Collectively Moving Electrons

1 November 2015

Photo: Stephan Klumpp, Deutsches Elektronen-Synchrotron DESY, Project A3

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) x-ray regime of the electromagnetic spectrum an inner-shell  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).

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.
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.
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.

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Stephan Klumpp, Deutsches Elektronen-Synchrotron DESY, Project A3