INTERACTION OF MUONIUM WITH OXYGEN ON SILICA POWDER SURFACES

Results of the first mu-SR 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 mu-s-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) cm2 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) x 10-10(-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.