We have recently begun an investigation of paramagnetic (free-radical) final states formed on metal nanoparticles by muonium (Mu) reactivity with surface-adsorbed molecules. The nanoparticles are incorporated into mesoporous silica, facilitating specific reaction steps in the silica host that involve H-atom transfer reactions important to studies in heterogeneous catalysis. Radio frequency (RF) methods are an essential tool for characterising final state species in these systems, and a non-metallic sample cell is essential for the RF field to penetrate the sample. Unfortunately, several significant problems were encountered during initial experiments using a cell made from PEEK polymer, the most serious being a temporal instability in the signals likely due to reactant molecules adsorbing on the PEEK. This paper discusses the problems encountered using the PEEK cell, and then considers the development of a ceramic cell designed to give better reproducibility in the measurements. The success of this new cell is demonstrated both through off-line tests and by muon measurements, including a series of TF 2G Mu spin precession measurements verifying the temporal stability of the experimental setup. Finally, an RF cavity was fashioned, and RF measurements made for muons stopped in bare silica, with signals from both diamagnetic and paramagnetic muon states clearly seen.

Precise measurements of the muonium (Mu) hyperfine interaction versus temperature are reported in a silica aerogel and mesoporous silica SBA-15 using a fast-timing spectrometer to detect the precession frequencies of Mu in a magnetic field of 1.14 T. The observed signals are a sensitive monitor of dynamics associated with the binding and unbinding of Mu from the silica surface. Above 100 K the Mu is effectively off the surface. Significant differences are observed in the way the lines show motional narrowing, and are attributed to differences in sample morphology. At room temperature the effective mean free path appears longer in SBA-15, suggesting it may offer advantages over aerogel as a source of Mu in vacuum.

We report the reaction of muonium (Mu = [μ+e−]), a light isotopic analog of hydrogen, with uncapped gold nanoparticles embedded in mesoporous silica. Using the radio-frequency muon spin rotation (RF-μSR) technique, we directly observe and characterize the resulting final state on the nanoparticle surface, showing conclusively its diamagnetic nature. The magnetic environment experienced by the reacted muons is only weakly perturbed compared to that of muons in a silica reference, consistent with the surface of the gold nanoparticles being metallic and non-magnetic. We demonstrate the potential of RF-μSR for the investigation of the surface properties of nanoparticles and show the feasibility of Knight shift measurements of muons on metal surfaces.

Gold nanoparticles (AuNPs) have been a subject of considerable interest in recent years due to both their magnetic and catalytic properties. This paper reports a two-fold study of the reactivity at 300 K of the isotopic hydrogen atom, muonium (Mu = mu(+)e(-)), (i) with bare uncapped AuNPs of different sizes encapsulated in mesoporous (SBA-15) silica hosts, forming a diamagnetic final state in the Mu + AuNP -> MuAuNP reaction, and (ii) with surface-adsorbed benzene on these NPs forming the muoniated cyclohexadienyl radical in the Mu + C6H6 -> MuC(6)H(6) addition reaction. The measured muon-spin relaxation rates, lambda(C), for the chemisorption reaction of Mu with the bare AuNPs show some variation with AuNP size. The Mu + C6H6 addition reaction has been studied over a range of benzene loadings both on bare silica and in the AuNP/silica samples. The measured muon-spin relaxation rates, lambda(Tot), exhibit a linear dependence on benzene concentration over the full range of loadings in both cases, in accord with an Eley-Rideal model of surface reactivity. Rate constants, k(Bz), were determined from this dependence which exhibit a 2-3-fold faster reaction rate on the AuNPs than on the bare silica, suggesting a catalytic effect due to benzene adsorbed on these NP surfaces.

Muon spin rotation/relaxation measurements show clear evidence for magnetism in 2.2 nm gold nanoparticles capped with butanethiol. At low temperatures (1.8 K), there is significant spin relaxation which decreases as a function of both the applied longitudinal magnetic field and increasing temperature. The results indicate that there are spatially inhomogeneous electronic moments that fluctuate with a wide distribution of correlation times. Possible explanations are discussed.

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.

mu SR and ALCR techniques have been used to investigate the structure and dynamics of the Mu-cyclohexadienyl radical interacting with Au and Pt metal nanoparticles (MNPs) supported in mesoporous silica (SBA-15). Surprisingly, coherent precession signals are observed and the isotropic hyperfine coupling constants are almost the same in loaded and unloaded samples, implying that the electronic structure of MuC(6)H(6) is only weakly perturbed by the presence of the MNPs. We propose the observed radicals are shielded from the metallic surfaces by a benzene coating on the MNPs. The Delta(1) resonance is observable in MNP-loaded samples at higher temperatures than in the unloaded SBA-15. This is attributed to stronger binding of MuC(6)H(6) to the benzene coated MNPs.

