While working with helium nanodroplets, scientists at the Department of Ion
Physics and Applied Physics led by Fabio Zappa and Paul Scheier have come
across a surprising phenomenon: When the ultracold droplets hit a hard
surface, they behave like drops of water. Ions with which they were
previously doped thus remain protected on impact and are not neutralized.
At the Department of Ion Physics and Applied Physics, Paul Scheier's
research group has been using helium nanodroplets to study ions with methods
of mass spectrometry for around 15 years. Using a supersonic nozzle, tiny,
superfluid helium nanodroplets can be produced with temperatures of less
than one degree Kelvin. They can very effectively be doped with atoms and
molecules. In the case of ionized droplets, the particles of interest are
attached to the charges, which are then measured in the mass spectrometer.
During their experiments, the scientists have now stumbled upon an
interesting phenomenon that has fundamentally changed their work. "For us,
this was a gamechanger," says Fabio Zappa from the nano-bio-physics team.
"Everything at our lab is now done with this newly discovered method." The
researchers have now published the results of their studies in Physical
Review Letters.
A surprising phenomenon
When charged particles are fired at a metal plate, the particles are
normally neutralized by the many free electrons on the metal surface. They
can then no longer be measured in the mass spectrometer. But when the ions
are packed in a helium nanodroplet, they remain protected on impact and fly
off in all directions with a few weakly bound helium atoms. "The ions are
apparently protected by the helium," Zappa says. He doesn't yet fully
understand the underlying mechanism. "But there is some evidence that the
helium loses its superfluid property before impact and then behaves like a
liquid, splashing away from the surface and only then partially evaporates."
Another possible reason could be that the first droplets evaporate at the
surface, forming a layer of gas that slows down subsequent droplets and, in
this way protects them from evaporation. Only further investigations will
show if one of these explanations is correct or if there are other reasons.
The fact that this method also works with negative ions, which are normally
very fragile, indicates to the scientists a strong effect of the previously
unknown phenomenon.
Nanotechnology benefits
With this discovery, Paul Scheier's team not only improved their own
measurement method, but also gained important insights for other research
groups that, for example, deal with the deposition of nanoparticles on
surfaces. "Metal nanoparticles are a great example of this," Scheier
recounts. "In many modern technologies, metal nanoparticles are found that
have very specific properties." The fact that the generation of such
nanofilms can often be very inefficient could also be related to the
phenomenon now discovered in Innsbruck.
Reference:
Paul Martini et al, Splashing of Large Helium Nanodroplets upon Surface
Collisions, Physical Review Letters (2021).
DOI: 10.1103/PhysRevLett.127.263401
Tags:
Physics