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Collectively Moving
Electrons
The simultaneous excitation of more than two
particles with one photon is a very faint, but
possible, process according to the laws of
quantum mechanics. By preparing an ion beam
of singly charged carbon ions it was possible to
measure the trace of the rare triple Auger decay
using soft x-rays delivered by the beamline P04 at
PETRA III, DESY, Hamburg.
After excitation of matter with light in the (soft) xray regime of the electromagnetic spectrum an innershell electron near the nucleus can leave the atom
where it was bound before, and, as a free electron,
can reach the vacuum (or scientists can catch it in a
detector and count it). This process is called
photoionization which was experimentally found by
Hertz in 1887 and explained by Einstein in 1905.
The produced hole in the atomic shell will not stay
there forever. It will be filled by an electron from an
upper shell. This electron can be compared in a
simple picture with a rain drop in the atmosphere
which will fall down to the ground (hole). When the
rain drop is falling down due to the gravitational
field of the earth the rain drop loses potential energy
and gains velocity. The transition of an electron
from an upper shell to a hole is more complex.
According to the laws of quantum mechanics the
electron won't gain velocity like the rain drop but
has to transfer its excessive energy to another
particle. In our case this is another electron of the
atom which for itself, can gain enough energy to
leave the atom. This process is called an Auger
decay named after Pierre Auger describing it in 1923
(but Lise Meitner found it, too, in 1922).
In the simplest approach only two particles interact
with each other causing many effects we can
observe in nature. However, on a closer look, all
particles are interacting with each other and these
interactions can result in exotic behavior, like
chaotic trajectories or entanglement. To study such
many-particle effects single atoms are ideally suited.
In terms of the Auger effect, not only two electrons
can interact, but also three, four, and so on.
Fig. 1: Illustration of the triple Auger decay
A core electron is resonantly excited into an upper
shell by absorbing an x-ray photon (blue wavy
arrow). The remaining hole can be filled with an
electron from an upper shell. Upon its way through
the high lying shells the decaying electron transfers
its excessive energy to the electrons there (light
blue shade) which can gain enough energy to leave
the atom.
A team of scientists using the high brilliant x-ray
source PETRA III at DESY was able to measure the
signature of the triple Auger process by preparing a
beam of singly charged carbon ions. The five
remaining electrons in the carbon are distributed
over two main shells, one near the nucleus and one
outer shell. By tuning the energy of the x-rays
carefully one inner-shell electron can be excited to
the upper shell. One electron decays back to the
inner shell and the other three are ejected
simultaneously into the vacuum leaving behind a
detectable four times charged ion. Because of the
low number of electrons within the carbon ion it was
proven that no other process except the triple Auger
decay can charge up the ion from one to four by
absorbing one photon.
Stephan Klumpp, Deutsches Elektronen-Synchrotron
DESY, Project A3
Original publication:
“Observation of a Four-Electron Auger Process in
Near-K-Edge Photoionization of Singly Charged
Carbon Ions”
Physical Review Letters 114, 013002 (2015)
Doi: 10.1103/PhysRevLett.114.013002
A. Müller, A. Borovik, Jr., T. Buhr, J. Hellhund,
K. Holste, A. L. D. Kilcoyne, S. Klumpp, M.
Martins, S. Ricz, J. Viefhaus, S. Schippers