The study of multiply-charged ion-solid interactions is of considerable technological importance for the understanding of material damage, surface modification, and plasma-wall interactions. The recent availability of sources for slow highly charged ions (HCI), namely electron cyclotron resonance (ECR) and electron beam ion sources (EBIS) has led to a flurry of research activities, both experimental and theoretical, in the field of HCI-solid interactions. On the most fundamental level, its importance is derived from the complex many-body response of surface electrons to the strong Coulomb perturbation.

Hollow atoms

From numerous experimental as well as theoretical studies the following scenario of the HCI-surface interaction has emerged: When an HCI approaches a solid surface, one or more electrons are resonantly captured at large distances into high Rydberg states of the projectile. As a result, so-called hollow atoms (ions) are formed where the atomic charge cloud transiently resides in shells with large diameters while the core is virtually empty. Direct observation of this short-lived state is complicated by the fact that the ion is always attracted towards the surface by its self-image potential. Consequently it will suffer close collisions upon impact on the surface and the memory of the hollow atom is all but erased. This problem has motivated the study of interactions of HCI with internal surfaces of microcapillaries and nanocapillaries as an alternative technique to study above surface processes. The use of capillary targets allows the extraction and study of hollow atoms in vacuum.

Interaction with thin films

To better study the electronic processes surrounding the impact of highly charged ions on solids, thin films have been increasingly used as targets in recent years. It has been shown that the stability of the layers after the interaction is closely related to the conductivity of the material. We are working on a simulation of the interaction that links the neutralization of the projectile with the motion of the target atoms.

Two luminous spheres penetrate a thin carbon layer

Highly charged atoms extract a large amount of electrons around the point of impact, which can lead to hole formation for certain materials. The size of the holes depends on the initial charge of the projectiles. (from Grossek et al., Nano Lett. 22, 9679 (2022), opens an external URL in a new window).