Muonium depolarization by electron spin exchange with O2 gas in the temperature range 90--500 K

The thermally averaged depolarization cross sections, sigma-bar/sub D/(T), for Mu+O2 electron spin exchange, have been measured by the muon spin relaxation (μSR) technique in a N2 moderator at total pressures near 1 atm over the temperature range 88--500 K. These values are related to the thermal spin-flip cross sections of interest (sigma-bar/sub SF/) by a simple numerical factor. At temperatures approx. >120 K, sigma-bar/sub D/(T) is essentially temperature independent [with sigma-bar/sub SF/ = (5.7 +- 0.5) x 10/sup -16/ cm2], though exhibiting a slight tendency to increase with temperature. At lower temperatures, sigma-bar/sub SF/(T) decreases noticeably. Comparison with the only currently available theoretical calculations of Mu(H)+O2 spin-flip cross sections by Aquilanti, Grossi, and Lagana [Hyperfine Interact. 8, 347 (1981)] on the potential-energy surface of Farantos et al. [Mol. Phys. 34, 947 (1977)] gives poor agreement with the data, particularly in their temperature dependence. The present results for sigma-bar/sub SF/(T) for Mu+O2 qualitatively exhibit the same trend with temperature as found by Desaintfuscien and Audoin [Phys. Rev. A 13, 2070 (1976)] for H-H spin exchange over a comparable temperature range, but the H-H cross sections are, surprisingly, about four times larger. Comparisons with the experimental H+O2 spin-exchange cross sections of Anderle et al. [Phys. Rev. A 23, 34 (1981)] and of Gordon et al. [JETP Lett. 17, 395 (1973)], indicate a significant isotope effect, with sigma-bar/sub SF/(H)approx. >1.5sigma-bar/sub SF/(Mu)