Spin-exchange and spin-destruction rates for the3He−Nasystem

Abstract

Optically pumped Na is used as a spin-exchange partner to polarize ${}^{3}\mathrm{He}.$ Polarizations around 20% have routinely been achieved in sealed spherical glass cells containing ${}^{3}\mathrm{He},$ ${\mathrm{N}}_{2},$ and a few droplets of Na. An optical technique has been developed to determine the $\mathrm{Na}{\ensuremath{-}}^{3}\mathrm{He}$ spin-exchange rate coefficient. By monitoring the Na spin relaxation ``in the dark,'' the average Na-Na spin-destruction cross section at $330\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ is estimated to be around $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}19}{\mathrm{cm}}^{2}.$ This value is 2--5 (15--30) times smaller than the previously reported values for the K-K (Rb-Rb) spin-relaxation cross section. In the temperature range $310--355\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ the spin-exchange rate coefficient is found to be $(6.1\ifmmode\pm\else\textpm\fi{}0.6)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}20}{\mathrm{cm}}^{3}/\mathrm{s}$ with no detectable temperature dependence. This value is in good agreement with a previous theoretical estimate reported by Walker and it is only slightly lower than the corresponding $\mathrm{Rb}{\ensuremath{-}}^{3}\mathrm{He}$ spin-exchange rate coefficient. The total $\mathrm{Na}{\ensuremath{-}}^{3}\mathrm{He}$ spin-destruction rate coefficient is, within errors, found to be the same as the $\mathrm{Na}{\ensuremath{-}}^{3}\mathrm{He}$ spin-exchange rate coefficient, thereby indicating that the maximum possible photon efficiency may approach unity for the $\mathrm{Na}{\ensuremath{-}}^{3}\mathrm{He}$ system. A technique, in which a charge-coupled device camera is used to take images of faint unquenched fluorescence light, has been utilized to allow for an instantaneous determination of the sodium number densities during the rate coefficient measurements.