Double Electron Capture and Loss by Helium Ions Traversing Gases

A beam of pure He⁺⁺ ions was passed through a cell in which the gas pressure could be varied, and the emergent beam was examined for its He⁰ and He⁺ content. With helium and air in the cell, it was observed that He⁰ persists to pressures so low that it could not have been formed except by double electron capture by He⁺⁺ in a single collision. In hydrogen the double capture event is much less probable.The analogous experiment was performed with a helium atomic beam, and at 250 kev and 450 kev in air He⁺⁺, formed by double electron loss, was detected. In hydrogen and helium only upper limits for double electron loss could be established.These results, with values of the sums (σ₂₀+σ₂₁) and (σ₀₁+σ₀₂) measured in previous researches, made it possible to compute values of σ₂₀ and σ₀₂, as in the following table. Cross sections in units of 10^-17 cm² per atom A table of the six charge-changing collision cross sections for helium ions is presented. The equations needed to compute the change in the charge composition of a helium ion beam due to passage through a gas film are given.