Influence of the excitation threshold of d electrons on electronic stopping of slow light ions

We present electronic stopping power data for H and He ions in Zn and In obtained from backscattering experiments in the range of low velocities; for In data are obtained up to the stopping maximum. These results in combination with previous data are analyzed to unravel the influence of d electrons in electronic stopping of slow light ions. We report on a systematic difference between the stopping behavior of H and He ions at velocities below 0.2 a.u. Furthermore, we find that the d-band offset is not correlated to the velocity at which deviations from velocity proportionality occur. This observation is in contrast to the state-of-the-art models of electronic stopping based on electron-hole pair excitation in a binary Coulomb collision. In this regard we discuss the possible relevance of electron promotion processes in atomic collisions.