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

Abstract

We present experiments demonstrating trajectory-dependent electronic excitations at low ion velocities, for which ions are expected to primarily interact with delocalized valence electrons. The energy loss of ${\mathrm{H}}^{+},$ ${{\mathrm{H}}_{2}}^{+},$ ${\mathrm{He}}^{+},$ ${\mathrm{B}}^{+},$ ${\mathrm{N}}^{+},$ ${\mathrm{Ne}}^{+},$ $^{28,29}\mathrm{Si}^{+},$ and ${\mathrm{Ar}}^{+}$ in self-supporting silicon membranes was analyzed along channeled and random trajectories in a transmission approach. For all ions, we observe a difference in electronic stopping dependent on crystal orientation. For heavier ions, the energy-loss difference between channeling and random geometry is generally found more pronounced, and, in contrast to 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 affect the energy loss. The strength of the effect seems to exhibit a ${Z}_{1}$ oscillation 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.

Topics & Concepts