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±0.4×${10}^{-14\mathrm{}}$ ${\mathrm{cm}}^2$.