Trajectory-dependent electronic excitations by light and heavy ions around and below the Bohr velocity

We present experiments demonstrating trajectory-dependent electronic excitations at low ion velocities, where ions are expected to primarily interact with delocalized valence electrons. Experiments were performed using pulsed beams of singly charged ions incident on self-supporting Si(100) nanomembranes, and energy was measured together with the angular distribution after transmission through the sample. The energy loss of H+, H2+, He+, N+, Ne+, 28/29Si+ and Ar+ was analyzed along channeled and random trajectories. For all ions, we observe a difference in electronic stopping dependent on crystal orientation. For protons, the difference between channeled and random trajectories increases with ion energy, which is explained by increasing contributions of core-electron excitations more likely to happen at small impact parameters accessible only in random geometry. For heavier ions, the energy loss difference between channeling and random geometry is generally found more pronounced, and, different from protons, increases for decreasing ion energy. Due to the inefficiency of core-electron excitations at employed ion velocities, we explain these results by reionization events occurring in close collisions of ions with target atoms, which are heavily suppressed for channeled trajectories. These processes result in trajectory-dependent mean charge states, which strongly affects the energy loss. The strength of the effect seems to exhibit an oscillation with Z1 with an observed minimum for Ne. We, furthermore, demonstrate that the simplicity of our experimental geometry leads to results that can serve as excellent benchmark systems for dynamic calculations of the electronic systems of solids using time-dependent density functional theory