Research team tracks ultra-high-speed quantum interferences in electron shells

Electrons in Motion

Laser Pulses in an atom

Laser pulses induce and track electronic quantum interferences in an atom. (Photo: Albert Ludwig University, Freiburg)

An international team of scientists, with the participation of DESY, has succeeded in observing ultra-fast quantum interferences between electrons in atoms of a noble gas in real time. The team headed by Frank Stienkemeier and Lukas Bruder, from the Albert Ludwig University in Freiburg, was able to record electronic oscillations having a period of just 150 attoseconds. An attosecond is a billionth of a billionth of a second. For the study, which has been published in the journal Nature Communications, rare gas atoms were stimulated using specially prepared laser pulses, and the response of the atoms was then observed using a new, highly sensitive measuring technique. This method allows fundamental quantum-mechanical effects to be observed in atoms and molecules on their natural timescale.

Many chemical reactions are induced by shining light on matter, breaking the bonds in the molecules, for example. Immediately after absorbing the light, the electronic structure of the atomic shell is modified, and this has a significant influence on the further course of the reaction. These changes occur very quickly and can take as little as a few attoseconds. Conventional spectroscopic techniques using visible laser pulses are not fast enough to track processes like these. That is why new types of laser sources, in the extreme ultraviolet and X-ray spectral range, are being developed all over world along with the corresponding spectroscopic techniques.

The team of scientists has now used a new technique to observed electronic quantum interferences in the atomic shell – a kind of oscillatory pattern of the electrons – directly on its natural timescale. To achieve this, they extended the technique of coherent pump-probe spectroscopy, which was already known from the visible spectral range, to the extreme ultraviolet (XUV), the frequency range between X-rays and ultraviolet light. A sequence consisting of two ultra-short XUV laser pulses was prepared at the free-electron laser FERMI in Trieste (Italy) using a procedure developed in Freiburg. The two pulses have a precisely defined time delay as well as a precisely defined phase relationship. The phase of an oscillation (such as a light wave) describes the state in which it is found at a given point in time.

The first pulse triggers the process in the electron shell (pumping), while the second pulse serves to determine the state of the electron shell at a later point in time (probing). By carefully adjusting the time delay and the phase relationship, it is possible to draw conclusions about how the electron shell changes over time. “The biggest challenge was to control the properties of the pulse sequences as precisely as possible, and to isolate the weak signals when measuring them,” explains the Freiburg PhD student Andreas Wituschek, who played a key role in carrying out the actual experiment.

“The excellent coherence properties of the ultra-short light pulses generated using so-called seeding at FERMI, in particular, are crucial for the experiments,” as DESY scientist Tim Laarmann points out, one of the co-authors of the study who is also involved in research at the “CUI: Advanced Imaging of Matter” cluster of excellence at the University of Hamburg and DESY. This special beam quality is also to be provided in future to researchers at DESY’s free-electron laser FLASH facility with a distinctly higher repetition rate as part of its FLASH2020+ upgrade.

By way of example, the scientists examined argon, in which the pump pulse creates a special configuration of two electrons inside the atomic shell. This configuration decays by emitting one electron from the atom within an extremely short period of time, ultimately leaving behind the atom in the state of an ion. The team succeeded for the first time in tracking the decay of the quantum interference over time as the single electron leaves the atom. “This experiment paves the way for many new applications in the study of atomic and molecular processes following specific excitation using high-energy coherent light fields,” adds Lukas Bruder.

The research project was funded, among other sources, by the Federal Ministry of Education and Research (BMBF) as part of the collaborative research project “Longitudinal Coherence on Free-Electron Lasers – Control, Analysis and Applications (LoKoFEL)” and involved scientists from the University of Freiburg, Lund University, the Elettra Synchrotron Trieste, IFN-CNR and the Politecnico di Milano in Italy, the Ecole Polytechnique Fédérale in Lausanne in Switzerland, the University of Gothenburg in Sweden, DESY, CUI, Aarhus University in Denmark, the University of Milan and La Sapienza University in Rome in Italy, as well as the Freiburg Institute of Advanced Studies at the University of Freiburg.

(from DESY News)

Tracking attosecond electronic coherences using phase-manipulated XUV pulses; A. Wituschek et al.; Nature Communications, 2020; DOI: 10.1038/s41467-020-14721-2