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Charge equilibration and energy loss of slow highly charged ions in single layer graphene

Wilhelm, R. A.; Gruber, E.; Smejkal, V.; Schwestka, J.; Kozubek, R.; Hierzenberger, A.; Schleberger, M.; Facsko, S.; Aumayr, F.


Processes in ion-solid interaction are subject to both fundamental research and technological application. The ion stopping force, i.e. the kinetic energy loss per unit length in a solid is here a key main parameter. The stopping force depends on the kinetic energy of the ions as well as on the ionic charge [1]. Usually the influence of the charge state is neglected, because after a few nanometers in a solid the ion accommodates an equilibrium charge state Qeq independent of it’s initial charge. Stopping force in charge equilibrium is well understood.
In this study we use slow (v≪v0) highly charged ions (Qeq≪Q) to study stopping force far from charge equilibrium [2,3]. Using novel two-dimensional materials as target material allows us to limit the ion interaction in the solid to only a single scattering event.
In our experiment an ion beam is transmitted through a freestanding single layer of graphene. The ion energy and charge state after transmission are measured with an electrostatic analyzer. Ions at low velocity (0.05v0 - 0.25v0) are not fully neutralized. Hence, they still capture and stabilize about 20 electrons within the collision time of less than 3 fs. Especially stabilization of the electrons is surprising, since the classical-over- barrier model for charge exchange [4] predicts the population of highly excited states with principal quantum numbers of n > 10 and a subsequent Auger electron cascade. Such a cascade would lead to the reemission of electrons and thus to a recharging of the ion.
This ultrafast charge exchange process is accompanied by a kinetic energy loss of up to ∆E /E ≈ 10 %, which is about 1 order of magnitude larger than predicted by the SRIM code.

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Related publications

  • Invited lecture (Conferences)
    27th International Conference on Atomic Collisions in Solid (ICACS), 24.-29.07.2016, Lanzhou, China
  • Lecture (Conference)
    Frühjahrestagung der Deutschen Physikalischen Gesellschaft, Sektion Kondensierte Materie, 07.-10.03.2016, Regensburg, Deutschland