20.06.2012

DESY’s X-ray laser FLASH observes quantum race of electrons

At DESY’s free-electron laser FLASH, scientists observed the quantum mechanical race of two electrons for the first time. The measurements which were published today in the research journal “Physical Review Letters” will improve the study of the properties of atoms and molecules. They will also increase the quality of so-called free-electron lasers as FLASH, which are new tools for many fields of science, from physics to biology.

View into the experimental hall of DESY´s free-electron laser FLASH.

A team of physicists led by Markus Drescher from the University of Hamburg and Michael Bonitz from Kiel University studied the so-called Auger effect, where two electrons are kicked out of an atom in quick succession: first, a photon of sufficiently high energy kicks an electron out of the atomic shell. Another electron fills the gap and emits energy. The released energy is absorbed by a third electron which is also kicked out of the atom. When it flies in the same direction, it chases the first electron.

This race is extremely fast, typically taking between one and hundred quadrillionths of a second (1 and 100 femtoseconds). “Until now, no experiment has ‘filmed’ the process of this race,” said Drescher, head of the experimental division of this project. In order to resolve the movements of the electrons within only 50 femtoseconds of time, they are deflected with a fast oscillating electromagnetic field. Such as with a streak camera, the deflection of the electrons changes depending on time. The scientists repeated the race many times observing it at slightly delayed intervals. From these individual observations it was possible to trace the timing of the race.

In their experiments, the scientists traced back how the Auger electron passes and exchanges energy with the first one. They discovered that the energy of the Auger electron changes in a characteristic time dependent way. The so-called chirped Auger electron emission is important for spectroscopic investigations. Detailed computer simulations at Christian-Albrechts-Universität zu Kiel confirmed the interpretation of the measured data. “We tested several possible explanations for the measured results,” said Bonitz. “Finally, our PhD student Sebastian Bauch verified that the experiment indeed observed the quantum race of two electrons."

The results are not only important for future measurements of ultrafast processes in atoms, molecules and solids but also for improving the quality and precision of free-electron lasers. “The aim is to further develop the technology for a more accurate measurement and adjustment of the laser pulse duration,” said DESY´s scientist Elke Plönjes.

Original publication: "Evidence for chirped Auger electron emission"; Schütte, B., Bauch, S., Frühling, U., Wieland, M., Gensch, M., Plönjes, E., Gaumnitz, T., Azima, A., Bonitz, M. and Drescher, M.; Phys. Rev. Lett. 108, 253003 (2012), DOI: 10.1103/PhysRevLett.108.253003