New Smallest Time Measurement: How Long It Takes a Photon to Cross a Hydrogen Molecule

Physicists have now calculated one of the shortest processes ever.
Loukia Papadopoulos

Back in 1999, Egyptian chemist Ahmed Zewail was awarded the Nobel Prize for measuring the speed at which molecules change their shape, founding femtochemistry in the process. His measurements were made in femtosecond where one femtosecond equals 0.000000000000001 seconds, or 10-15 seconds.


Now, nearly two decades later, atomic physicists at Goethe University led by Professor Reinhard Dörner have calculated a process that is shorter than femtoseconds for the first time ever: the measurement of how long it takes for a photon to cross a hydrogen molecule.

This is the shortest timespan that has ever been measured and amounts to about 247 zeptoseconds (a trillionth of a billionth of a second, or 10-21 seconds). To achieve this, the scientists irradiated a hydrogen molecule with X-rays from the X-ray laser source PETRA III at the Hamburg accelerator facility DESY. They set it up so that one photon was sufficient to eject both electrons out of the hydrogen molecule. 

The scientists then calculated the interference pattern of the first ejected electron using the COLTRIMS reaction microscope. This apparatus was developed partially by Dörner and it makes the super speedy reaction processes in atoms and molecules visible. 

"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," explained in a statement Sven Grundmann whose doctoral dissertation was the basis of the resulting scientific article published 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."

 "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 the information within the molecule only spreads at the speed of light. With this finding we have extended our COLTRIMS technology to another application," said Professor Reinhard Dörner.

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