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

Abstract

The thermally averaged depolarization cross sections, \ensuremath{\sigma}${\ifmmode\bar\else\textasciimacron\fi{}}_{D}$(T), for Mu+${\mathrm{O}}_{2}$ electron spin exchange, have been measured by the muon spin relaxation (\ensuremath{\mu}SR) 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 (\ensuremath{\sigma}${\ifmmode\bar\else\textasciimacron\fi{}}_{\mathrm{SF}}$) by a simple numerical factor. At temperatures \ensuremath{\gtrsim}120 K, \ensuremath{\sigma}${\ifmmode\bar\else\textasciimacron\fi{}}_{D}$(T) is essentially temperature independent [with \ensuremath{\sigma}${\ifmmode\bar\else\textasciimacron\fi{}}_{\mathrm{SF}=(5.7\ifmmode\pm\else\textpm\fi{}0.5)\ifmmode\times\else\texttimes\fi{}{10}^{\mathrm{\ensuremath{-}}16}}$ ${\mathrm{cm}}^{2}$], though exhibiting a slight tendency to increase with temperature. At lower temperatures, \ensuremath{\sigma}${\ifmmode\bar\else\textasciimacron\fi{}}_{\mathrm{SF}(\mathrm{T}}$) decreases noticeably. Comparison with the only currently available theoretical calculations of Mu(H)+${\mathrm{O}}_{2}$ spin-flip cross sections by Aquilanti, Grossi, and Lagan\`a [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 \ensuremath{\sigma}${\ifmmode\bar\else\textasciimacron\fi{}}_{\mathrm{SF}(\mathrm{T}}$) 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 \ensuremath{\sigma}${\ifmmode\bar\else\textasciimacron\fi{}}_{\mathrm{SF}(\mathrm{H})\mathit{\ensuremath{\gtrsim}}1.5\mathit{\ensuremath{\sigma}}{\ifmmode\bar\else\textasciimacron\fi{}}_{\mathrm{SF}}}$(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.