Temperature Dependence of Helium Metastability Exchange Cross Section

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×${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$ at 500°K to less than ${10}^{-17\mathrm{}}$ ${\mathrm{cm}}^2$ at 4.2°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.