Electronic energy loss of light ions transmitted through nanometer films of Al has been studied at very low ion velocities. For hydrogen, the electronic stopping power S is found to be perfectly proportional to velocity, as expected for a free electron gas. For He, the same is anticipated, but S shows a transition between two distinct regimes, in both of which S is velocity proportional-however, with remarkably different slopes. This finding can be explained as a consequence of charge exchange in close encounters between He and Al atoms, which represents an additional energy loss channel.

Paper accepted for publication by Physical Review A Link to the abstract: https://journals.aps.org/pra/accepted/af070N0aRc31281619294920dc695c1c385658bbf 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 timedependent 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.

No abstract