Anticusp and binary peak structures in the electronic spectra arising from proton- and antiproton-helium collisions

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Abstract
The spectra of electrons ejected in collisions of protons and antiprotons with helium at impact energies of 100 and 500 keV are reported and compared. These calculations extend recent studies of the electronic spectra by means of the classical-trajectory Monte Carlo method to consider more energetic ejected electrons. Specifically, these new calculations exhibit the well-known binary peak, around which protons and antiprotons are shown to yield similar results. On the other hand, as predicted by quantum-mechanical distorted-wave approximations, a clear dip or anticusp is found in the electronic spectra produced by antiproton impact at electron velocities close to the projectile velocity, unlike the enhancement or cusp found for proton impact. Our results are consistent with simple dynamical mechanisms that reflect the two-center nature of the collision.