Local Electronic Structure and Dynamics of Muon-Polaron Complexes in Fe_{2}O_{3}.

We perform detailed muon spin rotation (µSR) measurements in the classic antiferromagnet Fe_{2}O_{3} and explain the spectra by considering dynamic population and dissociation of charge-neutral muon-polaron complexes. We show that charge-neutral muon states in Fe_{2}O_{3}, despite lacking the signatures typical of charge-neutral muonium centers in nonmagnetic materials, have a significant impact on the measured µSR frequencies and relaxation rates. Our identification of such polaronic muon centers in Fe_{2}O_{3} suggests that isolated hydrogen (H) impurities form analogous complexes, and that H interstitials may be a source of charge carrier density in Fe_{2}O_{3}.

Communication: Chemisorption of muonium on gold nanoparticles: A sensitive new probe of surface magnetism and reactivity.

Chemisorption of muonium onto the surface of gold nanoparticles has been observed. Muonium (µ+e-), a light hydrogen-like atom, reacts chemically with uncapped 7 nm gold nanoparticles embedded in mesoporous silica (SBA-15) with a strong temperature-dependent rate. The addition rate is fast enough to allow coherent spin transfer into a diamagnetic muon state on the nanoparticle surface. The muon is well established as a sensitive probe of static or slowly fluctuating magnetic fields in bulk matter. These results represent the first muon spin rotation signal on a nanoparticle surface or any metallic surface. Only weak magnetic effects are seen on the surface of these Au nanoparticles consistent with Pauli paramagnetism.

Free radical formation in supercritical CO2, using muonium as a probe and implication for H atom reaction with ethene.

This report presents the first observation of an alkyl radical in supercritical CO(2) by any magnetic resonance technique. Muoniated ethyl radical has been detected in muon-irradiated supercritical CO(2) solutions. In the presence of a low concentration of ethene in supercritical CO(2), it is found that the addition of muonium to ethene is the only reaction channel, and that the yield of this process is enhanced compared to conventional solvents. The temperature dependence of the hyperfine coupling constants of the ethyl radical suggests that at a density of 0.3 g/cm(3) both the rotational motion of the methyl group and the electronic structure of the radical are similar to those in the gas phase, and therefore that the local environment around the ethyl radical is similar to the gas phase under these conditions. At higher densities, however, there is a remarkable and unexpected density dependence of the hyperfine coupling constant of the ethyl radical, which has never been observed in any environment. In this regime, the density dependence suggests that supercritical CO(2) has a significant effect on the electronic structure of the free radical. Thus, changing the density of CO(2) offers a possible means of tuning the radical reactivity. In addition, at a density of close to 0.4 g/cm(3), CO(2) molecules cluster around the ethyl radical, and this increases the local density around the ethyl radical by a factor of ~1.5.

Generation and detection of the cyclohexadienyl radical in phosphonium ionic liquids.

The formation of the cyclohexadienyl radical, C(6)H(6)Mu, in ionic and molecular solvents has been compared. This is the first time that a muoniated free radical is reported in an ionic liquid. In marked contrast to molecular liquids, free radical generation in ionic liquids is significantly enhanced. Comparison of the hyperfine interactions in the ionic liquid and in molecular solvents and with theoretical calculations, suggests significant and unforeseen solvent interaction with the cyclohexadienyl radical.

Depth-controlled beta-NMR of 8Li in a thin silver film.

Depth-controlled beta-NMR can be used to probe the magnetic properties of thin films and interfaces on a nanometer length scale. A 30 keV beam of highly spin-polarized 8Li+ ions was slowed down and implanted into a 50 nm film of Ag deposited on a SrTiO3 substrate. A novel high field beta-NMR spectrometer was used to observe two well resolved resonances which are attributed to Li occupying substitutional and octahedral interstitial sites in the Ag lattice. The temperature dependence of the Knight shifts and spin relaxation rates are consistent with the Korringa law for a simple metal, implying that the NMR of implanted 8Li reflects the spin suspectibility of bulk metallic silver.