Physicists have just discovered light acting on the “glue” part between atoms, in a kind of unbound molecule.
“For the first time we have succeeded in polarizing several atoms together in a controlled manner, creating a measurable force of attraction between them,” says University of Innsbruck physicist Matthias Sonneleitner.
Atoms are linked to form molecules in many ways, all of which involve the trade of charges as a kind of “super glue”.
Some share their negatively charged electrons, and form relatively strong bonds, like the simplest gases of two connected oxygen atoms we constantly breathe, to the complex hydrocarbons found in space. Some atoms are attracted to due to differences in their total charge.
Electromagnetic fields can change the arrangements of charges around an atom. Because light is a rapidly changing electromagnetic field, a barrage of appropriately oriented photons can push electrons into positions they can – in theory – see bound.
“If you now turn on an external electric field, this charge distribution changes slightly,” explains physicist Philip Haslinger of the Technical University of Vienna (TU Wien).
“The positive charge shifts a little in one direction, the negative charge a little in the other, and the atom suddenly has a positive and negative side, it’s polarized.”
Haslinger, TU Wien, atomic physicist Mira Maiwöger and colleagues used ultracold rubidium atoms to demonstrate that light can indeed polarize atoms in the same way, causing otherwise neutral atoms to become slightly sticky.
“This is a very weak force of attraction, so you have to do the experiment very carefully until you can measure it,” Mayuger says.
“If the atoms have a lot of energy and move quickly, the attractive force disappears instantly. That’s why a cloud of very cold atoms was used.”
The team trapped a cloud of about 5,000 atoms beneath a gold-coated wafer, in one plane, using a magnetic field.
This is where they cool the atoms to temperatures near absolute zero (273°C or 460°F), forming a quasi-condensation – so the rubidium particles begin to act collectively and share properties as if they were in the fifth state of matter, but not as much Entirely the same.
Atoms were struck by a laser, and the atoms experienced a variety of forces. For example, it can be driven by radiation pressure from incoming photons along a light beam. Meanwhile, responses in the electrons can pull the atom back toward the denser part of the beam.
To discover the subtle attraction thought to arise between atoms in this electromagnetic flow, the researchers needed to make some careful calculations.
When they shut off the magnetic field, the atoms fell freely for about 44 milliseconds before hitting the laser light field where they were also imaged using fluorescence optical paper microscopy.
During the fall, the cloud naturally expanded, so the researchers were able to take measurements at different densities.
At high densities, Mayuger and colleagues found that up to 18 percent of the atoms were missing from the observed images they were taking. They believe the reason for this absence is light-assisted collisions that expelled rubidium atoms from the cloud.
This showed part of what was going on – it wasn’t just the incoming light affecting the atoms, but the light being scattered from other atoms as well. When the light touches the atoms, it gives them polarity.
Depending on the type of light used, the atoms were either attracted or repelled by a greater light intensity. So they were pulled either towards the low light or high light area – in each case they ended up piling up together.
“A fundamental difference between the usual radiation forces and [light triggered] The interaction is that the latter is an efficient interaction between particles, mediated by scattered light,” Mayuger and colleagues write in their paper.
“It does not trap atoms in a fixed position (for example, the focus of a laser beam) but rather directs them toward regions of maximum particle density.”
While this force that holds atoms together is much weaker than the intermolecular forces we are more familiar with, it can accumulate on a large scale. This can alter emission patterns and resonance lines – features that astronomers use to enrich our understanding of celestial bodies.
It can also help explain how molecules form in space.
“In the vast space, small forces can play an important role,” Haslinger says.
“Here, we were able to show for the first time that electromagnetic radiation can generate a force between atoms, which may help shed new light on astrophysical scenarios that have yet to be explained.”
This research was published in X . physical review.
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