Physicists observe rare resonance in molecules for the first time

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

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

MIT physicists may have cracked part of this mystery with a new study appearing in the journal. Nature. The team reports that they have observed for the first time a resonance in colliding ultra cold molecules.

They found 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 tuned the particles into a resonance, causing them to react faster than normal.

The findings shed light on the mysterious forces that drive molecules to react chemically. They also suggest that scientists could one day take advantage of the natural resonances of particles to direct and control certain chemical reactions.

“This is the first time a resonance between two ultracold molecules has been seen,” says study author Wolfgang Ketterle, the John D. MacArthur Professor of Physics at MIT. “There were suggestions that molecules are so complicated that they’re like a dense forest, where you wouldn’t be able to recognize a single resonance. But we found a big tree that stands out, by a factor of 100. We 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 Snowfall.

a medium mystery

Within a cloud of molecules, collisions are constantly occurring. The particles can collide with each other like frenzied billiard balls or come together in a brief but crucial state known as a “complex intermediate” 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 get to that intermediate state,” says Jamison. “But when they’re in resonance, the rate of going to that state increases dramatically.”

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

Ketterle’s group has searched for signs of resonance in supercooled atoms and molecules, at temperatures just above absolute zero. Such ultracold conditions inhibit the random motion of the particles, driven by temperature, giving scientists a better chance of recognizing any more subtle signs of resonance.

In 1998, Ketterle made the first observation of such resonances in ultracold atoms. He observed that when a very specific magnetic field was applied to supercooled sodium atoms, the field enhanced the way the atoms scattered from one another, in an effect known as Feshbach resonance. Since then, he and others have searched for similar resonances in collisions between atoms and molecules.

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

needle in a haystack

Several years ago, Jamison, who at the time was a postdoc in Ketterle’s lab, proposed a similar experiment to see if signs of resonance could be observed in a mixture of atoms and molecules cooled to a millionth of a degree above absolute zero. . By varying a external magnetic fieldfound that they could, in fact, pick up various resonances between sodium atoms and sodium-lithium molecules, which reported last year.

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

“She found that one of those resonances didn’t involve atoms,” says Ketterle. “She blew away the atoms with laser light, and a resonance was still there, very sharp, and it only involved molecules.”

Park found that the molecules seemed to disappear, a sign that the particles underwent 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 which varied over a wide range of 1000 Gaussians. Park found that sodium-lithium molecules suddenly disappeared, 100 times faster than normal, within a small portion of this magnetic range, at about 25 milli-Gaussians. That is equivalent to the width of a human hair compared to a meter long stick.

“Careful measurements are needed to find the needle in this haystack,” says Park. “But we use a systematic strategy to approach this new resonance.”

In the end, the team observed a strong signal that this particular field resonated with the molecules. The effect enhanced the chance that the particles would come together in a brief intermediate complex that then triggered a reaction that caused the molecules to disappear.

Overall, the discovery provides a deeper understanding of molecular dynamics and chemistry. While the team doesn’t anticipate that scientists will be able to stimulate resonance and drive reactions at the level of organic chemistry, it might one day be possible to do so on a quantum scale.

“One of the main themes of quantum science is the study of systems of increasing complexity, especially when quantum control is potentially in the offing,” says John Doyle, a professor of physics at Harvard University, who was not involved in the research. of the group. “Observed first in simple atoms and then in more complicated ones, these kinds of resonances have led to amazing advances in atomic physics. Now that this is seen in molecules, we must first understand it in detail and then let the imagination wander.” and think what it could be good for, maybe build bigger ultracold moleculesperhaps studying interesting states of matter”.