New world record in short-term measurement as a physicist track the...

New world record in short-term measurement as a physicist track the...
New world record in short-term measurement as a physicist track the...

The photon (yellow, coming from the left) generates electron waves from the electron cloud (gray) of the hydrogen molecule (red: nucleus), which interfere with each other (interference pattern: violet-white). The interference pattern is inclined slightly to the right, so it can be calculated how long it takes the photon to travel from one atom to the next. Photo credit: Sven Grundmann, Goethe University Frankfurt

Physicists from Frankfurt, Hamburg and Berlin follow the propagation of light in a molecule.

In the global race for ever shorter periods of time, physicists from Goethe University Frankfurt have now taken the lead: together with colleagues from the accelerator facility DESY In Hamburg and at the Fritz Haber Institute in Berlin, they measured a process for the first time in the range of zeptoseconds: the propagation of light within a molecule. A zeptosecond is a trillionth of a billionth of a second (10th-21 Seconds).

In 1999, Egyptian chemist Ahmed Zewail received the Nobel Prize for measuring the speed at which molecules change shape. He founded femtochemistry with ultrashort laser flashes: the formation and breaking of chemical bonds takes place in the femtosecond range. A femtosecond is equal to 0.000000000000001 seconds or 10 seconds-fifteen Seconds.

Now atomic physicists at Goethe University in Professor Reinhard Dörner’s team have for the first time investigated a process that is orders of magnitude shorter than femtoseconds. They measured how long it takes for a photon to cross a hydrogen molecule: around 247 zeptoseconds for the average bond length of the molecule. This is the shortest time span that has been successfully measured so far.

The scientists measured the time on a hydrogen molecule (H2), which they irradiated with X-rays from the PETRA III synchrotron light source at the DESY accelerator center in Hamburg. The researchers adjusted the energy of the X-rays so that one photon was enough to eject both electrons from the hydrogen molecule.

Electrons behave like particles and waves at the same time, and therefore the ejection of the first electron caused electron waves to be released first in one and then in the second hydrogen molecule Atom in quick succession, the waves merging.

The photon behaved like a flat pebble that is thrown twice over the water: When a wave trough hits a wave crest, the waves of the first and second water contact cancel each other, which leads to a so-called interference pattern.

The scientists measured the interference pattern of the first electron ejected with the COLTRIMS reaction microscope, a device that Dörner was involved in developing and that makes ultrafast reaction processes in atoms and molecules visible. At the same time as the interference pattern, the COLTRIMS reaction microscope also made it possible to determine the orientation of the hydrogen molecule. The researchers took advantage of the fact that the second electron also left the hydrogen molecule, so that the remaining hydrogen nuclei flew apart and were detected.

“Since we knew the spatial alignment of the hydrogen molecule, we used the interference of the two electron waves to calculate exactly when the photon reached the first and the second hydrogen atom,” explains Sven Grundmann, whose dissertation is the basis of the scientific article in Science. “And that’s up to 247 zeptoseconds, depending on how far apart the two atoms were from each other from the perspective of the light in the molecule.”

Professor Reinhard Dörner adds: “We observed for the first time that the electron shell in a molecule does not react to light everywhere at the same time. The time delay occurs because information within the molecule only propagates at the speed of light. With this knowledge we have expanded our COLTRIMS technology to a different application. ”

Reference: “Zeptosecond birth time delay in molecular photoionization” by Sven Grundmann, Daniel Trabert, Kilian Fehre, Nico Strenger, Andreas Pier, Leon Kaiser, Max Kircher, Miriam Weller, Sebastian Eckart, Lothar Ph. H. Schmidt, Florian Trinter, Till Jahnke, Markus S. Schöffler and Reinhard Dörner, October 16, 2020, Science.
DOI: 10.1126 / science.abb9318

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