Charge Exchange Cross Sections for Helium Ions in Gases

The charge exchange cross sections have been determined for a helium ion beam in several stopping gases. The cross sections for electron loss by a fast helium atom (σ₀₁) and for electron capture by an ion (σ₁₀) are reported for energies between 4 and 200 kev. The target gases studied were hydrogen, helium, nitrogen, oxygen, neon, and argon. σ₀₁ increases monotonically throughout the energy range for all gases studied, obtaining values of 10^-6 cm² at 200 kev. In all stopping gases except helium, σ₁₀ passes through a maximum of approximately 3×{}10^-16 cm² near 50 kev, whereas for helium this cross section decreases throughout the energy range as expected for the resonant exchange reaction. Evidence is presented that the metastable excited state of the helium atom is of importance in the charge exchange process.