Atomic physics: Radiation pressure with recoil
Researchers deliver experimental proof for a 90 year-old theory

Date:
June 15, 2020
Source:
Goethe University Frankfurt
Summary:
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 have now observed this for the first time, confirming
a 90 year-old theory.



FULL STORY ========================================================================== 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 Goethe
University 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 -- 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 Do"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 -- which are called ions - -
with unprecedented precision.

Professor Reinhard Do"rner explains: "We were not only able to measure
the ion's momentum, but also see where it came from -- 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."

========================================================================== Story Source: Materials provided by Goethe_University_Frankfurt. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. 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. Scho"ffler, Till Jahnke,
Reinhard Do"rner.

Observation of Photoion Backward Emission in Photoionization of
He and N2. Physical Review Letters, 2020; 124 (23) DOI: 10.1103/
PhysRevLett.124.233201 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200615140846.htm

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