Low-Energy Ionization of Argon Atoms by Argon Atoms

A neutral atomic beam has been used to measure the total ionization cross section for argon atoms incident on low-density argon gas. The beam was produced by symmetric charge transfer of argon ions produced in an electron-impact ion source with 20-eV electrons. Beam energies varied from 30 eV (just below threshold for electron production in ground-state atom collisions) to 2900 eV laboratory energy. As in earlier work, the neutral beam intensity was determined by measurements of slow and scattered ion currents in the charge-transfer process. The intensities were checked by measurements of secondary electron emission from a gold target. Comparison of these measurements with those made at higher energies by other investigators suggests additional broad structure, with possibly a minimum near 8 keV.