The motion of the two electrons in the helium atom can be imaged and controlled with attosecond-timed laser flashes
Physicists are continuously advancing the control they can exert over matter. A German-Spanish team working with researchers from the Max Planck Institute for Nuclear Physics in Heidelberg has now become the first to image the motion of the two electrons in a helium atom and even to control this electronic partner dance.
Electronic pas de deux: Physicists in Heidelberg have filmed the pulsing motion of the electron pair in a helium atom. At 15.3 femtoseconds (fs) the two electrons are close to the nucleus (centre of image) and then move away from it. The colour indicates the probability of finding one electron at position A (vertical axis) and the second electron at position B (horizontal axis) on a line drawn through the atom (along the polarisation direction of the laser). At 16.3 femtoseconds they arrive back at their original position again; they thus move with a beat of around one femtosecond.
© MPI for Nuclear Physics
The scientists are succeeding in this task with the aid of different laser pulses which they timed very accurately with respect to each other. They employed a combination of visible flashes of light and extreme-ultraviolet pulses which lasted only a few hundred attoseconds. One attosecond corresponds to a billionth of a billionth of a second. Physicists aim to specifically influence the motion of electron pairs because they want to revolutionise chemistry: If lasers can steer the paired bonding electrons in molecules, they could possibly produce substances which cannot be produced using conventional chemical means.
Electrons are hard to get a hold of. Physicists cannot determine their precise location in an atom, but they can narrow down the region where the charge carriers are most probably located. When electrons move, this brings about a change to the regions where the electrons have the highest probability of being located. In some electronic states – physicists call them superposition states – this motion manifests itself as a pulsing with a regular beat.
It is precisely this pulsing motion which scientists working with Thomas Pfeifer, Director at the Max Planck Institute for Nuclear Physics, have recorded in a series of images of a helium atom. They observed how the electron pair danced close to the atomic nucleus one moment and slightly moved away from it the next moment. The researchers were not satisfied with the role of mere observers, however, and also actively intervened in the electronic choreography. They laid down the rhythm of the electronic partner dance, so to speak. “The motion of individual electrons in the atom has already been imaged quite often and even manipulated as well,” says Christian Ott, lead author of the study. “We have now achieved it for a pair of electrons which were bound together for a short time.”
On the one hand, the study of an electron pair is useful for physicists who want to gain a better understanding of how atoms and molecules interact with light as this interaction usually involves two or more electrons. It is useful for chemistry, on the other hand, if they are able to direct pairs of electrons, because the typical chemical bond consists of just such a pair; this means that chemists must always move at least two electrons when they want to create or break a molecular bond.
In order to choreograph and film electrons in a helium atom, the Heidelberg-based physicists sent two laser pulses through a cell with helium gas. It is not only the energy, i.e. the colour of the pulses, which is important here, but also their intensity and the interval between them. The researchers first move the electrons of the helium into the ultrafast pulsing state with the aid of an ultraviolet flash. They succeed only because the duration of this pulse is shorter than one femtosecond (one-millionth part of a billionth of a second), however. This is how long the pair of electrons needs for one cycle of the pulsing motion in which the pair is initially closer to the nucleus, then moves away from it and then returns to the nucleus again.
The researchers then use a weak, visible laser pulse to determine where the electrons are dancing at that particular moment. And by varying the interval between the ultraviolet attosecond pulse and the visible one, they produce a movie of the electronic dance: “Although we do not directly image where the electrons are,” explains Thomas Pfeifer, “the visible pulse provides us with the relative phase of the superposition state.” The phase describes the to and fro of an oscillation, and hence the rhythmic motion of the electron pair. In this case it tells the physicists at which point of their natural pas de deux around the helium atom the electrons are at a given moment.
