Projectile- and target-charge dependent effects in ionizing collisions of H⁺ and He²⁺ with He, Ne and Ar atoms

The spectra of electrons emitted in collisions between H⁺ and He²⁺ projectiles and He, Ne and Ar targets at energies of 50 and 100 keV amu^-1 have been studied. The data are in qualitative agreement with results of Irby el al. (1988), but are in disagreement with measurements of Bernardi et al. (1989, 1990). It is shown that the observed electron spectra have a dependence on both target-ion and projectile effective charge that can be understood qualitatively in terms of 'saddle-point' ionization. Several issues relevant to saddle-point ionization are discussed.