Scientists discover light as ‘glue’ in a loosely linked molecule

In several ways, atoms join together to form molecules, but in each case, a trade-off of charges acts as a sort of “super glue.”

Some of these molecules create rather strong connections by sharing their negatively charged electrons, ranging from the simplest gases (the two linked oxygen atoms we breathe continuously) to the intricate hydrocarbons found floating in space. Some atoms are attracted to one another due to variations in their total charge(s).

Charge configurations around an atom can change in the presence of electromagnetic forces. Since light is constantly chnaging electromagnetic fields, a suitable photon shower can move electrons into positions where they might, in theory, bond.

“If you now switch on an external electric field, this charge distribution shifts a little,” explained physicist Philipp Haslinger from the Technical University of Vienna (TU Wien).

“The positive charge is shifted slightly in one direction, the negative charge slightly in the other direction; the atom suddenly has a positive and a negative side. It is polarized,” he added.

Using ultracold rubidium atoms, Haslinger, atomic physicist Mira Maiwöger, and colleagues showed that light may polarize atoms like that of magnetic, which causes otherwise neutral atoms to become slightly sticky.

“This is a very weak attractive force, so you have to conduct the experiment very carefully to be able to measure it,” says Maiwöger.

“If atoms have a lot of energy and are moving quickly, the attractive force is gone immediately. This is why a cloud of ultracold atoms was used,” Maiwöger added.

How was the discovery made?

The scientists used a magnetic field to confine a cloud of about 5,000 atoms beneath a gold-coated chip to a single plane.

To create a quasicondensate, they cooled down the rubidium particles to temperatures close to absolute zero (273 °C or 460 °F). As a result, the rubidium particles started collaborating and sharing properties similar to those of the fifth state of matter, though not quite to the same extent.

The atoms were then hit by a laser and underwent a variety of stresses. For instance, the pressure from incoming photons’ radiation can force them to move along the light beam. As the atom moves away from the most intense region of the beam, reactions in the electrons may cause it to return.

The researchers performed thorough calculations to identify the slight attraction expected to form between atoms in this flood of electromagnetic.

After turning off the magnetic field, the atoms free-fell for around 44 milliseconds before arriving in the laser light field. They could be imaged using light sheet fluorescence microscopy.

The cloud spontaneously extended during the fall, allowing the researchers to collect data at various densities.

Maiwöger and colleagues discovered that at high densities, up to 18% of the atoms were missing from their observational photographs. They postulate that these absences resulted from collisions aided by light, which forced the rubidium atoms from their cloud.

The discovery illustrated a portion of what was going on. Light was scattering off other atoms, and the light coming in was having an impact on the atoms. The atoms acquired polarity when the light made contact with them.

Greater light intensity either attracted or repelled the atoms depending on the type of light employed. As a result, they were either drawn to an area of lower light or more light, and in both cases, they eventually gathered together.

“An essential difference between usual radiation forces and the [light triggered] interaction is that the latter is an effective particle-particle interaction, mediated by scattered light,” Maiwöger and colleagues write in their paper.

“It does not trap atoms at a fixed position (for example, the focus of a laser beam) but draws them toward regions of maximum particle density.”

Although the force pulling the atoms together is far smaller than the molecular forces we are more familiar with, it can still accumulate in vast sizes. Resonance lines and emission patterns, which astronomers use to help us understand celestial objects, may change as a result.

Additionally, it might clarify how molecules arise in space.

“In the vastness of space, small forces can play a significant role,” says Haslinger.

“Here, we were able to show for the first time that electromagnetic radiation can generate a force between atoms, which may help to shed new light on astrophysical scenarios that have not yet been explained.”

You can review the complete study for yourself in the journal Physical Review X.

Study abstract:

“Light-matter interaction is well understood on the single-atom level and routinely used to manipulate atomic gases. However, in denser ensembles, collective effects emerge that are caused by light-induced dipole-dipole interactions and multiple photon scattering. Here, we report on the observation of a mechanical deformation of a cloud of ultracold ⁸⁷Rb atoms due to the collective interplay of the atoms and a homogenous light field. This collective light scattering results in a self-confining potential with interesting features: It exhibits nonlocal properties, is attractive for both red- and blue-detuned light fields, and induces a remarkably strong force that depends on the gradient of the atomic density. Our experimental observations are discussed in the framework of a theoretical model based on a local-field approach for the light scattered by the atomic cloud. Our study provides a new angle on light propagation in high-density ensembles and expands the range of tools available for tailoring interactions in ultracold atomic gases.”