Projectile's Core Electrons and Large Values of Electronic Stopping

Light projectiles (H, He) shooting through solids lose energy at a rate of the order of a few eV/Ang, this value peaking at projectile velocities around 1% of the speed of light. These processes are well reproduced in simulations of electronic stopping using time-dependent density-functional theory in real time. Good agreement with non-trivial experimental results have already been obtained for a variety of hosts materials or liquids. Heavier projectiles as 3d transition metals, however, give rise to electronic stopping powers in the keV/Ang range, and the maximum stopping occurs at velocities several times higher. Here we present results of Ni projectiles shooting through bulk Ni using a plane-wave implementation of TD-DFT(t). The effect of core electrons is investigated, finding that the explicit and flexible consideration of deep core states of the projectile is crucial for an accurate description of the problem. Remarkably, instantaneous semilocal TD-DFT seems to capture the relevant physics in the electron excitation process relevant to electronic stopping.