Understanding atomic interactions in detail is essential to refine the current chemical models concerning the construction and structuring of molecules. Until now, to study these interactions, chemists had to be content with groups of atoms in which they calculated and determined mean correlations, giving a vague idea of the individual behavior of atoms during the formation of molecules. But recently, a team of researchers has developed a technique for manipulating individual atoms in order to observe them forming molecules.
One way to analyze such exchanges is to grab single atoms with the equivalent of a tiny pair of tweezers, immobilize them, and record the changes as they meet. Fortunately, such a pair of tweezers exists. Made from specially aligned polarized light, these laser tweezers can serve as optical traps for tiny objects.
Manipulate individual atoms with optical tweezers
With sufficiently short light waves, an experimenter has a good chance of trapping something as small as an individual atom in these clamps. Of course, the atoms must first be cooled to make them easier to catch, and then separated in an empty space."Our method involves the individual trapping and cooling of three atoms to a temperature of about one millionth of a Kelvin using highly focused laser beams in a hyper-evacuated (vacuum) chamber, the size of a grid. -bread. We slowly combine the traps containing the atoms to produce controlled interactions which we measure”, explains the physicist Mikkel F. Andersen.
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| Experimental procedure for directly observing collisions of cold atoms. The researchers isolate three 85Rb atoms in separate optical tweezers and confirm their presence by fluorescence imaging. A collision and compression stage allows the atoms to interact. Credits; LA Reynolds et al. 2020 |
The atoms in this case were all rubidium atoms, which bond to form dirubidium molecules, but two atoms are not enough to achieve this. "Two atoms alone cannot form a molecule, you need at least three to do chemistry," says physicist Marvin Weyland.
Better understand the formation of molecules on the atomic scale
Modeling how it works is a real challenge. It is clear that two atoms must get close enough to be able to form a bond, while a third one tears off part of this bond energy to leave them connected. The three-body recombination between atoms should, in theory, force them out of their trap, which usually adds another problem to physicists trying to study the interactions between several atoms. |
| Rubidium atomic cloud cooled by laser and observed via the camera developed by the researchers. Credits: University of Otago |
Using a special camera to amplify the changes, the team captured the moment the rubidium atoms moved closer together, revealing a different rate of loss than that predicted by the models. Indeed, it also means that molecules do not bind as quickly as existing models explain. The results were published in the journal Physical Review Letters .
"This is the first time that this basic process has been studied in isolation, and it turns out that it has produced several surprising results that were not expected from previous measurements in large clouds of atoms. With further development, this technique could provide a way to build and control unique molecules of particular chemicals,” says Weyland. Other experiments will help refine these models to better explain how groups of atoms work together to meet and bond under various conditions.
| Bibliography: Direct Measurements of Collisional Dynamics in Cold Atom Triads L. A. Reynolds, E. Schwartz, U. Ebling, M. Weyland, J. Brand, and M. F. Andersen Phys. Rev. Lett. 124, 073401 – Published 18 February 2020 DOI:https://doi.org/10.1103/PhysRevLett.124.073401 |
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Chemistry