Saddle-point electrons in ionizing ion-atom collisions

We have studied ${\mathrm{H}}^{+}$+He ionizing collisions, and find that electrons stranded between the two postcollision Coulomb centers dominate the ejected-electron spectra for intermediate (∼100 keV) projectile energies. The importance of these electrons is clearly established by our classical calculations which are in qualitative agreement, in both shape and magnitude, with experimental spectra taken by us, and with earlier measurements. These ‘‘saddle-point’’ electrons are produced by a classically understandable mechanism, and are unambiguously observed at nonzero scattering angles.