Differential cross section for electron impact excitation of metastable helium

By analyzing the time-of-flight distribution of metastable helium atoms produced by bombardment of a ground-state atomic beam by low-energy electrons, we have measured the differential cross section for excitation of the $2{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$ state over the energy range 19.9-20.4 eV. The results are compared with those determined from electron energy-loss experiments as well as with various theoretical predictions.