Cross Sections for Double Electron Capture by 2-50-keV Protons Incident upon Hydrogen and the Inert Gases

The cross section for capture of two electrons by protons in single collisions with hydrogen and inert gas targets has been measured over the energy range 2 to 50 keV. Cross-section values are given relative to the well-known single-electron-capture cross section by protons in each target gas. In hydrogen, a peak value of 1.0×{}10^-17 cm²/molecule at 17 keV is obtained for the double-capture cross section σ_1,-1. The double-peaked cross section versus velocity function, which is probably attributable to 10-12 and 10-13e processes, is confirmed. In He, the cross section σ_1,-1 reaches a maximum of 7.10^-19 cm²/atom at 30 keV and steadily decreases to 2.10^-20 cm²/atom at 2 keV. In Ne, the present values show a peak cross section of 1.2×{}10^-18 cm²/atom being reached at 14 keV. The present values when extrapolated above 50 keV agree well with Mittleman's corrected Born approximation calculations for H₂.