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We have simultaneously measured angular distributions and electronic energyloss of helium ions and protons directly transmitted through self-supporting,single-crystalline silicon foils. We have compared the energy loss alongchannelled and random trajectories for incident ion energies between 50 keV and200 keV. For all studied cases the energy loss in channelling geometry is foundlower than in random geometry. In the case of protons, this differenceincreases with initial ion energy. This behaviour can be explained by theincreasing contribution of excitations of core electrons, which are more likelyto happen at small impact parameters accessible only in random geometry. Forhelium ions we observe a reverse trend - a decrease of the difference betweenchannelled and random energy loss for increasing ion energy. Due to theinefficiency of core-electron excitations even at small impact parameters atsuch low energies, another mechanism has to be the cause for the observeddifference. We provide evidence that the observation originates fromreionisation events induced by close collisions of helium ions occurring onlyalong random trajectories.

Disparate energy scaling of trajectory-dependent electronic excitations for slow protons and He ions