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Ultracold matter

Ultracold matter

Giant two-atom molecules are the size of bacteria

25 Aug 2016
Montage comparing the size of a bacterium to that of a giant molecule
Bigger than a bacterium: comparing the size of a bacterium to that of a giant molecule made from weakly bound Rydberg atoms. The separation between the two atoms is about 1 micron. (Courtesy: Heiner Saszligmannshausen)

Enormous two-atom molecules about the size of ordinary bacteria have been made by two chemists in Switzerland. Comprising two caesium atoms, each “macrodimer” is about 1 μm in length – which is almost 10,000 times larger than common diatomic molecules such as oxygen. Although macrodimers were first spotted in 2009, this time the scientists were able to study the molecules more directly. They were also able to flesh out the existing theory describing these short-lived molecules and predict which types would have longer lifetimes. This allowed them to create macrodimers that could last about 1 μs before breaking apart into ions.

The macrodimers are so large because their constituent atoms are also huge – with each atom having an outermost electron that is excited into a far-flung atomic orbital. These are known as Rydberg atoms, and at room temperature they only exist for a very short time. This is because the outer electron is so weakly bound to the rest of the atom that collisions from nearby particles can easily knock it out of the atom. To minimize these collisions and extend the lifetime of the Rydberg atoms so molecules could be made, Heiner Saßmannshausen and Johannes Deiglmayr of the Swiss Federal Institute of Technology (ETH) in Zurich, Switzerland, created Rydberg atoms at extremely low temperatures.

They began with a diffuse cloud of caesium atoms that had been laser-cooled to below 40 μK. The average separation between atoms in the cloud was about 1 μm. The duo then used pulsed laser light to excite a small fraction of the caesium atoms into Rydberg states in the 44th energy level. Then they pulsed the gas cloud with a second laser, which had a photon energy slightly less than that required for caesium’s transition to the 43rd energy level. That difference in energy is equal to the binding energy of the macrodimer. That is the amount of energy that two caesium atoms in the 43rd and 44th energy levels would lose by joining together as a macrodimer.

Ions of distinction

This pulse excited pairs of atoms simultaneously into a state in which the two atoms behaved collectively as a molecule. To confirm that it had indeed created macrodimers, the team looked for the caesium ions that formed when the huge molecues break apart. The researchers found that these ions have distinctive properties that are predicted by a macrodimer model, which allowed the duo to conclude that they had indeed made the giant molecules.

The orbital overlap is basically zero

Heiner Saßmannshausen, ETH Zurich

The atoms in the micron-sized molecule interact with each other via van der Waals forces. This is a relatively weak interaction that arises when the outer electron of one atom deforms the shape of the other atom via electrostatic forces. This deformity can result in either attraction or repulsion between the two atoms, depending on the distance between them. This exotic molecular “bond” is different from the usual bonds that hold molecules together, such as covalent and ionic bonds, in which atoms in close proximity share or give up electrons to each other. “In our case, the atoms are really completely separated,” Saßmannshausen says. “The orbital overlap is basically zero.”

“This is a great achievement,” says Robin Côté, a theoretical physicist at the University of Connecticut, who was part of the team that first predicted the existence of macrodimers in 2002. This latest work expands on that prediction, which was based on a much simpler model.

Quantum gold mine

Côté says the work heralds a gold mine of new quantum-mechanical phenomena on a different scale. “The fact that these macrodimers exist is amazing,” he says. “It’s amazing that quantum mechanics is relevant between objects a micron apart. This is a new type of molecule that you could not observe under normal conditions.”

Côté and collaborators are already on to the next step: modelling a three-atom micron-scale molecule. In 2013, they published a paper predicting the existence of these macrotrimers, still yet to be created in the lab. In addition, he says that because macrodimers provide a new way to control two atoms at once, they could be used in quantum-information applications. “What’s next? Who knows?” he says. “There’s plenty of possibilities. Whenever you get a new toy, there are plenty of interesting new things to think about.”

The macrodimers are described in Physical Review Letters.

  • There is much more about the fascinating world of Rydberg atoms in this feature article by Keith Cooper: “The rise of Rydberg physics“.
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