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 start first in one and then in the second hydrogen molecule atom in quick succession, fusing the waves.
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 lag occurs because information within the molecule only propagates at the speed of light. With this knowledge we have expanded our COLTRIMS technology to another application. ”
Publication: 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, Reinhard Dörner: Zeptosecond Birth Time Delay in Molecular Photoionization. Science https://science.sciencemag.org/cgi/doi/10.1126/science.abb9318
Download picture: https://www.uni-frankfurt.de/93157222
Caption: Schematic representation of the zeptosecond measurement. 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: Sven Grundmann, Goethe University Frankfurt
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