EPITHERMAL MUONIUM PROCESSES IN METHANE AND PROPANE GASES

Gas-phase mu-SR studies of dense methane up to 10 mol L-1 and of propane at low densities, up to 0.2 mol L-1, are reported. It is found that the diamagnetic polarization, P(D), increases in both gases with pressure, with a related decrease seen in the muonium polarization, P(Mu). The initial increase seen in P(D) at low densities is attributed to hot-atom (Mu*) abstraction and substitution reactions. Comparisons with both hot-tritium (T*) and epithermal H* reactions in the alkanes reveal substantial isotope effects. In the case of CH4, a continued increase in P(D) at higher densities is likely due to a proton-transfer reaction from the molecular ion CH4Mu+ in the radiolysis track, forming CH3Mu, rather than to hot-atom reactions. Extrapolation of the present trend with density gives agreement with the value of P(D) found in liquid CH4, indicating that the diamagnetic muon polarization in dense alkanes depends primarily on density rather than on temperature or phase.