Spin Exchange between Optically Oriented Metastable Helium Atoms and Thermal Electrons in a Flowing Afterglow

Abstract

The effects of spin-exchange collisions between thermal electrons and optically oriented metastable helium (${2}^{3}{S}_{1}$) atoms have been observed in a flowing helium afterglow, and the exchange cross section has been measured. The source of electrons derives from Penning collisions between the 19.8-eV metastable atoms and impurity atoms which are titrated into the flow region. The presence of thermal electrons shortens the metastable-spin-state lifetime and reduces the magnitude of the optical-pumping resonance signal. The electron density is determined from the reduction of the metastable density. The exchange cross section is found to be 1.6\ifmmode\pm\else\textpm\fi{}0.4\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}14}$ ${\mathrm{cm}}^{2}$.