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

Abstract
The formation of fast metastable helium atoms in two-electron capture by 200 keV 4He2+ 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 He2++H2 to He(21S)+H++H+ which necessarily leads to 21S rather than 23S formation has been determined. The formation of He- ions in one-electron capture by 200 keV He(21S) and (23S) atoms has also been studied. The cross section for capture by 21S atoms is shown to be approximately one half that for capture by 23S atoms.