The team in Heidelberg uses findings from previous research to determine the dance moves. From this existing knowledge they determine where the electrons are when they are not moving. “With the information on the phase which we measured here and our prior knowledge we reconstruct where the electrons are at a given time,” says Pfeifer. He and his colleagues' experimental results are in good agreement with state-of-the art theoretical simulations by their cooperators Luca Argenti and Fernando Martín at Universidad Autónoma de Madrid in Spain, confirming the validity of the experimental and computational methodology.
The Heidelberg-based physicists also rely on these simulations to confirm the second part of their experiments. The visible laser pulse here serves them not only as a camera but also as a pacemaker for the pulsing motion of the electrons. For when they increase the intensity of the pulse, the points in time at which the electrons are close to the atomic nucleus or further away from it shift in time. The researchers also record in an image sequence how the rhythm and thus the choreography of the electronic dance changes.
Thomas Pfeifer and his colleagues have not yet been able to explain all the details which they observe in the experiments with intense laser pulses. They want to change this now with more comprehensive experiments on the effect of the pulses. In future experiments they also want to follow the subsequent fate of the pair of electrons in great detail, for the electronic dance in the superposition state ends with one of the two partners being ejected from the atom, with the consequence that the atom is ionised. These ionisations also play a role in many chemical reactions. A better understanding of such wild two-electron dances could thus tell chemists how a reaction can be steered into the desired direction and product channels. At this point, at the latest, attosecond physics would create new tools for chemistry as well.
Dr. Thomas Pfeifer | Max Planck Institute for Nuclear Physics, Heidelberg
What happens when we heat the atomic lattice of a magnet all of a sudden?
17.07.2018 | Forschungsverbund Berlin
Subaru Telescope helps pinpoint origin of ultra-high energy neutrino
16.07.2018 | National Institutes of Natural Sciences
For the first time ever, scientists have determined the cosmic origin of highest-energy neutrinos. A research group led by IceCube scientist Elisa Resconi, spokesperson of the Collaborative Research Center SFB1258 at the Technical University of Munich (TUM), provides an important piece of evidence that the particles detected by the IceCube neutrino telescope at the South Pole originate from a galaxy four billion light-years away from Earth.
To rule out other origins with certainty, the team led by neutrino physicist Elisa Resconi from the Technical University of Munich and multi-wavelength...
For the first time a team of researchers have discovered two different phases of magnetic skyrmions in a single material. Physicists of the Technical Universities of Munich and Dresden and the University of Cologne can now better study and understand the properties of these magnetic structures, which are important for both basic research and applications.
Whirlpools are an everyday experience in a bath tub: When the water is drained a circular vortex is formed. Typically, such whirls are rather stable. Similar...
Physicists working with Roland Wester at the University of Innsbruck have investigated if and how chemical reactions can be influenced by targeted vibrational excitation of the reactants. They were able to demonstrate that excitation with a laser beam does not affect the efficiency of a chemical exchange reaction and that the excited molecular group acts only as a spectator in the reaction.
A frequently used reaction in organic chemistry is nucleophilic substitution. It plays, for example, an important role in in the synthesis of new chemical...
Optical spectroscopy allows investigating the energy structure and dynamic properties of complex quantum systems. Researchers from the University of Würzburg present two new approaches of coherent two-dimensional spectroscopy.
"Put an excitation into the system and observe how it evolves." According to physicist Professor Tobias Brixner, this is the credo of optical spectroscopy....
Ultra-short, high-intensity X-ray flashes open the door to the foundations of chemical reactions. Free-electron lasers generate these kinds of pulses, but there is a catch: the pulses vary in duration and energy. An international research team has now presented a solution: Using a ring of 16 detectors and a circularly polarized laser beam, they can determine both factors with attosecond accuracy.
Free-electron lasers (FELs) generate extremely short and intense X-ray flashes. Researchers can use these flashes to resolve structures with diameters on the...
13.07.2018 | Event News
12.07.2018 | Event News
03.07.2018 | Event News
17.07.2018 | Information Technology
17.07.2018 | Materials Sciences
17.07.2018 | Power and Electrical Engineering