Pre-equilibrium stopping power mechanisms in proton-irradiated aluminum sheets

Although ion-irradiation is a standard tool for imaging and altering materials' surfaces, most existing knowledge concerning an energetic ion's interaction with solids is limited to bulk materials. As an ion approaches and traverses a surface, it is expected to exchange charge with the material and transition into bulk conditions resembling a steady-state. To gain insight into this so-called pre-equilibrium behavior, we use real-time time-dependent density functional theory to simulate protons with 6 - 60 keV of kinetic energy impacting few-layer aluminum sheets. We find up to 25% stopping power enhancement in aluminum sheets compared to bulk, particularly near the entrance surface, and we examine several possible explanations for this behavior including pre-equilibrium projectile charge dynamics, polarization induced in the sheet during the proton's approach, and surface plasmon excitations. Our analysis suggests that surface plasmons are a plausible source of stopping power enhancement, while the contribution of the other potential mechanisms is either small or inconclusive. Such pre-equilibrium behavior has particularly important implications for optimizing focused ion beam techniques for 2D materials.