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
