UBC Theses and Dissertations
Muonium and positronium as microprobes of surfaces and solids Kiefl, Robert Francis
The properties of muonium(μ⁺e⁻) and positronium(e⁺e⁻) are altered significantly in the presence of matter. The study of these exotic H-like atoms provides a unique perspective on atomic interactions with atoms, surfaces, and solids. This theme is explored in a variety of hosts. The cross section for spin 1 positronium to be converted to spin 0 positronium during collisions with O₂ molecules has been measured from 120 °K to 630 °K in an SiO₂ powder moderator using a positron lifetime technique. The results indicate that positronium does not thermalize in the powder below 450 °K. The spin conversion cross section increases slightly with temperature above 450 °K. A theory for spin conversion of positronium by a spin 1 molecule is developed and used to interpret the data. Muon Spin Rotation measurements, in SiO₂, Al₂O₃, and MgO powders at low temperature in an atmosphere of He indicate that muonium emerges from the surfaces regardless of the ambient temperature of the powder. The muonium spin relaxation rate in Al203 in a He(or Ne) atmosphere is found to be a linear function of the fraction of surface area not covered by adsorbed He(or Ne). The cross sections for muonium to scatter elastically off adsorbed He and Ne atoms have been measured to be 11.0±0.2 Å² and 8.9±0.2 Å², respectively. The first observations of muonium in the condensed phases of Ar, Kr, and Xe are presented. The data indicate that there is a high probability of muonium formation in all cases. The spin relaxation rate of muonium in solid Xe is ten times that in the liquid, where the random local fields from the nuclear moments of ¹³⁹Xe and ¹⁴¹e are averaged by additional translational motion.
Item Citations and Data