Einstein's photoelectric effect: The time it takes for an electron to be released
Researchers examine photoelectric effect with the aid of a COLTRIMS
reaction microscope

Date:
February 10, 2022
Source:
Goethe University Frankfurt
Summary:
When light hits a material, electrons can be released from this
material -- the photoelectric effect. Although this effect played a
major role in the development of the quantum theory, it still holds
a number of secrets: To date it has not been clear how quickly the
electron is released after the photon is absorbed. Researchers have
now been able to find an answer to this mystery with the aid of a
COLTRIMS reaction microscope: The emission takes place lightning
fast, namely within just a few attoseconds.



FULL STORY ==========================================================================
It is now one hundred years ago that Albert Einstein was awarded the
Nobel Prize in Physics for his work on the photoelectric effect. The jury
had not yet really understood his revolutionary theory of relativity --
but Einstein had also conducted ground-breaking work on the photoelectric effect. With his analysis he was able to demonstrate that light comprises individual packets of energy -- so-called photons. This was the decisive confirmation of Max Planck's hypothesis that light is made up of quanta,
and paved the way for the modern quantum theory.


========================================================================== Although the photoelectric effect in molecules has been studied
extensively in the meantime, it has not yet been possible to determine its evolution over time in an experimental measurement. How long does it take
after a light quantum has hit a molecule for an electron to be dislodged
in a specific direction? "The length of time between photon absorption
and electron emission is very difficult to measure because it is only
a matter of attoseconds," explains Till Jahnke, the PhD-supervisor of
Jonas Rist. This corresponds to just a few light oscillations. "It has
so far been impossible to measure this duration directly, which is why
we have now determined it indirectly." To this end the scientists used a COLTRIMS reaction microscope -- a measuring device with which individual
atoms and molecules can be studied in incredible detail.

The researchers fired extremely intense X-ray light -- generated by the synchrotron radiation source BESSY II of Helmholtz-Zentrum Berlin -- at a sample of carbon monoxide in the centre of the reaction microscope. The
carbon monoxide molecule consists of one oxygen atom and one carbon
atom. The X-ray beam now had exactly the right amount of energy to
dislodge one of the electrons from the innermost electron shell of the
carbon atom. As a result, the molecule fragments. The oxygen and carbon
atoms as well as the released electron were then measured.

"And this is where quantum physics comes into play," explains Rist. "The emission of the electrons does not take place symmetrically in all
directions." As carbon monoxide molecules have an outstanding axis, the emitted electrons, as long as they are still in the immediate vicinity
of the molecule, are still affected by its electrostatic fields. This
delays the release slightly -- and to differing extents depending upon
the direction in which the electrons are ejected.

As, in accordance with the laws of quantum physics, electrons not only
have a particle character but also a wave character, which in the end
manifests in form of an interference pattern on the detector. "On the
basis of these interference effects, which we were able to measure with
the reaction microscope, the duration of the delay could be determined indirectly with very high accuracy, even if the time interval is
incredibly short," says Rist. "To do this, however, we had to avail of
several of the possible tricks offered by quantum physics." On the one
hand the measurements showed that it does indeed only take a few dozen attoseconds to emit the electron. On the other hand, they revealed that
this time interval is very heavily dependent on the direction in which
the electron leaves the molecule, and that this emission time is likewise greatly dependent on the velocity of the electron.

These measurements are not only interesting for fundamental research in
the field of physics. The models which are used to describe this type of electron dynamics are also relevant for many chemical processes in which electrons are not released entirely, but are transferred to neighbouring molecules, for instance, and trigger further reactions there. "In the
future such experiments could also help to better understand chemical
reaction dynamics therefore," says Jahnke.

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


========================================================================== Journal Reference:
1. Jonas Rist, Kim Klyssek, Nikolay M. Novikovskiy, Max Kircher, Isabel
Vela-Pe'rez, Daniel Trabert, Sven Grundmann, Dimitrios Tsitsonis,
Juliane Siebert, Angelina Geyer, Niklas Melzer, Christian Schwarz,
Nils Anders, Leon Kaiser, Kilian Fehre, Alexander Hartung,
Sebastian Eckart, Lothar Ph. H. Schmidt, Markus S. Scho"ffler,
Vernon T. Davis, Joshua B.

Williams, Florian Trinter, Reinhard Do"rner, Philipp V. Demekhin,
Till Jahnke. Measuring the photoelectron emission delay in
the molecular frame. Nature Communications, 2021; 12 (1) DOI:
10.1038/s41467-021-26994- 2 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220210114102.htm

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