Physicists observe rare resonance in molecules for the first time
The findings could provide a new way to control chemical reactions.
Peer-Reviewed PublicationIf she hits just the right pitch, a singer can shatter a wine glass. The reason is resonance. While the glass may vibrate slightly in response to most acoustic tones, a pitch that resonates with the material’s own natural frequency can send its vibrations into overdrive, causing the glass to shatter.
Resonance also occurs at the much smaller scale of atoms and molecules. When particles chemically react, it’s partly due to specific conditions that resonate with particles in a way that drives them to chemically link. But atoms and molecules are constantly in motion, inhabiting a blur of vibrating and rotating states. Picking out the exact resonating state that ultimately triggers 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 for the first time observed a resonance in colliding ultracold molecules.
They found that a cloud of super-cooled sodium-lithium (NaLi) molecules disappeared 100 times faster than normal when exposed to a very specific magnetic field. The molecules’ rapid disappearance is a sign that the magnetic field tuned the particles into a resonance, driving them to react more quickly than they normally would.
The findings shed light on the mysterious forces that drive molecules to chemically react. They also suggest that scientists could one day harness particles’ natural resonances to steer and control certain chemical reactions.
“This is the very first time a resonance between two ultracold molecules has ever 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 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.”
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 middle mystery
Within a cloud of molecules, collisions occur constantly. Particles may ping off each other like frenetic billiard balls or stick together in a brief yet crucial state known as an “intermediate complex” that then sets off a reaction to transform the particles into a new chemical structure.
“When two molecules collide, most of the time they don’t make it to that intermediate state,” says Jamison. “But when they’re in resonance, the rate of going to that state goes up dramatically.”
“The intermediate complex is the mystery behind all of chemistry,” Ketterle adds. “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 group has looked for signs of resonance in atoms and molecules that are super-cooled, to temperatures just above absolute zero. Such ultracold conditions inhibit the particles’ random, temperature-driven motion, giving scientists a better chance of recognizing any subtler 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 super-cooled sodium atoms, the field enhanced the way the atoms scattered off each other, in an effect known as a Feshbach resonance. Since then, he and others have looked 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 was not clear if molecules would show resonances at all.”
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 whether signs of resonance could be observed in a mixture of atoms and molecules cooled down to a millionth of a degree above absolute zero. By varying an external magnetic field, they found they could indeed pick up several resonances amid sodium atoms and sodium-lithium molecules, which they reported last year.
Then, as the team reports in the current study, graduate student Park took a closer look at the data.
“She discovered that one of those resonances did not involve atoms,” Ketterle says. “She blew away the atoms with laser light, and one resonance was still there, very sharp, and only involved molecules.”
Park found that the molecules seemed to disappear — a sign that the particles underwent a chemical reaction — much more quickly than they normally would, when they were exposed to a very specific magnetic field.
In their original experiment, Jamison and colleagues applied a magnetic field that they varied over a wide, 1,000-Gaussian range. Park found that molecules of sodium-lithium suddenly disappeared, 100 times faster than normal, within a tiny sliver of this magnetic range, at about 25 milli-Gaussian. That’s equivalent to the width of a human hair compared to a meter-long stick.
“It takes careful measurements to find the needle in this 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 that this particular field resonated with the molecules. The effect enhanced the particles’ chance of binding in a brief, intermediate complex that then triggered a reaction that made the molecules disappear.
Overall, the discovery provides a deeper understanding of molecular dynamics and chemistry. While the team does not anticipate scientists being able to stimulate resonance, and steer reactions, at the level of organic chemistry, it could one day be possible to do so at the quantum scale.
“One of the main themes of quantum science is studying systems of increasing complexity, especially when quantum control is potentially in the offing,” says John Doyle, professor of physics at Harvard University, who was not involved in the group’s research. “These kind of resonances, first seen in simple atoms and then more complicated ones, led to amazing advances 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.”
This research was supported, in part, by the National Science Foundation, and the U.S. Air Force Office of Scientific Research.
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Written by Jennifer Chu, MIT News Office
JOURNAL
Nature
ARTICLE TITLE
“A Feshbach resonance in collisions between triplet ground state molecules”
The bubbling universe: A previously unknown phase transition in the early universe
IMAGE: AI GENERATED ILLUSTRATION OF COLLIDING BUBBLES IN EARLY UNIVERSE view more
CREDIT: BIRGITTE SVENNEVIG, UNIVERSITY OF SOUTHERN DENMARK
Think of bringing a pot of water to the boil: As the temperature reaches the boiling point, bubbles form in the water, burst and evaporate as the water boils. This continues until there is no more water changing phase from liquid to steam.
This is roughly the idea of what happened in the very early universe, right after the Big Bang, 13.7 billion years ago.
The idea comes from particle physicists Martin S. Sloth from the Center for Cosmology and Particle Physics Phenomenology at University of Southern Denmark and Florian Niedermann from the Nordic Institute for Theoretical Physics (NORDITA) in Stockholm. Niedermann is a previous postdoc in Sloth’s research group. In this new scientific article, they present an even stronger basis for their idea.
