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Robust Approach Analyzes Complex Processes on a Femtosecond Scale

In the past, it has been difficult to understand the highly complicated processes that occur when ions penetrate a substance because they happen so quickly. However, modern measurements have made it possible.

TU Wien authors of the study from left to right: Friedrich Aumayr, Anna Niggas, and Richard Wilhelm. Image Credit: Vienna University of Technology.In the past, it has been difficult to understand the highly complicated processes that occur when ions penetrate a substance because they happen so quickly. However, modern measurements have made it possible.

How do various substances respond to the action of ions? This is a crucial question in many scientific fields, such as nuclear fusion, where the walls of the fusion reactor are bombarded with high-energy ions, as well as semiconductor technology, where semiconductors are subjected to ion beam bombardment to create intricate architectures.

Retrospective analysis of an ion impact’s effects on a substance is simple. However, it can be challenging to comprehend the temporal order of such operations. The effects of ion penetrating materials such as graphene or molybdenum disulfide on the individual particles have now been studied on a time scale of one femtosecond by a research team at TU Wien.

It was essential to thoroughly examine the electrons released throughout the process: In a sense, the measurement turns into an “electron slow-motion” when they are utilized to reconstruct the temporal sequence of the operations. Physical Review Letters has recently published the findings, and they were even chosen as “Editors’ Suggestion.”

Twenty to Forty Times Charged Particles

Highly charged ions are used by the research team of Professor Richard Wilhelm at the TU Wien Institute of Applied Physics. Xenon atoms have 54 electrons in their neutral state. They are stripped of 20 to 40 electrons and the remaining significantly positively charged xenon ions are then focused onto a tiny layer of material from xenon atoms.

We are particularly interested in the interaction of these ions with the material graphene, which consists of only a single layer of carbon atoms. This is because we already knew from previous experiments that graphene has very interesting properties. Electron transport in graphene is extremely fast.

Anna Niggas, Study First Author, Vienna University of Technology

It is impossible to directly monitor the processes because the particles react so quickly. However, there are unique techniques that can be applied.

During such processes, a large number of electrons is usually released as well. We were able to measure the number and energy of these electrons very precisely, compare the results with theoretical calculations contributed by our co-authors from Kiel University, and this allowed us to unravel what happens on a femtosecond scale.

Anna Niggas, Study First Author, Vienna University of Technology

Femtosecond Journey Through Graphene

The strongly charged ion first moves toward the thin layer of substance. Because of its positive charge, it creates an electric field that affects the material’s electrons, causing them to travel in the direction of the impact site even before it makes contact.

The material’s electrons are eventually ripped out by the material’s strong electric field and captured by the negatively charged ion. The ion then immediately strikes the material’s surface and pierces it. This leads to a complicated interaction in which electrons are released and a significant amount of energy is quickly transferred to the substance by the ion.

The positive charge is still present in the material if electrons are absent. However, electrons moving in from other material parts rapidly make up for this. This procedure occurs very quickly in graphene, resulting in the brief formation of powerful atomic-scale currents. This process moves a little more slowly in molybdenum disulfide.

However, in both situations, the distribution of electrons within the material affects the already released electrons. As a result, if these emitted electrons are monitored closely, they can reveal information about the impact’s temporal structure. Only quickly moving electrons can escape the substance; slower electrons reverse direction, are trapped again, and do not enter the electron detector.

The ion can pass through a layer of graphene in just one femtosecond. Ultrashort laser pulses have been used to measure processes on such short time scales, but in this situation, they would deposit a lot of energy in the material and fundamentally alter the process.

With our method, we have found an approach that allows quite fundamental new insights. The results help us to understand how matter reacts to very short and very intense radiation exposure—not only to ions, but ultimately also to electrons or light.

Richard Wilhelm, Head, FWF START Project, Vienna University of Technology

The “Innovative Projects” program, the TU-D doctoral college at TU Wien, and the FWF, provided funding for the research.

Journal Reference:

Niggas, A., et al. (2022) Ion-Induced Surface Charge Dynamics in Freestanding Monolayers of Graphene and MoS2 Probed by the Emission of Electrons. Physical Review Letters.

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