Projectile Charge Effects in Multiple Ionization of Argon by Positron and Electron Impact

Differential single-And multiple-ionization cross section ratios have been measured for 750 eV positron and electron impact on argon as a function of the projectile energy-loss from threshold up to 85% of the initial projectile energy. In addition, differential multiple ionization cross section ratios have been obtained for 500 eV electron impact as a function of both the scattering angle and projectile energy loss. The differential single-ionization cross sections obtained with positrons and electrons are found to have the same shape, i.e., the same energy loss dependence, in agreement with distorted wave Born predictions. The differential double-And triple-ionization cross sections obtained by electron impact, however, are systematically larger than those obtained with positron impact. This charge effect has been attributed to the quantal interference between the mechanisms leading to double ionization.