Trajectory-dependent electronic excitations by light and heavy ions around and below the Bohr velocity

S Lohmann,D Primetzhofer,R Holeňák

doi:10.1103/physreva.102.062803

Abstract

We present experiments demonstrating trajectory-dependent electronic excitations at low ion velocities, where ions are expected to primarily interact with delocalized valence electrons. The energy loss of H$^+$, H$_2 ^+$, He$^+$, B$^+$, N$^+$, Ne$^+$, $^{28/29}$Si$^+$ and Ar$^+$ in self-supporting silicon membranes was analysed along channelled and random trajectories in a transmission approach. For all ions, we observe a difference in electronic stopping dependent on crystal orientation. For heavier ions, the energy-loss difference between channelling and random geometry is generally found more pronounced, and, in contrast to protons, increases for decreasing ion energy. Due to the inefficiency of core-electron excitations at employed ion velocities, we explain these results by reionization events occurring in close collisions of ions with target atoms, which are heavily suppressed for channelled trajectories. These processes result in trajectory-dependent mean charge states, which strongly affects the energy loss. The strength of the effect seems to exhibit a Z$_1$ oscillation with an observed minimum for Ne. We, furthermore, demonstrate that the simplicity of our experimental geometry leads to results that can serve as excellent benchmark systems for dynamic calculations of the electronic systems of solids using time-dependent density functional theory.

Highlights

Energy transferred by energetic charged particles to matter governs a number of astrophysical phenomena [1] and leads to radiation damage in extreme environments [2]
A significant difference in the energy loss observed along random and channeled trajectories for the shown example becomes already apparent from Figs. 1 and 2. To study this observation more in detail and compare with our previous results for protons and He ions, we evaluate the energy loss along rather straight trajectories ending in circular regions of interest (ROIs) with 1-mm radius around the initial beam position for channeled ( Ech) and random incidence ( Er)
The comparison with the random incidence curve at the largest studied deflection angle indicates that the peak structure featuring a high-energy loss consists of ions that have traveled on completely random trajectories, i.e., have been dechanneled very close to the sample entry point

Summary

Introduction

Energy transferred by energetic charged particles to matter governs a number of astrophysical phenomena [1] and leads to radiation damage in extreme environments [2]. Detailed understanding of the energy-deposition mechanisms allows for predicting the mentioned effects and for using these processes for a number of scientific and technological applications ranging from hadron therapy for cancer treatment [3] to materials characterization and modification [4]. For semiconductors, the use of ion-beam irradiations is widely employed to manipulate material properties via implantation or controlled defect creation [5,6]. The energy deposition of ions is commonly denoted by the stopping power S, which is defined as the average energy loss per unit path length, i.e., S = −dE /dx. Many experimental studies determine S, in accordance with its definition, as an effective average along the ion trajectory, often by employing amorphous or polycrystalline samples. Several theoretical approaches have successfully predicted S without even taking the atomic structure or electronic binding of the target material into account [8,9]

Methods

Results

Discussion

Conclusion