In the global race to measure ever shorter time spans, a German team of physicists has now taken the lead: researchers from Goethe University Frankfurt, DESY in Hamburg and the Fritz-Haber-Institute in Berlin have measured a process at DESY´s synchrotron lightsource PETRA III that lies within the realm of zeptoseconds for the first time: the propagation of light within a molecule. A zeptosecond is a trillionth of a billionth of a second (10^-21 seconds). The group reports about their experiments today in the journal Science.

Schematic representation of zeptosecond measurement. The photon (yellow, coming from the left) produces electron waves out of the electron cloud (grey) of the hydrogen molecule (red: nucleus), which interfere with each other (interference pattern: violet-white). The interference pattern is slightly skewed to the right, allowing the calculation of how long the photon required to get from one atom to the next (picture: S Grundmann/Univ. Frankfurt).

Now a researchers´ team around Reinhard Dörner (Goethe University Frankfurt) have for the first time studied a process that is shorter than femtoseconds by magnitudes. They measured how long it takes for a photon to cross a hydrogen molecule. Their result: about 247 zeptoseconds for the average bond length of the molecule. This is the shortest timespan that has been successfully measured to date.

The scientists carried out the time measurement on a hydrogen molecule (H₂) which they irradiated with X-rays from the synchrotron lightsource PETRA III. The researchers set the energy of the X-rays so that one photon was sufficient to eject both electrons out of the hydrogen molecule.

Electrons behave like particles and waves simultaneously, and therefore the ejection of the first electron resulted in electron waves launched first in the one, and then in the second hydrogen molecule atom in quick succession, with the waves merging.

The photon behaved here much like a flat pebble that is skimmed twice across the water: when a wave trough meets a wave crest, the waves of the first and second water contact cancel each other, resulting in what is called an interference pattern.

The scientists measured the interference pattern of the first ejected electron using a special reaction microscope which makes ultrafast reaction processes in atoms and molecules visible. Simultaneously with the interference pattern, the microscope also allowed the determination of the orientation of the hydrogen molecule. The researchers here 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. “PETRA III will certainly like that: The new world record in time-resolution is no longer for lasers or free-electron lasers, but thanks to our reaction microscope at the best synchrotron lightsource in the world,” enthuses Florian Trinter, who conducts research at DESY and at the University of Frankfurt.

“Since we knew the spatial orientation of the hydrogen molecule, we used the interference of the two electron waves to precisely calculate when the photon reached the first and when it reached the second hydrogen atom,” explains Sven Grundmann (Frankfurt University) whose doctoral thesis forms the basis of the scientific article in Science. “And this is up to 247 zeptoseconds, depending on how far apart in the molecule the two atoms were from the perspective of light.”

In these experiments, the team was able to observe 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 spreads at the speed of light.

Reference:

Zeptosecond Birth Time Delay in Molecular Photoionization; 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; Science