We report muon spin rotation/relaxation measurements of muonium in mesoporous silica (SBA-15) with a high specific surface area of 600 m(2)/g. Up to 70 percent of the incoming muons form muonium and escape efficiently into the open pores at all temperatures between 3 and 300K. We present evidence that the interaction with the silica surfaces involves both spin exchange and a transition to a diamagnetic state, possibly due to dangling bonds on the surface. At very low temperatures, below 20K, the interaction between muonium and the silica surfaces is suppressed due to a He film coating the surfaces. These results indicate that it should be possible to use muonium to probe the surfaces of uncapped nanoparticles supported in silica.

Longitudinal muonium spin relaxation/modulation in zero magnetic field (zf-MSR) has been used to study muonium (μ+e−) atoms in single-crystal α-quartz between 5 K and room temperature. At 6 K, three frequencies are observed, corresponding to a triaxial hyperfine matrix whose principal values are close to those observed for hydrogen atoms frozen into known sites. For intermediate temperatures the Mu atoms “hop” between sites, causing a relaxation whose rate first increases with the hop rate and then decreases due to motional narrowing. Finally, at room temperature, a single-frequency oscillation is observed, corresponding to a uniaxial motionally-averaged hyperfine interaction.

Diamagnetic and muonium (Mu) fractions formed in low-pressure inert gases, by energetic muon implantation, have been studied using the technique of time-delayed radio frequency muon spin resonance (RF-muSR). Results obtained establish the validity of the long-held view that formation of these species is due only to prompt processes, and in turn confirms that the diamagnetic environment is due to a muon molecular ion, MMu(+), and not a bare mu(+). In addition, polarization fractions for the diamagnetic and Mu environments have been determined at different pressures, thereby complementing earlier data, and demonstrating that the RF-muSR technique provides polarization fractions in good accord with those obtained using conventional transverse-field muon spin resonance measurements.

Muonium (Mu = mu(+)e(-)), which can be considered a light isotope of the H atom, has been observed for the first time in supercritical CO2 (ScCo2). It is Unreactive on a time scale of a few microseconds and over a wide density range from well below to well above the CO2 critical density rho(c) = 0.47 g/cm(3). The fraction of muon polarization in muonium, P-Mu, does not vary significantly at low densities but changes quickly at the highest densities, approaching zero. This density dependence is reflected in a concomitant increase observed in the lost fraction of polarization, P-L, demonstrating that the dynamics of Mu formation and depolarization in ScCO2 is a direct probe of radiolysis effects in the terminal muon radiation track. In marked contrast to previous studies in hydrogen-containing solvents, C2H6 and H2O, over comparable density ranges, the diamagnetic fraction, P-D, was found to be almost independent of density in CO2, attributed to the formation of the stable solvated MuCO(2)(+) molecular ion in this hydrogen-free solvent. The differing density dependences of both the Mu and the diamagnetic fraction in Co-2, in comparison with the rather similar trends seen for both in C2H6 and H2O, supports previous claims of a significant role played by proton (muon) transfer reactions in the competing processes involved in Mu formation in hydrogen-containing solvents. In addition to this being the first report of radiolysis effects accompanying energetic positive muons stopping in ScCO2, it is the only report of end of track effects in this solvent, which has many applications in nuclear waste management and green chemistry. With a mass intermediate between that of the electron, which has provided most radiation-chemistry studies in ScCO2 to date, and the proton (or alpha-particle), implanted muons provide a unique data set, characteristic of higher LET radiation, that may be relevant to radiolysis effects induced in ScCO2 by alpha decay from heavy nuclei, for which there are no comparable studies.

The precession signals of the muoniated ethyl radical have been studied as a function of ethene pressure in pure ethene, ethene/nitrogen mixtures and ethene/helium mixtures. The purpose was to investigate the kinetics and mechanism of the radical formation process. It is possible to fit the signal amplitudes with a model involving a single reaction step—Mu addition to ethene. However, the rate constant deduced from the model fit is significantly higher than the literature value for a thermal reaction, as determined by direct study of Mu decay in low partial pressures of ethene. The conclusion is that at partial pressures above 1bar the reaction occurs before the incoming (beam) muons are fully thermalized.

