Negative Ion Formation of Hydrogen and Helium in Low Energy Collisions of H+ and He+ with Potassium Atoms

Mechanism for negative ion formation of hydrogen and helium by electron capture of H + and He + in the passage through potassium vapor has been studied. A 60° sector mass spectrometer equipped with two potassium cells arranged along the beam axis is used. The translational energy analysis by the magnetic field of the formed negative ions has been made. The main processes of the H - and He - formation are double-collision processes via H(1s) and He(2 3 S), respectively, confirming the description by Donnally et al. It is suggested that H - ions in the (2p 2 ) 3 P state are formed by single-collision process. In the case of K target, H - ions formed from H(2s) are negligible compared with those from H(1s). The metastable fraction of neutral H beams produced by passage of H + beams through thin K vapor is found to be 0.27±0.02 at 3 keV.