FRANKFURT. Light exerts a certain amount of pressure onto a body:
sun sails could thus power space probes in the future. However, when light
particles (photons) hit an individual molecule and knock out an electron, the
molecule flies toward the light source. Atomic physicists at 51猎奇
have now observed this for the first time, confirming a 90 year-old theory.
As
early as the 16th century, the great scholar Johannes Kepler
postulated that sunlight exerted a certain pressure, as the tail of the comets
he observed consistently pointed away from the sun. In 2010 the Japanese space
probe Ikaros used a sun sail for the first time in order to use the power of
sunlight to gain a little speed.
Physically and intuitively, the pressure
of light or radiation can be explained by the particle characteristic of light:
light particles (photons) strike the atoms of a body and transfer a portion of their
own momentum (mass times speed) onto that body, which
thus becomes faster.
However, when in the 20th
century physicists studied this momentum transfer in the laboratory during
experiments on photons of certain wavelengths which knocked individual
electrons out of atoms, they were met by a surprising phenomenon: the momentum of
the ejected electron was greater than that of the photon that struck it. This
is actually impossible 鈥� since Isaac Newton it has been known that within a
system, for every force there must exist an equal but opposite force: the
recoil, so to speak. For this reason, the Munich scientist Arnold Sommerfeld
concluded in 1930 that the additional momentum of the ejected electron must
come from the atom it left. This atom must fly in the opposite direction; in
other words, toward the light source. However, this was impossible to measure with
the instruments available at that time.
Ninety years later the physicists in the
team of doctoral student Sven Grundmann and Professor Reinhard D枚rner from the
Institute for Nuclear Physics have succeeded for the first time in measuring
this effect using the COLTRIMS reaction microscope developed at Goethe
University Frankfurt. To do so, they used X-rays at the accelerators DESY in
Hamburg and ESRF in French Grenoble, in order to knock electrons out of helium
and nitrogen molecules. They selected conditions that would require only one
photon per electron. In the COLTRIMS reaction microscope, they were able to
determine the momentum of the ejected electrons and the charged helium and
nitrogen atoms 鈥� which are called ions 鈥� with unprecedented precision.
Professor Reinhard D枚rner explains: 鈥淲e
were not only able to measure the ion鈥檚 momentum, but also see where it came
from 鈥� namely, from the recoil of the ejected electron. If photons in these
collision experiments have low energy, the photon momentum can be neglected for
theoretical modelling. With high photon energies, however, this leads to imprecision.
In our experiments, we have now succeeded in determining the energy threshold
for when the photon momentum may no longer be neglected. Our experimental breakthrough
allows us to now pose many more questions, such as what changes when the energy
is distributed between two or more photons.鈥�
Publication:
Sven Grundmann, Max Kircher, Isabel
Vela-Perez, Giammarco Nalin, Daniel Trabert, Nils Anders, Niklas Melzer, Jonas
Rist, Andreas Pier, Nico Strenger, Juliane Siebert, Philipp V. Demekhin, Lothar
Ph. H. Schmidt, Florian Trinter, Markus S. Sch枚ffler, Till Jahnke, and Reinhard
D枚rner: Observation of Photoion Backward
Emission in Photoionization of He and N2. Phys. Rev. Lett. 124, 233201
Further information:
Prof.
Dr. Reinhard D枚rner
Institute for Nuclear Physics
Tel. +49 69 798-47003
doerner@atom.uni-frankfurt.de
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