Ionization of helium and molecular hydrogen by slow antiprotons

Abstract

Measurements of the cross sections for single and double ionization or helium as well as for the creation of H⁺₂ and H⁺ ions from H₂ for impact of antiprotons in the energy range 13-500 keV are presented. The results are compared with our earlier, less accurate data and with data for equivelocity proton impact. The single ionization cross section of helium agrees remarkably well with a continuum distorted wave (CDW-EIS) calculation. The ratio between the double and the single ionization cross sections of helium increases dramatically with decreasing projectile energy. New theoretical calculations are called for.