Muonium has been directly detected over a range of temperatures and fields by transverse field μSR in different zeolites: 3A, 13X, USY, ZSM-5, and S-115 (a high-silica form of ZSM-5), as well as in silica gel. The polarizations determined from data at 75 and 150 G were independent of both field and temperature. The amounts of Mu seen vary from ∼20% to 40%, with a large missing fraction seen in every case, which may be partly due to slow Mu formation. There is also a fast Mu relaxation rate seen in all samples. This is the first direct observation of Mu in zeolites.

The adsorption and dynamical behavior of the muonated cyclohexadienyl radical (C(6)H(6)Mu) in NaY zeolite, formed by muonium (Mu) addition on adsorbed benzene. was investigated by the muon spin resonance (muSR) technique, primarily at loadings of 2-3 C6H6 molecules per supercage of NaY. The dynamics of this radical are expected to be the same as its isotopic analogue, C6H7, for which there are no similar data available. Both TF-muSR and ALC-muSR spectra were recorded, with the most detailed information provided by the positions and line widths of the avoided level crossing resonances. In concert with H-2 NMR, neutron diffraction and molecular dynamics studies of the parent benzene molecule, as well as current theoretical calculations, the dominant adsorption site for the C(6)H(6)Mu radical is believed to be the S-II Na cation, within a supercage, which gives rise to three observed ALC lines, corresponding to two different orientations for the muon (proton) of the CHMu methylene group: pointing toward (endo) and away (exo) from the Na cation. The cation interaction gives rise to unprecedentedly large (approximate to20%) shifts in hyperfine coupling constants, indicative of a strong bond formed with the T electrons of the C(6)H(6)Mu radical. An additional but weaker resonance line is also seen, which is interpreted as being due to adsorption at the window sites between supercages. The ALC lines associated with the CH,,Mu radical bound to both the Na cation and window sites are all broad, approximate to1 kG, change little with temperature and exhibit mainly static line shapes over the whole temperature range studied, from 3 to 322 K. This indicates a much stronger host-guest interaction for C(6)H(6)Mu, particularly with the Na cation. than is known for benzene, to the extent that this site acts as an effective trap for the free radical, over the critical muSR time scale of 50 ns.

The hyperfine interactions of the MuC6H6 radical in NaY zeolites at low to moderate benzene loadings have been measured by both the FT-μSR and ALC-μSR techniques over a wide temperature range. From a preliminary interpretation of the data, the hyperfine coupling constants for the muon and proton of the – CHMu methylene group in two different orientations for the radical bound to the SII cation site have been determined. At 322 K these values are: Aμ(1)=606±2MHz and Ap(1)=108±2MHz, for the muon on the opposite side of the ring from the cation; and Aμ(2)=430±2MHz and Ap(2)=70±5MHz, for the muon on the same side. These results as well as their trends with temperature demonstrate that the MuC6H6 radical adopts a non-planar equilibrium geometry due to the strong interaction of the π electron density with the Na cation.

The rate of the addition reaction H+O2→HO2 is measured at high O2 concentrations (up to 40 bar of O2 at room temperature) through its competition with rapid spin exchange, to give kch0=8.7±0.8×10−33cm6s−1.

Pyridinium tetrafluoroborate is a ferroelectric with a paraelectric-ferroelectric phase transition of second order, which is exceptional for multidirectional ferroelectrics, Avoided-level-crossing muon-spin resonance has been used to show that the Mu substituted aza-cyclohexadienyl radical derived from the pyridinium ions undergoes fast uniaxial rotation around the axis perpendicular to the molecular plane, confirming earlier results by Czarnecki et al. The changes in reorientational dynamics between 239 and 244 K also verify the existence of a phase transition in this range, as found by Czarnecki. Furthermore, the results give strong evidence that superimposed on the rotation there is a wobbling motion of the rotational axis which freezes in at the first transition temperature, T-1 = 238.7 K. (C) 2000 Elsevier Science B.V. All rights reserved.

The hyperfine coupling constant A of a hydrogen isotope confined in a cage differs from its vacuum value, demonstrating that the atom acts as a probe of its environment. Room-temperature values of A for muonium in different cube-shaped Si8O12 units (T8 units or octasilsesquioxanes), in H2O, and in D2O were determined with an accuracy of about 1 MHz. The results are compared with those obtained from high-resolution ESR on H in H2O, and D in D2O, and in different silsesquioxanes in the temperature range 40–300 K. Both the strong isotope effect and the temperature dependence are well described by a single-oscillator model of Roduner et al. (J. Chem. Phys. 102 (1995) 5989). Striking differences seen between the different silsesquioxanes and between the porous Optipur and bulk Suprasil reflect varying spatial constraints and electronic interactions.