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

TL;DR

This study demonstrates how the electronic excitations and energy loss of ions in silicon depend on their trajectory and velocity, highlighting the role of reionization events and crystal orientation effects at low ion velocities.

Contribution

It provides experimental evidence of trajectory-dependent electronic stopping in silicon at low velocities, serving as benchmarks for theoretical models using time-dependent density functional theory.

Findings

Energy loss varies with crystal orientation for all ions.

Heavier ions show more pronounced trajectory dependence, increasing at lower velocities.

Reionization events in close collisions influence charge states and energy loss.

Abstract

We present experiments demonstrating trajectory-dependent electronic excitations at low ion velocities, where ions are expected to primarily interact with delocalized valence electrons. The energy loss of H$^+$, H$_2 ^+$, He$^+$, B$^+$, N$^+$, Ne$^+$, $^{28/29}$Si$^+$ and Ar$^+$ in self-supporting silicon membranes was analysed along channelled 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 channelling 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 channelled trajectories. These processes result in trajectory-dependent mean charge states, which strongly affects 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.