Dec 24 2018
Scientists from the University of Paderborn and the Fritz Haber Institute Berlin have shown that they can observe the movements of electrons during a chemical reaction. They have reported this in a recent publication in the journal Science.
A simulation of indium wire atoms (red) on top of a silicon substrate. The glowing regions highlight where indium bonds have been excited and photoholes have formed. (Image credit: Andreas Lücke)
For many years, scientists have analyzed the atomic-scale processes controlling chemical reactions. However, they have never before able to view the movement of electrons as it happened.
Electrons dwell on the smallest scales, have a diameter of less than one-quadrillionth of 1 m, and orbit an atom at femtosecond speeds (one-quadrillionth of 1 second). Experimentalists who intend to observe the behavior of electrons use laser pulses to interact with the electrons. The energy and momentum of the electrons can be calculated by investigating the characteristics of the electrons ejected out of the probe by the laser light.
The problem faced by scientists is to record events that take place on a femtosecond scale—first, a system must be excited using a laser pulse; then, it must be observed for the next few femtoseconds. Subsequently, a second laser pulse is sent with a short time delay of a few femtoseconds. It is hard to realize this level of resolution since femtoseconds are very short—in 1 second, light can travel 300,000 km, but it can travel only 300 nm in 1 fs.
Upon excitation with the first laser pulse, valence electrons of the atoms, that is, the electrons on the outer orbit of an atom that help in forming chemical bonds, may re-arrange to form new chemical bonds, leading to new molecules. However, researchers have been able to only hypothesize the way this re-arrangement occurs due to the speed and scale of these interactions.
Besides experimental methods, high-performance computing (HPC) has turned out to be a highly significant tool for gaining insights into these atomic-level interactions, validating experimental observations, and analyzing the behavior of electrons in more detail during a chemical reaction. A University of Paderborn team headed by Prof. Dr Wolf Gero Schmidt has been partnering with chemists and physicists to complement experiments with computational models.
To gain better insights about the behavior of electrons at the time of a chemical reaction, Schmidt and his colleagues have been using supercomputing resources at the High-Performance Computing Center Stuttgart (HLRS) to model this phenomenon.
The experimental group at the Fritz Haber Institute came to us about this research, and we had actually already done the simulation. In this case, theory was ahead of experiment, as we had made a prediction and the experiment confirmed it.
Dr Wolf Gero Schmidt, Professor, University of Paderborn.
Laser-Like Focus
Last year, Schmidt’s team collaborated with experimentalists from the University of Duisburg-Essen to excite an atomic-scale system and view photo-induced phase transitions (PIPTs) in real time. Phase transitions (the change of a substance from one physical state to another, for example, change of water to ice) are crucial for studying and designing materials since the properties of a substance may change wildly based on the state in which it exists.
For instance, the researchers discovered that upon exciting with a laser pulse, indium-based nanoscale wires would essentially transition from being an insulator into an electrical conductor. Although these indium wires are not necessarily of direct technological interest for electronic applications, they serve as a perfect test case and a solid foundation for validating simulations through experiments.
This year, the researchers intended to use the knowledge they acquired about the indium wires previously and analyze chemical reactions on a more basic level—their goal was to track the behavior of the constituent electrons upon excitation with a laser pulse.
Last year, we published a Nature article that demonstrated the measurement of the atomic movement on this scale. We could show how the atoms moved during the chemical reaction. This year, we were even able to monitor the electrons while the reaction took place.
Dr Wolf Gero Schmidt, Professor, University of Paderborn.
Literally, electrons act as the adhesive that chemically binds atoms together. However, a laser pulse has the potential to eject out an electron, forming the so-called “photohole.” Although the photoholes last only for a few femtoseconds, they can cause the breaking of chemical bonds and the formation of new bonds. Upon hitting the indium nanowire with a laser pulse, a metallic bond is formed by the system, accounting for its phase change into an electrical conductor.
Supercomputing simulations enable researchers to put the paths of the electrons in motion, eventually helping them analyze the entire reaction “pathway.” Scientists run first-principles simulations, that is, they start with no assumptions on the working of an atomic system; then, they computationally model atoms and their electrons under the experimental conditions. Such intensive, first-principles calculations necessitate the use of leading-edge supercomputing resources, for example, those offered by the Gauss Centre for Supercomputing at HLRS.
From its previous work to its current project, the researchers now have better insights about the significant role played by photoholes in shaping the distribution of energy across a system, eventually giving the scientists a reliable computational technique for simulating extremely fast phase transitions.
Complex Chemistry
The current simulations of the researchers consist of about 1000 atoms, which are small yet enable them to obtain a representative sample of the interaction of the atoms in a system and their constituent electrons. The Paderborn team received assistance from the HLRS team in optimizing its code, enabling it to run efficiently on up to 10,000 cores in parallel. Schmidt described that although growing the system size to the order of 10,000 atoms would be advantageous to the overall research, the subsequent phase of the researchers’ study is to work on highly complicated systems.
The current research is a complex calculation, but a simple system. Our next step is to develop this research as it relates to photocatalysts or systems that are relevant for large-scale energy production—we want to apply this to a real system.
Dr Wolf Gero Schmidt, Professor, University of Paderborn.
Better perception of the behavior of electrons at the atomic level would enable the researchers to design better materials for the conversion, transportation, and storage of energy.
Source: https://www.gauss-centre.eu/