Research: Unexpected Type of Energy Transmission from Ions to Materials

2020-05-14

Uppsala researchers Svenja Lohmann and Daniel Primetzhofer have by irradiating silicon crystals with ion beams been able to describe how ions may interact with electrons in various materials in a way that leads to transmission of unexpectedly large amounts of energy. The results have recently been published in Physical Review Letters.

Today there are a number of macroscopic models describing the interaction of ions with materials, for example within astrophysics, materials science and cancer therapy. There are just very limited models for what this interaction looks like on the nanometer scale though, which would give a more complete picture of what really happens.

The researchers have now come a step on the road to a model for interaction on the nanometer scale where they have conducted experiments at the Ångström laboratory in Uppsala. In the experiments, nanometer thin foils of silicon crystals have been irradiated with ion beams of hydrogen and helium ions with a current of only a few femtoampere. The ions have then been transmitted, i.e. come out on the other side, since the silicon crystal is so thin. When the ions have passed through the crystal, they have interacted with it and lost a part of their energy, which later has been measured with so called 3D-transmission measurements.

Svenja Lohmann and Daniel Primetzhofer discuss 3D-spectra of particles which have passed through nanometer foils of silicon crystals. Photo: Mauricio Sortica.

“We measured both energy and direction of the particles that left the crystal on the other side”, says Svenja Lohmann, PhD at the Department of Physics and Astronomy.

By measuring both energy and direction of the particles, and based on the geometry of the crystal used in the experiment, the researchers could reconstruct the ions’ interaction with the silicon crystal. Above all they could decide if the ion’s orbit had come close to the atomic nuclei or not, and thus been able to find out which specific electrons they may interact with in the complex electronic systems of solid matter.

In the experiment, the researchers made measurements on both helium and hydrogen ions and made use of energies of 50-200 keV, which are relatively low energies compared to previous measurements. At the same time, the examined energies are particularly relevant for many technical applications of ion beams. Apart from this, the results from such measurements may be used as a test scenario for completely new calculation models of how the electrons in various materials react on different time dependent perturbations. In previous experiments that have had higher input energies of the order MeV, one has been able to observe that ions that come close to the atomic nuclei loose much more energy than the ions that do not come close to the nucleus. Namely, ions of high energy can excite electrons with high binding energies. However, this effect cannot occur to a large extent for low energies since the ions cannot transmit enough energy in the collisions and the electrons close to the nuclei are left unaffected.

The geometric distribution of the ions’ energy transmission to the crystal. The figure shows peaks and valleys depending on the paths of the ions. The corresponding figure according to previous theories would instead lack structure. Image: Radek Holenak.

The Uppsala researchers could anyway see that the helium ions in the experiment lost more energy when they came close to the atomic nuclei. On the other hand, this effect was not observed for the hydrogen ions. The energy difference between the different ion paths also becomes larger for the slower helium ions. The explanation may again be coupled to complex perturbations of the material’s electronic system. For ions which have lower speed and come close to the atomic nucleus, electrons can namely under certain circumstances jump between atom and ion. The process leads to a steady fluctuation in the ions’ charge number and means an average higher charge number. Ions with higher charge number interact more with the electrons and loose more energy. The reason to that this effect could not be observed for the hydrogen ions depends on that there are not the same electron exchange processes for the hydrogen ions.

“In previous experiments in more complicated systems, one has already been able to observe that helium sometimes behaves differently to hydrogen. But it has been unclear how relevant this has been for interactions between ions and matter in large. In our experiment we have been able to identify the specific ways in which helium ions may transmit energy to what was previously known”, says Daniel Primetzhofer, Senior Lecturer at the Department of Physics and Astronomy and Director at Tandem Laboratory.

The researchers’ new result how ions can transmit energy to electrons is important to be able to understand radiation effects in solid matter within a number of research areas, such as the previously mentioned areas astrophysics, materials science and cancer therapy.

The Uppsala researchers expect that the energy distribution of the electrons created during the ionic irradiation, the so called secondary electrons, are clearly affected by the newly discovered way of energy transmission. It is namely the secondary electrons which most often stand for radiation effects of materials.

“These new research results now need to be included in calculations to be able to improve models for ionic irradiation on earth, but also for how for example stones on the moon or comets change by the solar wind, which just consists of ions of the examined energy”, says Daniel Primetzhofer.

To get a clearer picture of which research areas are largely affected by the new results, the researchers have already started new experiments. In the new experiments they examine, among other things, to what extent the results also applies to heavier ions.

The experiments have had the character of an international competition when research groups in, for instance Canada, Luxembourg and Germany also study similar systems, for instance with helium ion microscopes.

Article reference

Svenja Lohmann, & Daniel Primetzhofer, Disparate Energy Scaling of Trajectory-Dependent Electronic Excitations for Slow Protons and He IonsPhys. Rev. Lett. 124, 096601(2020). Publication Date: March 2, 2020, DOI: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.124.096601

Camilla Thulin
Translation: Johan Wall

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