The Coldest Reaction ever shows the moment molecules bond

A record supercold chemical reaction has allowed researchers to observe the very moment molecules bond and transform.

Amongst a chaotic mess of lasers the coldest known chemical reaction — one with temperatures millions of times colder than interstellar space — has been achieved by Kang-Kuen Ni of the Ni Group. In the process, Ni and her team have observed the very moment that molecules bond. Something no human has seen before.

This feat of extraordinary precision was achieved by forcing together two ultracold molecules and causing them to react, resulting in the breaking and formation of the coldest bonds in the history of molecular coupling.

“Probably, for the next couple of years, we are the only lab that can do this,” Ming-Guang Hu, a postdoctoral scholar in the Ni lab and first author of a paper published in Science detailing the experiment.

Chemical reactions transform reactants to products through an intermediate state where bonds break and form. Often too short-lived to observe, this phase has so-far eluded intimate investigation. By “freezing out” the rotation, vibration, and motion of the reactants (here, potassium-rubidium molecules) to a temperature of 500 nanokelvins (barely above absolute zero temperature), the number of energetically allowed exits for the products is limited. “Trapped” in the intermediate for far longer, researchers can then observe this phase directly with photoionization detection. This technique paves the way towards the quantum control of chemical reactions with ultracold molecules. (Ming-Guang Hu)

Performing the coldest reaction ever recorded will be something of a relief for Ni and her fellow researchers at Ni labs. Five years ago, the Morris Kahn Associate Professor of Chemistry and Chemical Biology — a pioneer of ultracold chemistry — set about building an apparatus in her lab that could theoretically achieve the lowest temperature chemical reactions available to any existing technology.

But even with the intricate engineering work that the project required, the researchers couldn’t be sure that the feat they desired was achievable. Now, not only has the apparatus delivered the reaction it was designed to perform, but it has also delivered a massive surprise to the team. They were able to catch a chemical reaction in its most critical and elusive moment.

In the intense cold — 500 nanokelvins or just a few millionths of a degree above absolute zero — that they engineered, molecules move with glacially slow speeds. In these conditions, the researchers were stunned to see something no one ever had before. The very moment that two molecules meet to form two new molecules.

As chemical reactions are so utterly ubiquitous — responsible for everything from breathing, the pharmaceuticals that we depend on, to the creation of energy — understanding how they work at such fundamental levels could help researchers design all kinds of applications and combinations previously unknown.

Some revolutionary new applications of this new glimpse into fundamental chemistry suggested by the researchers range from more efficient energy production methods to the creation of the building blocks for quantum computing. But the applications could be almost infinite.

You’re getting colder (and slower)!

Ni used colder and colder temperatures in her previous work to forge molecules from atoms that would otherwise never react with each other.

When cooled to such extremes, atoms and molecules slow to a quantum crawl, their lowest possible energy state. This means that Ni can then manipulate molecular interactions with utmost precision. Despite this, Ni could still only see the reactions begin, events after the start of the reaction — its the middle and end — were still shrouded in mystery. Only accessible by theory alone.

As chemical reactions occur in just millionths of a billionth of a second — or femtoseconds — even the most sophisticated technology can’t capture them.

The development of laser technology in the last two decades has allowed scientists to use ultrafast lasers as fast-action cameras — snapping rapid images of reactions as they occur. But even then, they can’t capture the whole picture.

“Most of the time, you just see that the reactants disappear and the products appear in a time that you can measure,” says Ni. “There was no direct measurement of what actually happened in these chemical reactions.”

That is, until now, with Ni’s ultracold temperatures forcing reactions to a comparatively much-subdued speed.

“Because [the molecules] are so cold, now we kind of have a bottleneck effect,” continues Ni.

You’re as cold as ice. Actually, much colder…

Ni and her team used two potassium rubidium molecules — chosen for their pliability — for their reaction, with the ultracold temperatures forcing the molecules to linger in the intermediate stage for microseconds.

This may seem like a painfully short moment of time, but it’s still millions of times longer than usual. Long enough for Ni and her team to investigate the phase when bonds break and form — in essence, how one molecule turns into another.

According to Ni, with this level of scrutiny, her team can test theories that predict what happens in a reaction’s hidden phases to confirm if they got it right. From there the team should be able to craft new theories — using actual data — to more precisely predict what happens during other chemical reactions. Possibly, even those that take place in the quantum realm.

Already, the team is exploring what else they can learn from conducting experiments in their ultracold test lab. One possibility is the manipulation of reactants — exciting them before they react to see how their heightened energy impacts the outcome. The researchers believe they could even influence the reaction as it occurs, nudging one molecule or the other.

“With our controllability, this time window is long enough, we can probe,” Hu concludes. “Now, with this apparatus, we can think about this. Without this technique, without this paper, we cannot even think about this.”