On the disappearance of Z₁ oscillations in electronic stopping power with velocity from first principles

The electronic stopping power for projectiles going through metals is known to oscillate as a function of the projectile atomic number (Z₁) at low velocity, v, related to the number of valence electrons of the projectile when increasing Z₁. For sufficiently high values of v, that oscillations disappear, which is rationalized in terms of core electrons becoming active in the dissipation process. Here we present first principles calculations of electronic stopping power for oxygen and magnesium projectiles, which are the typical maximum and minimum of Z₁ oscillations, in aluminium as a prototypical metal. We use real-time time-dependent density-functional theory. We observe that the crossing is extremely sensitive to trajectory. We correlate the difference in stopping power with the charge state, which we characterize using several population analysis methods, Voronoi and Hirshfeld, and also looking at deformation density plots, towards understanding the change in behaviour.