  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.
