Taking Pictures of Electrons Moving with World's Fastest Camera in Quadrillionths of a Second
During the conversion of light into electricity, such as in solar cells, a large part of the input light energy is lost. This is due to the behaviour of electrons inside of materials. If light hits a material, it stimulates electrons energetically for a fraction of a second, before they pass the energy back into the environment. Because of their extremely short duration of a few femtoseconds – a femtosecond is one quadrillionth of a second – these processes have hardly been explored to date.
The properties of a material depend on the behaviour of its constituent electrons and atoms. A basic model to describe the behaviour of electrons is the concept of the so-called Fermi gas, named after the Nobel Prize winner Enrico Fermi. In this model, the electrons in the material are considered to be a gaseous system. In this way, it is possible to describe their interactions with each other.
Developed in Kiel:one of the world’s fastest systems
The special feature of the Kiel system is its extremely high temporal resolution of 13 femtoseconds. This makes it one of the fastest electron cameras in the world. “Thanks to the extremely short duration of the light pulses used, we are able to film ultrafast processes live. Our investigations have shown that there is a surprising amount of stuff happening here,” explained Michael Bauer, professor of ultrafast dynamics at the CAU. He developed the system, together with the working group of Kai Roßnagel, professor of solid state research with synchrotron radiation.
In their current experiment, the research team irradiated a graphite sample with a short, intense light pulse of only seven femtoseconds duration. Graphite is characterised by a simple electronic structure. Thus, fundamental processes can be observed particularly clearly. In the experiment, the impacting light particles – also called photons – disturbed the thermal equilibrium of the electrons. This equilibrium describes a condition in which a precisely-definable temperature prevails amongst the electrons. The Kiel research team then filmed the behaviour of the electrons, until a balance was restored after about 50 femtoseconds.
Numerous interactions within an extremely short period
In doing so, the scientists observed numerous interaction processes of excited electrons with the impacting photons, as well as atoms and other electrons in the material. On the basis of the film footage, they could even distinguish different phases within this ultrashort period: first of all, the irradiated electrons absorbed the light energy of the photons in the graphite, and thereby transformed it into electrical energy. Then the energy was distributed to other electrons, before they passed it on to the surrounding atoms. In this last process, the electrical energy is ultimately permanently converted into heat; the graphite warms up.
The study was funded by the German Research Foundation (DFG).It was highlighted by the publishers of the journal in Physics, the American Physical Society’s highlight journal.
Contacts and sources:
Prof. Dr sc. nat. Michael Bauer
Institute of Experimental and Applied Physics
Ultrafast Dynamics Group
Julia Siekmann / , Dr Boris Pawlowski
Science communication
Priority research area Kiel Nano, Surface and Interface Science (KiNSIS)
Kiel University
Citation: Ultrafast Formation of a Fermi-Dirac Distributed Electron Gas, G. Rohde, A. Stange, A. Müller, M. Behrendt, L.-P. Oloff, K. Hanff, T.?J. Albert, P. Hein, K. Rossnagel, and M. Bauer, Phys. Rev. Lett. 121, 256401 (2018), Published December 19, 2018. DOI: 10.1103/PhysRevLett.121.256401https://doi.org/10.1103/PhysRevLett.121.256401
Source: http://www.ineffableisland.com/2018/12/taking-pictures-of-electrons-moving.html
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Could they catch the actual “stimulation” and “absorption” on a molecular basis?