Double Electron Capture and Loss by Helium Ions Traversing Gases

A beam of pure ${\mathrm{He}}^{++}$ ions was passed through a cell in which the gas pressure could be varied, and the emergent beam was examined for its ${\mathrm{He}}^0$ and ${\mathrm{He}}^{+}$ content. With helium and air in the cell, it was observed that ${\mathrm{He}}^0$ persists to pressures so low that it could not have been formed except by double electron capture by ${\mathrm{He}}^{++}$ in a single collision. In hydrogen the double capture event is much less probable.