Ultracold chemistry captures an elusive act

US researchers say they have generated the coldest chemical reaction ever by forcing two ultracold molecules to meet and react. But that’s only part of the story.

To their surprise, they found that at 500 nanokelvin – just a few millionths of a degree above absolute zero – their new equipment slowed molecules to such glacial speeds that they could witness the moment when the two molecules met to form two new molecules. 

In essence, they write in the journal Science, they captured, for the first time, a chemical reaction in its most critical and elusive act.

In previous work, Harvard University’s Kang-Kuen Ni used colder and colder temperatures to forge molecules from atoms that would otherwise never react. Cooled to such extremes, atoms and molecules slow to their lowest possible energy state. 

This allowed Ni and her colleagues to manipulate molecular interactions with precision, but they could only see the start of the reactions, which occur in femtoseconds (millionths of a billionth of a second). What happened in the middle and the end was just theory. 

Even ultra-fast lasers working like rapid-action cameras can’t capture the whole picture, Ni says. 

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

Using the new apparatus, Ni’s ultracold temperatures force reactions to slow to what she describes as a comparatively numbed speed. 

When she and colleagues reacted two potassium rubidium molecules (chosen for their pliability), the molecules lingered in the intermediate stage for microseconds (millionths of a second) – long enough for them to investigate the phase when bonds break and form; in essence, how one molecule turns into another.

With this intimate vision, Ni says, they can test theories that predict what happens in a reaction’s black hole to confirm if they got it right. Then, they can craft new theories, using actual data to more precisely predict what happens during other chemical reactions – even those that take place in the mysterious quantum realm.

They could, for example, manipulate the reactants, exciting them before they react to see how their heightened energy impacts the outcome, they say. Or, they could even influence the reaction as it occurs, nudging one molecule or the other.