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. Jonas Rist, a Ph.D. student working within an international
team of researchers at the Institute for Nuclear Physics at Goethe University
Frankfurt, has now been able to find an answer to this mystery with the aid of
a COLTRIMS reaction microscope which had been developed in Frankfurt: The
emission takes place lightning fast, namely within just a few attoseconds – within
a billionths of billionths of a second.
FRANKFURT. It is
now exactly 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
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
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.
Jonas Rist, Kim Klyssek, Nikolay M.
Novikovskiy, Max Kircher, Isabel Vela-Pérez, 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,1 Markus S. Schöffler, Vernon T. Davis, Joshua B.
Williams, Florian Trinter, Reinhard Dörner,1 Philipp V. Demekhin, Till Jahnke: Measuring the photoelectron emission delay
in the molecular frame. Nat Commun 12, 6657 (2021). https://doi.org/10.1038/s41467-021-26994-2
COLTRIMS reaction microscope at electron storage ring BESSY II,
Helmholtz-Zentrum Berlin für Materialien und Energie (HZB). Photo: Miriam Weller, Goethe University Frankfurt
student Jonas Rist, Goethe University Frankfurt. Photo: Alexander Hartung, Goethe University Frankfurt
Prof. Dr. Till Jahnke
European XFEL and
Institute for Nuclear Physics, Goethe University Frankfurt, Germany
Tel.: + 49 (0)69-798 47023 (Office)
Prof. Dr. Reinhard Dörner
Institute for Nuclear Physics
Goethe University Frankfurt, Germany
Tel. +49 (0)69 798-47003