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.

• Received 12 November 1999

DOI:https://doi.org/10.1103/PhysRevLett.85.1634