title: Electrons ejected with half the projectile velocity and the saddle point mechanism in ion-atom collisions
abstract: Full three-dimensional ejected electron momentum distributions for proton impact ionization of atomic hydrogen are calculated for impact energies 10 through 50 keV. The distributions show a peak in the longitudinal momentum at half the projectile impact velocity: the v/2 peak. A quantitative assessment of saddle point ionization, based on quantum and classical analysis, reveals that the v/2 peak is a false indicator for this mechanism. The influence of the potential saddle on ionization is seen to decrease rapidly from 10 to 50 keV.
authors.affiliation: Department of Physics, Cardwell Hall, Kansas State University, Manhattan, Kansas 66506, USA.
authors.initials: EY
authors.lastname: Sidky
authors.initials: C
authors.lastname: Illescas
authors.initials: CD
authors.lastname: Lin