Experimental tests of reaction rate theory: Mu+H2 and Mu+D2

Copyright @ 1987 American Institute of Physics.Bimolecular rate constants for the thermal chemical reactions of muonium (Mu) with hydrogen and deuterium—Mu+H2→MuH+H and Mu+D2→MuD+D—over the temperature range 473–843 K are reported. The Arrhenius parameters and 1σ uncertainties for the H2 reaction are log A (cm3 molecule-1 s-1)=-9.605±0.074 and Ea =13.29±0.22 kcal mol-1, while for D2 the values are -9.67±0.12 and 14.73±0.40, respectively. These results are significantly more precise than those reported earlier by Garner et al. For the Mu reaction with H2 our results are in excellent agreement with the 3D quantum mechanical calculations of Schatz on the Liu–Siegbahn–Truhlar–Horowitz potential surface, but the data for both reactions compare less favorably with variational transition-state theory, particularly at the lower temperatures.NSERC (Canada) and the Petroleum Research Foundation of the Americal Chemical Society

Reaction kinetics of muonium with the halogen gases (F2, Cl2, and Br2)

Copyright @ 1989 American Institute of PhysicsBimolecular rate constants for the thermal chemical reactions of muonium (Mu) with the halogen gases—Mu+X2→MuX+X—are reported over the temperature ranges from 500 down to 100, 160, and 200 K for X2=F2,Cl2, and Br2, respectively. The Arrhenius plots for both the chlorine and fluorine reactions show positive activation energies Ea over the whole temperature ranges studied, but which decrease to near zero at low temperature, indicative of the dominant role played by quantum tunneling of the ultralight muonium atom. In the case of Mu+F2, the bimolecular rate constant k(T) is essentially independent of temperature below 150 K, likely the first observation of Wigner threshold tunneling in gas phase (H atom) kinetics. A similar trend is seen in the Mu+Cl2 reaction. The Br2 data exhibit an apparent negative activation energy [Ea=(−0.095±0.020) kcal mol−1], constant over the temperature range of ∼200–400 K, but which decreases at higher temperatures, indicative of a highly attractive potential energy surface. This result is consistent with the energy dependence in the reactive cross section found some years ago in the atomic beam data of Hepburn et al. [J. Chem. Phys. 69, 4311 (1978)]. In comparing the present Mu data with the corresponding H atom kinetic data, it is found that Mu invariably reacts considerably faster than H at all temperatures, but particularly so at low temperatures in the cases of F2 and Cl2. The current transition state calculations of Steckler, Garrett, and Truhlar [Hyperfine Interact. 32, 779 (986)] for Mu+X2 account reasonably well for the rate constants for F2 and Cl2 near room temperature, but their calculated value for Mu+Br2 is much too high. Moreover, these calculations seemingly fail to account for the trend in the Mu+F2 and Mu+Cl2 data toward pronounced quantum tunneling at low temperatures. It is noted that the Mu kinetics provide a crucial test of the accuracy of transition state treatments of tunneling on these early barrier HX2 potential energy surfaces.NSERC (Canada), Donors of the Petroleum Research Fund, administered by the American Chemical Society, for their partial support of this research and the Canada Council

Influence of the magnetic sublattices in the double perovskite LaCaNiReO6

The magnetism of double perovskites is a complex phenomenon, determined from intra- or interatomic magnetic moment interactions, and strongly influenced by geometry. We take advantage of the complementary length and timescales of the muon spin rotation, relaxation, and resonance (μ+SR) microscopic technique and bulk ac/dc magnetic susceptibility measurements to study the magnetic phases of the LaCaNiReO6 double perovskite. As a result, we are able to discern and report ferrimagnetic ordering below TC=102K and the formation of different magnetic domains above TC. Between TC<T<270K, the following two magnetic environments appear, a dense spin region and a static-dilute spin region. The paramagnetic state is obtained only above T>270K. An evolution of the interaction between Ni and Re magnetic sublattices, in this geometrically frustrated fcc perovskite structure, is revealed as a function of temperature through the critical behavior and thermal evolution of microscopic and macroscopic physical quantities

Quantum critical behavior of the hyperkagome magnet Mn3CoSi

β-Mn-type family alloys Mn3TX (T = Co, Rh, and Ir; X = Si and Ge) have a three-dimensional antiferromagnetic (AF) corner-shared triangular network, i.e., the hyperkagome lattice. The antiferromagnet Mn3RhSi shows magnetic short-range order over a wide temperature range of approximately 500 K above the Néel temperature TN of 190 K. In this family of compounds, as the lattice parameter decreases, the long-range magnetic ordering temperature decreases. Mn3CoSi has the smallest lattice parameter and the lowest TN in the family. The quantum critical point (QCP) from AF to the quantum paramagnetic state is expected near a cubic lattice parameter of 6.15 Å. Although the Néel temperature of Mn3CoSi is only 140 K, the emergence of the quantum critical behavior in Mn3CoSi is discussed. We study how the magnetic short-range order appears in Mn3CoSi by using neutron scattering, μSR, and bulk characterization such as specific heat capacity. According to the results, the neutron scattering intensity of the magnetic short-range order in Mn3CoSi does not change much at low temperatures from that of Mn3RhSi, although the μSR short-range order temperature of Mn3CoSi is largely suppressed to 240 K from that of Mn3RhSi. Correspondingly, the volume fraction of the magnetic short-range order regions, as shown by the initial asymmetry drop ratio of μSR above TN, also becomes small. Instead, the electronic-specific heat coefficient γ of Mn3CoSi is the largest in this Mn3T Si system, possibly due to the low-energy spin fluctuation near the quantum critical point

