Assessing electronic stopping cross sections of light ions at low ion energies: The impact of crystallinity and surface orientation

• MD simulations reveal crystal orientation effects on low-energy He scattering. • Close-packed surfaces yield higher backscattering and smaller impact parameters. • fcc and bcc metals show distinct orientation-dependent scattering behavior. • Structural effects explain biases in low-energy electronic stopping measurements using the relative approach. • Impact-parameter selectivity challenges the concept of a single global stop cross section. Accurate knowledge of the energy deposition of slow ions in solids is essential for modelling plasma-material interactions. While nuclear stopping can be reliably predicted through simulations electronic stopping at very low energies remains challenging to determine. Using molecular dynamics simulations, we investigate how crystallographic structure and surface orientation affect the backscattering probability and impact parameters of low-energy He in fcc and bcc metals. On this basis we evaluate challenges for typical approaches to assess electronic stopping. Close-packed surface orientations yield higher backscattering and smaller mean impact parameters due to reduced channel sizes, with characteristic differences for different crystal structures. For fcc Au, the (1 1 1) surface behaves similarly to a pseudo-amorphous target, whereas for bcc W, the (1 1 0) surface shows a significantly lower backscattering probability. These structural effects can explain the observed energy scaling of the electronic stopping power in some bcc material systems extracted from relative measurements using fcc reference materials. The results furthermore highlight that crystallographic orientation and impact-parameter selectivity can strongly bias measurements of electronic stopping at low energies, severely challenging the applicability of a single global electronic stopping cross section.