potential barrier for methyl group rotation is lower in the liquid due to simultaneous inversion at the radical centre, and that the inversion mode is somewhat inhibited in the solid. The best fit of the liquid-phase data indicates a V 2 barrier of 1.8 kJ mol −1 , and is consistent with a long CMu bond and a tilt of the CH 2 Mu group in the direction that brings the Mu atom closer to the radical centre.

Measurement of the 13C hyperfine constants of the cyclohexadienyl radical using muon level-crossing resonance

Abstract The 13 C hyperfine constants of the cyclohexadienyl radical, formed by muonium addition to 13 C-enriched benzene, have been measured using a novel muon level-crossing resonance technique. The constants are compared with a recent ab initio calculation, Karplus-Fraenkel theory, and with ESR results on the 1,2,3,4,5-pentacarboxyl cyclohexadienyl radical.

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.

The reaction of muonium with hydrated electrons

Abstract When positive muons stop in water roughly 60% are incorporated in diamagnetic compounds, and the remainder form muonium. The two fractions can be distinguished by their muon spin rotation (μSR) signals, but the signal amplitudes indicate that part of the initial muon spin polarization is missing. Earlier work showed that the depolarization occurs as a result of encounters between muonium atoms and hydrated electrons formed at the end of the muon track. A reexamination of the field dependence of the muon polarization in the diamagnetic signal has revealed a small increase at low fields, which is consistent with formation of MuH from Mu on the same time-scale as the muonium depolarization process. This has been explored in a variety of experiments at various temperatures, pressures, and concentrations of paramagnetic ions.

Hot muonium and muon spur processes in nitrogen and ethane

Muon polarizations are reported for nitrogen and ethane over a wide pressure range from below 1 to 200 atm for N2 and up to 245 atm for C2H6. The N2 measurements were made at ambient temperature, while those for C2H6 were made at temperatures both above and below the critical temperature (305.3 K). This is the first μSR study of muonium and diamagnetic muon formation to cover the entire range from a low pressure gas to densities typical of liquids. The data are discussed in terms of hot atom and spur models. In the lowest pressure range, below 1.5 atm for N2 and about 10 atm for C2H6, the muonium polarization increases with pressure. This is well understood in terms of epithermal charge exchange. In N2 there is a small diamagnetic fraction, which is ascribed to the N2Mu+ molecular ion. This fraction approaches zero as the pressure is increased to 200 atm, with a corresponding increase in the muonium fraction, consistent with charge neutralization of the molecular ion by electrons from the radiolysis track...

Surface Dynamics of the Cyclohexadienyl Radical Adsorbed on Silica Gel Investigated Using Avoided Level-Crossing Muon Spin Resonance

The dynamic behaviour of the cyclohexadienyl radical on a silica surface has been investigated using the avoided level-crossing muon spin resonance technique. Translational motion of the radical on a curved surface leads to partial averaging of the muon-electron hyperfine anisotropy and thus to motional narrowing of the experimental signal. A theoretical model based on a stochastic Liouville formalism allows interpretation of the data in terms of radical diffusion. Arrhenius behaviour with an activation energy of (10.8 ±

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.