Electron-emission processes in highly charged Ar and Xe ions impinging on highly ordered pyrolytic graphite at energies just above the kinetic threshold

At keV energies, many electronic processes contribute to the emission of secondary electrons in the interaction of highly charged ions on surfaces. To unravel contributions resulting from isolated hollow atoms in front of the surface or embedded in the electron gas of the target, heavy highly charged Ar and Xe ions are collided on highly ordered pyrolytic graphite (HOPG). A light target material as HOPG assures straight-line trajectories in the subsurface range. By a systematic change of incidence angle and energy of low-charged Ar and Xe ions, the kinetic electron emission component is determined. Separating out the kinetic energy contribution for the highly charged ions yields pure potential-energy-driven secondary-electron yields. From these yields it is concluded that in contrast to metallic targets, only a small fraction of the secondary electrons stem from above the surface. The lack of above-surface emission is likely due to the semimetallic electronic structure of HOPG. The subsurface emission is found to scale with the increase in binding energy of the inner-shell hole when incrementing the charge state of the projectile Ar or Xe ions.