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 (${\sigma }_{01}$) and for electron capture by an ion (${\sigma }_{10}$) are reported for energies between 4 and 200 kev. The target gases studied were hydrogen, helium, nitrogen, oxygen, neon, and argon. ${\sigma }_{01}$ increases monotonically throughout the energy range for all gases studied, obtaining values of ${10}^{-6\mathrm{}}$ ${\mathrm{cm}}^2$ at 200 kev. In all stopping gases except helium, ${\sigma }_{10}$ passes through a maximum of approximately 3×${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$ 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.