  We have simultaneously measured angular distributions and electronic energy
loss of helium ions and protons directly transmitted through self-supporting,
single-crystalline silicon foils. We have compared the energy loss along
channelled and random trajectories for incident ion energies between 50 keV and
200 keV. For all studied cases the energy loss in channelling geometry is found
lower than in random geometry. In the case of protons, this difference
increases with initial ion energy. This behaviour can be explained by the
increasing contribution of excitations of core electrons, which are more likely
to happen at small impact parameters accessible only in random geometry. For
helium ions we observe a reverse trend - a decrease of the difference between
channelled and random energy loss for increasing ion energy. Due to the
inefficiency of core-electron excitations even at small impact parameters at
such low energies, another mechanism has to be the cause for the observed
difference. We provide evidence that the observation originates from
reionisation events induced by close collisions of helium ions occurring only
along random trajectories.
