Collisions of highly charged ions with 2D materials
- What we learn from ion transmission spectroscopy -

Slow ions in high charge states impacting a solid surface represent a far-from-equilibrium system. Upon impact, the ions capture dozens of electrons and these electrons decay into the atomic ground state already during the collision driven by non-radiative de-excitation processes such as Interatomic Coulombic Decay [1]. The neutralization and electronic decay of the ion leads to the release of its potential energy, which amounts up to several 10 keV facilitating nanostructure formation in suscep- tible materials (mainly insulators with strong electron-phonon coupling).
When a freestanding 2D material is used as a solid target, the ions are still available for spectroscopic measurements after the ion-surface interaction. We performed charge state and kinetic energy analy- sis of ions transmitted through freestanding single layer graphene (SLG) [2], amorphous 1 nm thick Carbon Nanomembranes (CNM) [3], freestanding single layer MoS2, SLG/MoS2 heterostructures and others. As a first result we found an ultrafast (sub-10 fs) neutralization taking place, much faster than established models would have anticipated. Further, kinetic energy loss is significantly en- hanced over the expected value from singly charged ions under the same conditions. We are able to find charge exchange patterns, utilizing angle-resolved charge exchange spectroscopy [4]. To facilitate a comprehensive understanding of the plethora of observed phenomena and their interplay, we developed an atomistic model for ion stopping, charge exchange and electronic decay taking the time-dependent ion charge state explicitly into account [5].
In this contribution I will show that charge exchange pattern together with an atomistic and local model for charge exchange can be used to determine the structure of 2D materials on a sub-nm level, especially important for amorphous materials where atomically-resolved microscopy is hard to perform.