Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf
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Ion-electron dynamics studied in a 3D-transmission approach
We present experiments demonstrating trajectory-dependent electronic excitations at low ion velocities attributed to charge-exchange events. Experiments were performed with the time-of-flight medium energy ion scattering set-up at Uppsala University . We employed pulsed beams of singly charged ions with masses from 1 (H+) to 40 u (Ar+) and energies between 20 and 300 keV. Ions are transmitted through self-supporting Si(100) nanomembranes and detected behind the sample. Fig. 1 demonstrates our experimental approach, in which ion energy loss is measured together with angular distributions for different beam-crystal alignments. We have analysed both trajectory-dependent electronic stopping and electronic energy-loss straggling. Our results show higher electronic stopping for random than for channelled trajectories for all studied ions . For ions heavier than protons, direct core-electron excitations at employed ion velocities are inefficient. We, therefore, explain our observation by reionisation events occurring in close collisions of ions with target atoms mainly accessible in random geometry . These events result in trajectory-dependent mean charge states, which heavily affects the energy loss. The electronic energy-loss straggling likewise exhibits a strong dependence on ion type, velocity and trajectory. For all ions, straggling in random geometry is higher than in channelling orientation. While for He straggling increases with ion velocity, for B travelling along random trajectories a minimum is observed in the studied velocity range. We compare experimental results for these two ions with predictions by the Chu model and transport cross section calculations (Penn-TCS model). We provide strong evidence that electron-hole pair creation alone cannot explain electronic excitations by slow ions other than protons. Especially for heavy ions, additional energy-loss processes such as charge exchange and autoionisation including possible alterations of the scattering potential  have to be taken into account.
 M. A. Sortica et al., Nucl. Instrum. Methods Phys. Res. B, 463 (2020) 16-20.
 S. Lohmann et al., Phys. Rev. A, 102 (2020) 062803.
 S. Lohmann and D. Primetzhofer, Phys. Rev. Lett., 124, (2020) 096601.
 R. A. Wilhelm and P. L. Grande, Communications Physics, 2 (2019) 89.
Invited lecture (Conferences)
27th International Symposium on Ion-Atom Collisions (ISIAC), 14.-16.07.2021, Online, Online