Zeptoseconds: New world record in short time measurement
Physicists track the propagation of light in a molecule

Date:
October 16, 2020
Source:
Goethe University Frankfurt
Summary:
In the global race to measure ever shorter time spans, physicists
have now measured a process 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.



FULL STORY ==========================================================================
In the global race to measure ever shorter time spans, physicists
from Goethe University Frankfurt have now taken the lead: together
with colleagues at the accelerator facility DESY in Hamburg and the Fritz-Haber-Institute in Berlin, they have measured a process 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).


==========================================================================
In 1999, the Egyptian chemist Ahmed Zewail received the Nobel Prize for measuring the speed at which molecules change their shape. He founded femtochemistry using ultrashort laser flashes: the formation and breakup
of chemical bonds occurs in the realm of femtoseconds. A femtosecond
equals 0.000000000000001 seconds, or 10-15 seconds.

Now atomic physicists at Goethe University in Professor Reinhard Do"rner's
team 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: 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 X-ray laser source PETRA
III at the Hamburg accelerator facility DESY. 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 the COLTRIMS reaction microscope, an apparatus that Do"rner helped develop and which makes ultrafast reaction processes in atoms
and molecules visible. Simultaneously with the interference pattern,
the COLTRIMS reactions 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.

"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 whose doctoral dissertation 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." Professor Reinhard Do"rner 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 spreads at the speed
of light. With this finding we have extended our COLTRIMS technology to
another application."

========================================================================== Story Source: Materials provided by Goethe_University_Frankfurt. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. 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. Scho"ffler, Reinhard Do"rner. Zeptosecond Birth
Time Delay in Molecular Photoionization. Science, 2020 DOI:
10.1126/science.abb9318 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/10/201016090209.htm

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