6/15/2023 0 Comments Helium atomThe researchers also record in an image sequence how the rhythm and thus the choreography of the electronic dance changes. 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 visible laser pulse here serves them not only as a camera but also as a pacemaker for the pulsing motion of the electrons. The Heidelberg-based physicists also rely on these simulations to confirm the second part of their experiments. Intense visible laser pulses change the rhythm of the electronic dance 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. “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. From this existing knowledge they determine where the electrons are when they are not moving. The team in Heidelberg uses findings from previous research to determine the dance moves. 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. 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. The researchers then use a weak, visible laser pulse to determine where the electrons are dancing at that particular moment. 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. They succeed only because the duration of this pulse is shorter than one femtosecond (one-millionth part of a billionth of a second), however. The researchers first move the electrons of the helium into the ultrafast pulsing state with the aid of an ultraviolet flash. the colour of the pulses, which is important here, but also their intensity and the interval between them. 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 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. 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. “We have now achieved it for a pair of electrons which were bound together for a short time.” When electrons are shifted, molecular bonds can be created “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. They laid down the rhythm of the electronic partner dance, so to speak. The researchers were not satisfied with the role of mere observers, however, and also actively intervened in the electronic choreography. They observed how the electron pair danced close to the atomic nucleus one moment and slightly moved away from it the next moment. 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. In some electronic states – physicists call them superposition states – this motion manifests itself as a pulsing with a regular beat. When electrons move, this brings about a change to the regions where the electrons have the highest probability of being located. Physicists cannot determine their precise location in an atom, but they can narrow down the region where the charge carriers are most probably located.
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