  The electronic stopping power of H and He moving through gold is obtained to
high accuracy using time-evolving density-functional theory, thereby bringing
usual first-principles accuracies into this kind of strongly coupled, continuum
non-adiabatic processes in condensed matter. The two key unexplained features
of what observed experimentally have been reproduced and understood: (i) The
non-linear behaviour of stopping power versus velocity is a gradual crossover
as excitations tail into the d-electron spectrum; and (ii) the low-velocity
H/He anomaly (the relative stopping powers are contrary to established theory)
is explained by the substantial involvement of the d electrons in the screening
of the projectile even at the lowest velocities where the energy loss is
generated by s-like electron-hole pair formation only.
