A singer can break a wine glass by hitting the right pitch, due to resonance. When the glass vibrates in response to a sound wave, it will shatter if the pitch matches the glass’s natural frequency.

The same principle applies to atoms and molecules on a smaller scale, where chemical reactions occur when conditions resonate with particles and cause them to bond.

But since they live in a haze of vibrating and spinning states, atoms and molecules are continuously in motion. It has been almost impossible to figure out the exact state of resonance that causes molecules to react.

With new research that was published in the journal Nature, MIT physicists may have solved some of this riddle. The group claims to have seen a resonance in crashing ultracold molecules for the first time.

They discovered that when subjected to a particularly particular magnetic field, a cloud of super-cooled sodium-lithium (NaLi) molecules vanished 100 times more quickly than usual. The sudden disappearance of the molecules indicates that the magnetic field tuned the particles into resonance, causing them to respond faster than they would typically.

The results explain the mysterious forces that cause molecules to react chemically. They also imply that one day, scientists may be able to use the inherent resonances of particles to direct and regulate certain chemical processes.

“This is the very first time a resonance between two ultracold molecules has ever been seen,” remarks study author Wolfgang Ketterle. “There were suggestions that molecules are so complicated that they are like a dense forest, where you would not be able to recognize a single resonance. But we found one big tree standing out, by a factor of 100. We observed something completely unexpected.”

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

The mystery behind all of chemistry

Collisions between molecules inside a cloud happen often. Particles may bounce off of one another like frantic pool balls or create a “intermediate complex,” a fleeting but important condition that triggers a reaction to change the particles into a new chemical structure.

According to Jamison, most of the time when two molecules collide, that intermediate state is not reached. However, when they are in resonance, the pace at which they reach that condition increases considerably.

“The intermediate complex is the mystery behind all of chemistry,” Ketterle says. “Usually, the reactants and the 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 middle state.”

Ketterle’s team has searched for traces of resonance in super-cooled atoms and molecules, at temperatures close to absolute zero. These ultracold temperatures prevent the random, temperature-driven mobility of the particles, providing scientists a greater opportunity to detect any finer signals of resonance.

Ketterle produced the first record-breaking detection of these resonances in ultracold atoms in 1998. He discovered that a Feshbach resonance—a phenomenon known as increased atom-to-atom scattering—occurs when a particular precise magnetic field is applied to supercooled sodium atoms. Since then, he and other researchers have sought for resonances of a similar kind in collisions of atoms and molecules.

Ketterle asserts that molecules are far more complex than atoms. They exist in a wide variety of rotational and vibrational states. Therefore, it was unclear whether or not molecules would exhibit resonances.

New resonance

Jamison, who was a postdoc in Ketterle’s lab at the time, suggested a similar experiment to see if evidence of resonance might be found in a combination of atoms and molecules chilled to one millionth of a degree above absolute zero. They confirmed their previous findings that they could detect multiple resonances between sodium atoms and sodium-lithium molecules by altering the external magnetic field.

Then, as reported in the present research, graduate student Park examined the data in further detail.

“She discovered that one of those resonances did not involve atoms,” Ketterle adds. “She blew away the atoms with laser light, and one resonance was still there, very sharp, and only involved molecules.”

When subjected to a particular precise magnetic field, Park discovered that the molecules looked to vanish much more rapidly than they typically would, indicating that the particles experienced a chemical reaction.

In the first part of their experiment, Jamison and his team used a magnetic field that they changed over a wide range of 1,000 Gauss. Within a very narrow window of this magnetic range, at roughly 25 milli-Gaussian, Park discovered that sodium-lithium molecules abruptly vanished 100 times quicker than usual. When compared to a meter-long stick, it is comparable to the width of a human hair.

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

In the end, the researchers saw a significant signal that indicated the molecules resonated with this specific field. The result increased the likelihood of the particles forming a quick, intermediate complex, which in turn started a process that caused the molecules to vanish.

Overall, the discovery helps us learn more about how molecules move and how chemistry works. Although the team does not believe that scientists will be able to trigger resonance and direct reactions at the level of organic chemistry, this may one day be achievable at the quantum level.

John Doyle, a professor of physics at Harvard University who was not involved in the group’s research, notes that one of the central topics of quantum science is the study of systems that get more complicated, particularly when quantum control may be on the horizon. These kinds of resonances, which were first seen in simple atoms and then in atoms with more parts, led to a lot of amazing progress in atomic physics. 

“Now that this is seen in molecules, we should first understand it in detail, and then let the imagination wander and think what it might be good for, perhaps constructing larger ultracold molecules, perhaps studying interesting states of matter.”

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