  We have measured the electronic energy-loss straggling of protons, helium,
boron and silicon ions in silicon using a transmission time-of-flight approach.
Ions with velocities between 0.25 and 1.6 times the Bohr velocity were
transmitted through single-crystalline Si(100) nanomembranes in either
channelling or random geometry to study the impact parameter dependence of
energy-loss straggling. Nuclear and path length contributions to the straggling
were determined with the help of Monte Carlo simulations. Our results exhibit
an increase in straggling with increasing ion velocity for channelled
trajectories for all projectiles as well as for protons and helium in random
geometry. In contrast for heavier ions, electronic straggling at low velocities
does not decrease further but plateaus and even seems to increase again. We
compare our experimental results with transport cross section calculations. The
satisfying agreement for helium shows that electronic stopping for light ions
is dominated by electron-hole pair excitations, and that the previously
observed trajectory dependence can indeed be attributed to a higher mean charge
state for random trajectories. No agreement is found for boron and silicon
indicating the breakdown of models based solely on electron-hole pair
excitations, and that local electron-promotion and charge-exchange events
significantly contribute to energy loss at low velocities.
