Experimental Identification of Saddle-Point Electrons

Experimental and theoretical velocity distributions of electrons ejected in 1-6 keV ionizing ${\mathrm{H}}^{+}$-H collisions are presented. Both the experimental velocity distributions and the relative total ionization cross sections can be explained by the theory in a fully consistent way. Strong evidence is found for the existence of a saddle-point ionization mechanism at collision energies of 4 keV and higher.