Many bubbles crashing into each other
- One must imagine that bubbles arose in various places in the early universe. They got bigger and they started crashing into each other. In the end, there was a complicated state of colliding bubbles, which released energy and eventually evaporated, said Martin S. Sloth.
The background for their theory of phase changes in a bubbling universe is a highly interesting problem with calculating the so-called Hubble constant; a value for how fast the universe is expanding. Sloth and Niedermann believe that the bubbling universe plays a role here.
The Hubble constant can be calculated very reliably by, for example, analyzing cosmic background radiation or by measuring how fast a galaxy or an exploding star is moving away from us. According to Sloth and Niedermann, both methods are not only reliable, but also scientifically recognized. The problem is that the two methods do not lead to the same Hubble constant. Physicists call this problem “the Hubble tension”.
Is there something wrong with our picture of the early universe?
- In science, you have to be able to reach the same result by using different methods, so here we have a problem. Why don't we get the same result when we are so confident about both methods?, said Florian Niedermann.
Sloth and Niedermann believe they have found a way to get the same Hubble constant, regardless of which method is used. The path starts with a phase transition and a bubbling universe - and thus an early, bubbling universe is connected to "the Hubble tension".
- If we assume that these methods are reliable – and we think they are – then maybe the methods are not the problem. Maybe we need to look at the starting point, the basis, that we apply the methods to. Maybe this basis is wrong.
An unknown dark energy
The basis for the methods is the so-called Standard Model, which assumes that there was a lot of radiation and matter, both normal and dark, in the early universe, and that these were the dominant forms of energy. The radiation and the normal matter were compressed in a dark, hot and dense plasma; the state of the universe in the first 380.000 years after Big Bang.
When you base your calculations on the Standard Model, you arrive at different results for how fast the universe is expanding – and thus different Hubble constants.
But maybe a new form of dark energy was at play in the early universe? Sloth and Niedermann think so.
If you introduce the idea that a new form of dark energy in the early universe suddenly began to bubble and undergo a phase transition, the calculations agree. In their model, Sloth and Niedermann arrive at the same Hubble constant when using both measurement methods. They call this idea New Early Dark Energy – NEDE.
Change from one phase to another – like water to steam
Sloth and Niedermann believe that this new, dark energy underwent a phase transition when the universe expanded, shortly before it changed from the dense and hot plasma state to the universe we know today.
- This means that the dark energy in the early universe underwent a phase transition, just as water can change phase between frozen, liquid and steam. In the process, the energy bubbles eventually collided with other bubbles and along the way released energy, said Niedermann.
- It could have lasted anything from an insanely short time – perhaps just the time it takes two particles to collide – to 300,000 years. We don't know, but that is something we are working to find out, added Sloth.
Do we need new physics?
So, the phase transition model is based on the fact that the universe does not behave as the Standard Model tells us. It may sound a little scientifically crazy to suggest that something is wrong with our fundamental understanding of the universe; that you can just propose the existence of hitherto unknown forces or particles to solve the Hubble tension.
- But if we trust the observations and calculations, we must accept that our current model of the universe cannot explain the data, and then we must improve the model. Not by discarding it and its success so far, but by elaborating on it and making it more detailed so that it can explain the new and better data, said Martin S. Sloth, adding:
- It appears that a phase transition in the dark energy is the missing element in the current Standard Model to explain the differing measurements of the universe's expansion rate.
AI generated illustration of colliding bubbles in the universe
CREDIT
Birgitte Svennevig, University of Southern Denmark
SIDE BAR: How fast is the universe expanding?
The Hubble constant is a value for how fast the universe is expanding.
In Martin S. Sloth and Florian Niedermann's model, the Hubble constant is 72. Approximately. After all, large distances are being calculated, so we must allow for uncertainty of a few decimals.
What does 72 mean? It means 72 km per second per Megaparsec. Megaparsecs are a measure of the distance between, for example, two galaxies, and one megaparsec is 30,000,000,000,000,000,000 km. For every megaparsec between us and, for example, a galaxy, the galaxy moves away from us at 72 km per second.
When you measure the distance to galaxies by supernovas, you get a Hubble constant of approx. 73 (km/s)/megaparsec. But when measuring on the first light particles (the cosmic background radiation), the Hubble constant is 67.4 (km/s)/megaparsec.
When Sloth and Niedermann changed the basis of these calculations by introducing the existence of a new, early, dark energy that undergoes a phase transition – as described in the article – both types of calculations come to a Hubble constant of about 72.
JOURNAL
Physics Letters B
METHOD OF RESEARCH
Computational simulation/modeling
SUBJECT OF RESEARCH
Not applicable
ARTICLE TITLE
Hot new early dark energy: Towards a unified dark sector of neutrinos, dark energy and dark matter
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