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Chemists Orchestrate the Molecular Union of Two Single Atoms

Imagine an atom as a tiny nucleus immersed in a giant diffuse cloud that is its electrons. When two atoms get close together, each one’s electron cloud pushes the other’s around, and sometimes the two atoms start to behave as a molecule. But experts still can’t describe this process in detail.

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It’s a molecular Parent Trap: Stick two atoms in a vacuum chamber, and steer them together with lasers., Emily Waite

THE MAIN ACT of Kang-Kuen Ni’s experiment could fit on the tip of a needle—and it happens in a fraction of a second. The Harvard chemist takes two individual atoms, a sodium and a cesium, each about 10,000 times smaller than a bacterium. Then, very carefully, she brings them together to become a single molecule: sodium cesium.

It’s an unlikely pairing. In the cosmic rom-com that is nature, sodium rarely goes for cesium; both atoms tend to become positively charged ions that actually repel each other. But after years of work, Ni’s team has figured out how to Parent Trap this union: stick the two atoms in a vacuum chamber with as few other atoms as possible, and steer them with lasers into forced proximity. They published the resultsin Science earlier this month.

And with that, these matchmakers have a new way of studying one of the most basic processes on Earth: the formation of a chemical bond. It’s the atomic relationship that determines whether a mixture of carbon, hydrogen, and oxygen atoms is sugar, alcohol, or formaldehyde. “Making a single chemical bond is one of the most fundamental chemical reactions there is,” says physicist Daniel Slaughter of Lawrence Berkeley National Laboratory, who was not involved with the work. “In a way, they’ve made the purest type of chemical reaction.”

It took Ni and her team years to pull off—because a reaction between just two atoms isn’t an ordinary chemistry experiment. Chemists commonly assemble new molecules by mixing and heating powders and solutions in specific concentrations and orders, trusting that the 1023 atoms will join up through random collisions. They might engineer reactions so that collisions between certain atoms are more likely, but they don’t painstakingly assemble each bond, one by one.

But Ni’s team wasn’t trying to make a big batch of chemicals. They wanted to show they could set up one particular match—between two single atoms.

To picture a chemical bond, imagine an atom as a tiny nucleus immersed in a giant diffuse cloud that is its electrons. (They’re not really the Tinker toy models you played with in chemistry class.) When two atoms get close together, each one’s electron cloud pushes the other’s around, and sometimes the two atoms start to behave as a unit: a molecule.

But experts still can’t describe this process in detail: what it looks like, in slo-mo, for one atom to edge closer to another until two become one. “One of the dreams we have in molecular physics and chemistry is to really image bonds, to really understand what a bond is,” says Slaughter. For his research, Slaughter actually does Ni’s experiment in reverse: He breaks molecules apart. “I start with a small molecule and blow it up with a laser, and then I look at the fragments,” he says. The forensics of the explosion gives him information about the bond.

To make a single molecule, Ni’s group built a bespoke contraption: a machine consisting of lasers and lenses, a vacuum chamber, detectors, and coils of wire. It took a lot of testing. Before they could make a molecule, they had to figure out how to move single atoms. And before they could move single atoms, they had to figure out how to grab them.

“Grabbing a single atom isn’t like grabbing a macroscopic object,” says Ni. They start with several small containers each with a solid form of sodium and cesium, placed inside a small chamber under high vacuum. They heat the containers, which turns sodium and cesium atoms into a vapor. Then, they use tightly focused lasers to move individual atoms in the vapor. Essentially, the photons from the laser pelt the atoms, nudging them in a specific direction until they are confined to specific areas inside the chamber designed to hold only a single atom. Once they’ve isolated one sodium and one cesium atom, they then move them close to each other. They also use a laser to give the sodium and cesium some extra energy to form the bond. To get everything to work in sequence, they automate it on a computer. “There are too many little details that all have to be tweaked correctly,” says Ni.

Ni’s machine is specifically designed for making sodium cesium, which they chose partly because the two atoms are relatively simple, each with just one free electron to participate in chemical reactions. Past researchers have also studied these atoms a lot—so Ni’s group could piggyback on lasers developed to manipulate the atoms.

But Ni’s techniques could be adapted to make other molecules with more complicated atoms, too. Slaughter, for example, thinks someone could use it to make molecules of carbon dioxide or nitrogen gas. Even though these molecules form easily in real life, its individual atoms are much more complicated to control than sodium and cesium.

For now, though, Ni is sticking with sodium cesium—because she thinks it could be useful in future technology. “These molecules already have nice properties we want to push,” she says. It’s relatively easy to manipulate a sodium cesium molecule into a specific configuration and have it stay like that for a while. If the molecule turns out to be an obedient quantum particle, it could potentially be useful as a component for a—buzzword alert—quantum computer. Sodium cesium: The chemistry is undeniable.