He(21S) and He-formation in fast charge-changing collisions and evidence for a long-lived doublet state of He-

The formation of fast metastable helium atoms in two-electron capture by 200 keV 4 He 2+ ions in passage through hydrogen has been studied by means of the beam-attenuation technique. The results have been compared with metastable formation in one-electron capture by He + ions. The cross section for the process He 2+ +H 2 to He(2 1 S)+H + +H + which necessarily leads to 2 1 S rather than 2 3 S formation has been determined. The formation of He - ions in one-electron capture by 200 keV He(2 1 S) and (2 3 S) atoms has also been studied. The cross section for capture by 2 1 S atoms is shown to be approximately one half that for capture by 2 3 S atoms.