Physicists observe rare resonance in molecules for the first time

Three-state model for resonance and optical analogs. Credit: Nature (2023). DOI: 10.1038/s41586-022-05635-8

If it hits just the right tone, a singer can break a wine glass. The reason is resonance. While glass may vibrate slightly in response to most acoustic tones, a tone that resonates with the material’s natural frequency can send its vibrations into overdrive, causing the glass to shatter.

Resonance also occurs on a much smaller scale of atoms and molecules. When particles react chemically, it is partly due to specific conditions that resonate with the particles in a way that causes them to chemically bond. But atoms and molecules are constantly in motion, inhabiting a blur of states of vibration and rotation. It was nearly impossible to determine the exact resonance state that ultimately triggers the molecules to react.

MIT physicists may have solved some of this mystery with a new study in the journal Nature. The team reports that they have for the first time observed resonance in the collision of ultracold molecules.

They discovered that a cloud of supercooled sodium-lithium (NaLi) molecules disappeared 100 times faster than normal when exposed to a very specific magnetic field. The rapid disappearance of the molecules is a sign that the magnetic field has put the particles into resonance, causing them to react faster than they normally would.

The discoveries have shed light on the mysterious forces that cause molecules to react chemically. They also suggest that scientists may one day harness the natural resonances of particles to direct and control certain chemical reactions.

“This is the very first time that a resonance between two ultracold molecules has been observed,” says study author Wolfgang Ketterle, John D. MacArthur Professor of Physics at MIT. “There were suggestions that the molecules are so complicated that they look like a dense forest, where you couldn’t recognize a single resonance. But we found a large tree that stood out, by a factor of 100. We found observed something completely unexpected.”

Ketterle’s co-authors include lead author and MIT graduate student Juliana Park, graduate student Yu-Kun Lu, former MIT postdoc Alan Jamison, who is currently at the University of Waterloo, and Timur Tscherbul at the University of Nevada.

A medium mystery

Within a cloud of molecules, collisions are constantly occurring. The particles can bump into each other like frenzied billiard balls or stick together in a brief but crucial state known as an “intermediate complex” which then triggers a reaction to transform the particles into a new chemical structure.

“When two molecules collide, most of the time they don’t reach that intermediate state,” says Jamison. “But when they’re in resonance, the rate of going into that state increases dramatically.”

“The intermediate complex is the mystery behind all the chemistry,” adds Ketterle. “Usually the reactants and products of a chemical reaction are known, but not how one leads to the other. Knowing something about the resonance of molecules can give us a fingerprint of this mysterious intermediate state.”

Ketterle’s group looked for signs of resonance in atoms and molecules that are supercooled, at temperatures just above absolute zero. Such ultra-cold conditions inhibit the random motion of particles with temperature, giving scientists a better chance of recognizing any more subtle signs of resonance.

In 1998, Ketterle made the first-ever observation of such resonances in ultracold atoms. He observed that when a very specific magnetic field was applied to supercooled sodium atoms, the field increased the way the atoms scatter from each other, in an effect known as Feshbach resonance. Since then, he and others have searched for similar resonances in collisions involving both atoms and molecules.

“Molecules are much more complicated than atoms,” says Ketterle. “They have so many different vibrational and rotational states. Therefore, it wasn’t clear if the molecules would show resonances.”

Needle in a haystack

Several years ago, Jamison, then a postdoc in Ketterle’s lab, proposed a similar experiment to see if signs of resonance could be seen in a mixture of atoms and molecules cooled to a millionth of a degree above. above absolute zero. By varying an external magnetic field, they found they could indeed pick up multiple resonances among sodium atoms and sodium-lithium molecules, which they reported last year.

Then, as the team reports in this study, graduate student Park took a closer look at the data.

“She discovered that one of these resonances did not involve atoms,” Ketterle explains. “She blasted the atoms with the laser light, and a resonance was still there, very clear, and only involved molecules.”

Park found that the molecules seemed to disappear – a sign that the particles had undergone a chemical reaction – much faster than they normally would, when exposed to a very specific magnetic field.

In their original experiment, Jamison and his colleagues applied a magnetic field that they varied over a wide range of 1,000 Gaussians. Park found that sodium-lithium molecules suddenly vanish, 100 times faster than normal, in a tiny part of this magnetic range, at around 25 milli-Gaussian. This is equivalent to the width of a human hair compared to a meter long stick.

“It takes careful measurements to find the needle in that haystack,” Park says. “But we used a systematic strategy to zoom in on this new resonance.”

In the end, the team observed a strong signal indicating that this particular field was resonating with the molecules. The effect increased the chances of the particles bonding in a brief intermediate complex which then triggered a reaction that caused the molecules to disappear.

Overall, the discovery provides insight into molecular dynamics and chemistry. Although the team does not anticipate that scientists will be able to stimulate resonance and direct reactions at the level of organic chemistry, it may one day be possible to do so at the quantum scale.

“One of the main themes of quantum science is the study of systems of increasing complexity, especially where quantum control is potentially in sight,” says John Doyle, professor of physics at Harvard University, who does not did not participate in the group’s research. “These kind of resonances, first seen in simple atoms and later in more complicated atoms, have led to amazing advances in atomic physics. Now that it’s seen in molecules, we first need to understand it in detail. , and then let the imagination wander and think about what that might be good for, maybe building bigger ultracold molecules, maybe studying interesting states of matter.”

More information:
Juliana Park, A Feshbach Resonance in Collisions Between Triplet Ground-State Molecules, Nature (2023). DOI: 10.1038/s41586-022-05635-8.

Provided by Massachusetts Institute of Technology

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