Temperature Dependence of Helium Metastability Exchange Cross Section

Abstract

The cross section for exchange of metastability in collisions between ${\mathrm{He}}^{3}$ atoms in the ground ($1^{1}S_{0}$) state and metastable ($2^{3}S_{1}$) state has been measured as a function of temperature by optical pumping techniques. The exchange cross section decreases from about 5.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$ at 500\ifmmode^\circ\else\textdegree\fi{}K to less than ${10}^{\ensuremath{-}17}$ ${\mathrm{cm}}^{2}$ at 4.2\ifmmode^\circ\else\textdegree\fi{}K. These results suggest a thermally activated exchange process in qualitative agreement with the predicted existence of a long-range repulsive interaction between metastable and ground-state helium atoms as proposed by Buckingham and Dalgarno.