Influence of the electronic density of states on the trajectory-dependent energy deposition of keV ions in Silicon

We investigate effects of the density of states of condensed matter on the electronic excitations triggered by penetrating keV ions, in a systematic study of energy deposition along multiple well-defined channeling trajectories. We measure the specific energy deposition of ions with keV energies transmitted through Si - a band gap material - in the form of single-crystalline, self-supporting membranes. Energy transfers observed for Ne ions along the ⟨100⟩, ⟨211⟩, and ⟨111⟩ channeling orientations agree well in magnitude with predictions from density functional theory for the expected unperturbed electron densities in an electron gas. This agreement indicates that, along channeling trajectories, the interaction is dominated by conduction and valence electrons, with atomic (core-electron) processes largely suppressed. In contrast, for H and He ions, the predicted values are found systematically higher than the measured values. Non-linearities in the energy dependence of the specific energy deposition of Ne ions are found along all studied low-index orientations, with an inverted behavior observable for random in comparison to channeling orientation. In this context, we discuss the experimental challenges of limiting selected trajectories for the lowest velocities studied, which can mask effects of electronic excitation thresholds in the target electronic system. The new insights shed also light on earlier studies reporting a complex scaling of energy deposition with excitation thresholds, or even their apparent absence.