The evaluation of the signatures of the interaction of highly charged ions (HCI) with surfaces (e.g. electron or X-ray emission, charge state analysis of scattered particles) are of vital importance for diverse applied fields such as catalysis, plasma-wall interaction, materials processing, nanostructuring of surfaces, or in astrophysics. Established surface-structure analysis tools include low energy ion scattering (LEIS), secondary ion mass spectroscopy (SIMS), and Auger electron spectroscopy (AES). Current experimental interest at TU Wien focuses on the interaction of HCI with very thing targets (2D-structures, graphene) or the restructuring of surfaces by single ion impact.

Theoretical interest in HCI-surface interactions is triggered by the need for combining and developing methods from many different areas. At large distances from the surface, the projectile can be satisfactorily described by atomic physics methods which are used to determine energy  levels of ionic projectiles in external fields. To describe the response of surfaces to the presence of the HCI, methods of many-particle physics have to be applied. Elementary excitations in Fermi gases (plasmons and electron-hole pairs) and the polarization of surface areas to shield the inside of the target against the external field are at the center of calculations. The polarized surface gives rise to image charges known from electrostatics but also to the slowing down of projectiles moving parallel to or below the surface ("stopping power"). As the HCI approaches the surface, electron transfer channels open starting the neutralization sequence of the projectile [1]. Under special conditions, even over-neutralization of projectiles can be reached. This effect is used for the production of negative ions in scattering processes from caesiated surfaces. In this case the over-neutralization of the incoming projectile (usually hydrogen) is facilitated by the small workfunction of the material. Upon impact on the surface different processes related to conversion of its kinetic energy take place. The kinetic energy of the projectile is not only transferred to target electrons but also target atoms. Sputter-cleaning of surfaces is a well-known and efficient cleaning techniques. Lately, much smaller velocities are used to restrict the interaction to the close vicinity of the impact area causing localized restructuring in an interplay of kinetic and potential energies carried into the collision. A schematic illustration of the different stages of HCI-surface interactions is shown in Fig. 1.

© Friedrich Aumayr

Figure 1: At large distances from the surface, the HCI induces an electronic response in the target ("image charge"). At smaller distances, electron transfer to highly excited states of the projectile leads to the formation…

Figure 1: At large distances from the surface, the HCI induces an electronic response in the target ("image charge"). At smaller distances, electron transfer to highly excited states of the projectile leads to the formation of so-called “hollow atoms” which shrinks as the ion approaches the surface. Quick neutralization upon impact of the ion on the surface leads to the emission of electrons but may also initiate restructuring of the impact area due to local heating of the crystal above boiling temperature (from left to right).

Our current research focuses on the interaction of highly charged ions with two-dimensional structures (graphene, MoS₂) and the simulation of electron emission [2], as well as the creation of holes with diameters of a few nanometres for the functionalisation of the layers as nano-sieves [3].

Furthermore, we are simulating the restructuring of insulator surfaces for various materials, whilst attempting to derive a threshold value for the minimum amount of energy required to be deposited [4]. To this end, we are carrying out extensive MD simulations using the LAMMPS software package [5] in order to account for the thermodynamic properties of the target materials as accurately as possible.

[1] J. Burgdörfer, P. Lerner, F.W. Meyer, Phys. Rev. A 44, 5674 (1991), opens an external URL in a new window.

[2] V. Vojtech, C. Lemell, A. Niggas, M. Werl, F. Vukovic, F. Aguilar-Galindo, A.G. Borissov, F. Aumayr, and R.A. Wilhelm, Phys. Rev. Lett., under review (2026).

[3] A. Sagar Grossek, A. Niggas, R.A. Wilhelm, F. Aumayr, C. Lemell, Nano Lett. 22, 9679 (2022), opens an external URL in a new window.

[4] A. S. El-Said, R. A. Wilhelm, R. Heller, S. Facsko, C. Lemell, G. Wachter, J. Burgdörfer, R. Ritter, F. Aumayr, Phys. Rev. Lett. 109, 117602 (2012), opens an external URL in a new window, A. S. El-Said et al., in preparation (2026).

[5] A. P. Thompson, et al., Comp. Phys. Comm. 271, 108171 (2022), opens an external URL in a new window, https://www.lammps.org, opens an external URL in a new window