Formation of He- ions in charge-changing collisions during passage of fast helium beams through hydrogen

The role of fast metastable helium atoms in the formation of He - ions by charge-changing collisions during the passage of 100 kev helium beams through hydrogen has been investigated quantitatively. It is shown that He - ions are formed by collisions involving one-electron capture by fast metastable helium atoms while all other possible production mechanisms are found to be relatively insignificant. A lower bound of 5·4 ± 0·4 × 10 -18 cm 2 is obtained for the cross section for He - formation from He(2 3 S) atoms.