Core electrons in the electronic stopping of heavy ions

Electronic stopping power in the $\mathrm{keV/\AA}$ range is accurately calculated from first principles. The energy loss to electrons in self-irradiated nickel, a paradigmatic transition metal, using real-time time-dependent density functional theory is studied. Different core states are explicitly included in the simulations to understand their involvement in the dissipation mechanism. The experimental data are well reproduced in the projectile velocity range of $1.0 - 12.0~\mathrm{atomic~units}$. The core electrons of the projectile are found to open additional dissipation channels as the projectile velocity increases. Almost all of the energy loss is accounted for, even for high projectile velocities, when core electrons as deep as $2s$ are explicitly treated. In addition to their expected excitation at high velocities, a flapping dynamical response of the core electrons is observed at intermediate projectile speeds.