Scattering Mechanisms in Ionization of Helium by Electron Vortex Projectiles*

For electron-impact ionization of helium into the azimuthal plane with symmetric energy sharing, the triple differential cross sections (TDCSs) show qualitative features that can be used to distinguish single and double scattering mechanisms.  For traditional, non-vortex projectiles, emission of electrons into the azimuthal plane requires a double scattering mechanism, and this mechanism becomes more important with increasing projectile energy.  At low projectile energies, the dominant ionization mechanism is a single binary collision.  In this work, we examine scattering mechanisms for electron vortex projectiles through the calculation of TDCSs using the distorted wave Born approximation.  Our results predict that the ionization mechanisms for vortex projectiles are similar to those of non-vortex projectiles.  However, some key differences are observed.  For vortex projectiles, a double scattering mechanism is not required for electron emission into the azimuthal plane, although the shape of the TDCS is still significantly influenced by double scattering, particularly at higher projectile energies.  At low projectile energies, ionization proceeds primarily through a single scattering mechanism, although unlike non-vortex projectiles, double scattering is also important.  Our results lead to several predictions that can be experimentally tested.