Muonium depolarization by electron spin exchange withO2gas in the temperature range 90–500 K

The thermally averaged depolarization cross sections, σ${¯}_D$(T), for Mu+${\mathrm{O}}_2$ electron spin exchange, have been measured by the muon spin relaxation (μSR) technique in a ${\mathrm{N}}_2$ 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 (σ${¯}_{\mathrm{SF}}$) by a simple numerical factor. At temperatures ≳120 K, σ${¯}_D$(T) is essentially temperature independent [with σ${¯}_{\mathrm{SF}=(5.7\mathrm{}\pm 0.5)\times {10}^{\mathrm{-}16}}$ ${\mathrm{cm}}^2$], though exhibiting a slight tendency to increase with temperature. At lower temperatures, σ${¯}_{\mathrm{SF}(\mathrm{T}}$) decreases noticeably. Comparison with the only currently available theoretical calculations of Mu(H)+${\mathrm{O}}_2$ spin-flip cross sections by Aquilanti, Grossi, and Laganà [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 σ${¯}_{\mathrm{SF}(\mathrm{T}}$) for Mu+${\mathrm{O}}_2$ 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+${\mathrm{O}}_2$ 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 σ${¯}_{\mathrm{SF}\left(\mathrm{H}\right)\mathit{\gtrsim }1.5\mathit{\sigma }{¯}_{\mathrm{SF}}}$(Mu). While this effect can be qualitatively understood in terms of the differing numbers of partial waves involved, detailed theoretical calculations on more recent potential-energy surfaces for ${\mathrm{HO}}_2$ are called for.