German physicists took a hydrogen molecule and fired an X-ray photon from a PETRA III laser source in DESY. From the observed interference between the photon and the electrons of this molecule, they ingeniously derived the time it takes for a photon to fly from one atom to another. It is a world record in measuring the shortest time.
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How to measure in questionsseconds? Credit: Sven Grundmann / Goethe University Frankfurt.
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Egyptian chemist Ahmed Zewail received the 1999 Nobel Prize for measuring the rate of change of shape of molecules. He founded a completely new field of femtochemistry, which works with ultrashort laser beams. With their help, it is possible to measure the femtosecond times required to form or break a chemical bond.
The year is now 2020, and a team of physicists led by Reinhard Dörner of Johann Wolfgang Goethe-Universität Frankfurt in Germany has gone much deeper into femtoseconds. In their groundbreaking study, they measured the time it takes for a photon to fly the distance corresponding to a hydrogen molecule. It takes up to 247 questions per second, ie 247 trillionths (10-21) seconds. This is the shortest time we have ever measured reliably.
Reinhard Dörner. Credit: / Goethe University Frankfurt.
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What did such an experiment look like? They used hydrogen molecules H for this2, which were irradiated with X – rays from the PETRA III X – ray laser source at the German research facility DESY in Hamburg. At the same time, they adjusted the energy of the X-rays so that one X-ray photon “kicked out” both electrons from the molecule with hydrogen, which the molecule normally contains.
Goethe University Frankfurt, logo.
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Electrons behave like particles as well as waves. And in this case, the photon acts like a plaid thrown to the surface. The result is specific interference that can be analyzed. Dörner’s team used a special device (COLTRIMS reaction microscope) that can visualize ultrafast processes in atoms and molecules. COLTRIMS also determines how oriented the hydrogen molecule is.
When researchers knew the orientation of the hydrogen molecule under study, they could deduce from the X-ray photon interference pattern exactly when the photon “kicked” the first electron of the molecule and when the second electron. As a result, it takes up to 247 seconds, depending on the orientation of the molecule relative to the incoming photon. Dörner adds that they were the first to observe how the electron envelope did not react to the whole light beam in a single moment. The slight time lag is due to the fact that information cannot propagate through a molecule faster than the speed of light.
Video: Reinhard Dörner – 2019 Schawlow-Townes Symposium
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Goethe University Frankfurt October 16, 2020.
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