  Electronic stopping cross sections (SCS) of nickel, silicon and
nickel-silicon alloys for protons and helium (He) ions are studied in the
regime of medium and low energy ion scattering, i.e., for ion energies in the
range from 500 eV to 200 keV. For protons, at velocities below the Bohr
velocity the deduced SCS is proportional to the ion velocity for all
investigated materials. In contrast, for He ions non-linear velocity scaling is
observed in all investigated materials. Static calculations using density
functional theory (DFT) available from literature accurately predict the SCS of
Ni and Ni-Si alloy in the regime with observed velocity proportionality. At
higher energies, the energy dependence of the deduced SCS of Ni for protons and
He ions agrees with the prediction by recent time dependent DFT calculations.
The measured SCS of the Ni-Si alloy was compared to the SCS obtained from
Bragg's rule based on SCS for Ni and Si deduced in this study, yielding good
agreement for protons, but systematic deviations for He projectiles, by almost
20%. Overall, the obtained data indicate the importance of non adiabatic
processes such as charge exchange for proper modelling of electronic stopping
of in particular medium energy ions heavier than protons in solids.
