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 billionth of billionths of a second.
Exactly 100 years ago, 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 Ph.D. 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 center 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 neighboring
molecules, for instance, and trigger further reactions there. "In the
future, such experiments could also help to better understand chemical
reaction dynamics," says Jahnke.
Reference:
Jonas Rist et al, Measuring the photoelectron emission delay in the
molecular frame, Nature Communications (2021).
DOI: 10.1038/s41467-021-26994-2
Tags:
Physics