Muonium formation in some vapors

The fractions of polarized positive muons thermalizing in diamagnetic environments (fD) and as muonium atoms (fM) have been measured in gas phase water, methanol, hexane, cyclohexane, tetramethylsilane, and the chloro-methanes. In almost every case, fM=0.8 and f=0.2, in contrast to the corresponding fractions measured in condensed media where PM=0.2 and PD=0.6. Unlike condensed phases, there is generally no "lost" polarization in the vapors. Any missing fraction is understood in terms of hyperfine dephasing of Mu during thermalization; a distinctly gas phase effect which disappears at moderately high pressures. Carbon tetrachloride is anomalous in having an unusually low muonium fraction (fM=0.5) in the vapor, but having no muonium in the liquid phase (PD=1.0). Furthermore, the vapor also has a true missing fraction while the liquid does not. The vapor phase results are interpreted in terms of a hot atom/ion reaction model giving either pressure independent yields (fD) as seen in water and the chloro-methanes or pressure dependent values as measured in the hexanes and TMS. That interpretation indicates that hot atom reactions do not account for more than about 30% of the much larger diamagnetic fractions seen in condensed phases, suggesting that radiation-induced spur effects are predominant in determining thermal fractions in condensed media.Science, Faculty ofChemistry, Department ofGraduat

The formation and reactivity of positive Muon molecular ions

Thermal (117—445 K) ion—molecule reaction rates are measured, using the µSR technique, for the muonated molecular ions HeMu+, NeMu+, ArMu+, and N²Mu+ reacting with a wide variety of polar and non-polar neutral species. Mu is a light (0.11 amu) isotope of H with a positive muon replacing the proton. In almost all cases, both charge- and muon-transfer reactions are observed. Since charge transfer is endothermic in many cases, the reaction is believed to occur from rovibrationally excited states, (HeMuj* and (NeMu+)*, in accordance with the low efficiencies of He and Ne moderators for collisional deactivation. The total experimental rate constants are generally in good agreement with capture theories (Langevin, ADO, AADO) and in excellent agreement with the few corresponding protonated ion measurements, regardless of the degree of internal excitation. The reacting muonated ions are found to form by association of a µ+ with the bath gas at muon kinetic energies <1 eV, and much of the binding energy is retained as rovibrational excitation. Collisional deactivation was investigated by varying the bath gas pressure (500~3000 torr) and by adding 0~2 torr Ar. A mechanism of de-excitation of (NeMuX+)* (for reactive gas X) is suggested, while direct quenching of (NeMu+)* and (HeMu+)* is less important, though it does occur.Science, Faculty ofChemistry, Department ofGraduat

Muonium Formation as a Probe of Radiation Chemistry in Sub- and Supercritical Carbon Dioxide

Muonium (Mu = μ+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 pc = 0.47 g/cm 3. The fraction of muon polarization in muonium, PMu, 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, PD, was found to be almost independent of density in CO2, attributed to the formation of the stable solvated MuCO2+ molecular ion in this hydrogen-free solvent. The differing density dependences of both the Mu and the diamagnetic fraction in CO2, in comparison with the rather similar trends seen for both in C2H2, 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 ScCO 2, 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

Hyperfine Interactions and Molecular Motion of the Mu−Ethyl Radical in Faujasites: NaY, HY, and USY

The adsorption and dynamical behavior of the Mu-ethyl radical (MuC 2H 4) in NaY, HY, and USY faujasites was investigated by the muon spin resonance (μSR) technique, at loadings of one to five ethene molecules per supercage and over a temperature range of ca. 5-500 K (for NaY). The temperature dependences of both the muon and proton hyperfine coupling constants (Hfc's) are reported and compared with similar studies of MuC 2H 4 in different environments. Both transverse field (TF) μSR and avoided level crossing resonance (ALC) μSR spectra were recorded, with information on molecular motion mainly provided by the ALC line shapes. The muon Hfc's show only a small sensitivity to different frameworks and loadings but exhibit significant (∼10%) shifts at low temperatures, in comparison with bulk values, due to binding of the ethyl radical to cations at S II sites in NaY and to framework hydroxyls in the case of HY(USY). The Δ 1 resonances are symmetric and quite broad at the lower temperatures studied, but dramatically further broaden near room temperature, seen also in the TF relaxation rates, suggesting that the Mu-ethyl radical either desorbs from or hops between its binding sites at the higher temperatures. An Arrhenius estimate of the activation energy for desorption gives ∼ 20 kJ/mol, consistent with the dipolar interaction energy between the Mu-ethyl radical and an NaY cluster. The observation of such highly broadened Δ 1 ALC lines at the higher temperatures contrasts with the largely static line widths reported previously for the Mu-cyclohexadienyl radical (MuC 6H 6) in NaY. Sharper Δ 0 ALC lines for both the α and β protons of MuC 2H 4 appear near the same temperatures at which the Δ 1 lines overly broaden, and which persist to the highest temperatures (350 K). For NaY, the α proton resonances also broaden further at these temperatures. For both NaY and particularly HY, the temperature dependence of the α proton Hfc's indicates considerable distortion of the Mu-ethyl radical geometry, due to its binding to zeolite sites. Recently published calculations of binding energies and Hfc's for ethyl radicals in NaY and HY suggest a much stronger binding of the MuC 2H 4 radical than seems warranted by the data and pose as well a conundrum in comparison with earlier results for MuC 6H 6 in NaY. On the other hand, the temperature dependence of the isotropic muon Hfc's found from the T-atom model for NaY employed in these calculations is in excellent agreement with experiment. © 2007 American Chemical Society