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