Interaction of muonium with oxygen on silica powder surfaces

Results of the first μSR studies using Merck FO Optipur silica powder, which contains paramagnetic impurities at the ppb level and has a surface area of 610±20 m2/g, are reported. Above 20 K, the transverse field muonium relaxation rate is roughly constant at 0.5 μs-1. Upon the addition of oxygen at ppm levels, the relaxation rate increases linearly with O2 concentration in the temperature range from 40-100 K yielding two-dimensional depolarization rate constants on the order of 10-4 cm-2 molecule-1 s-1. As the temperature is increased further, both oxygen and muonium desorb from the surface yielding a three-dimensional rate constant at 300 K of 3.1(3)x10-10-10 cm3 molecule-1 s-1, in agreement with the gas phase value. Longitudinal field measurements suggest that MuO2 is formed and is able to spin exchange with other oxygen molecules. (orig.)