With ultracold chemistry,
researchers get first look at exactly what happens during a chemical reaction
Harvard University – November 28, 2019
-- In temperatures millions of times colder than interstellar space,
researchers have performed the coldest reaction in the known universe. But
that's not all. In such intense cold, their molecules slowed to such glacial
speeds, they could see something no one has been able to see before: the moment
when two molecules meet to form two new molecules. In essence, they captured a
chemical reaction in its most critical and elusive act.
The coldest chemical reaction in the
known universe took place in what appears to be a chaotic mess of lasers. The
appearance deceives: Deep within that painstakingly organized chaos, in
temperatures millions of times colder than interstellar space, Kang-Kuen Ni
achieved a feat of precision. Forcing two ultracold molecules to meet and
react, she broke and formed the coldest bonds in the history of molecular
couplings.
"Probably in the next couple of
years, we are the only lab that can do this," said Ming-Guang Hu, a
postdoctoral scholar in the Ni lab and first author on their paper published
today in Science. Five years ago, Ni, the Morris Kahn Associate
Professor of Chemistry and Chemical Biology and a pioneer of ultracold
chemistry, set out to build a new apparatus that could achieve the lowest
temperature chemical reactions of any currently available technology. But they
couldn't be sure their intricate engineering would work.
Now, they not only performed the coldest
reaction yet, they discovered their new apparatus can do something even they
did not predict. In such intense cold -- 500 nanokelvin or just a few
millionths of a degree above absolute zero -- their molecules slowed to such
glacial speeds, Ni and her team could see something no one has been able to see
before: the moment when two molecules meet to form two new molecules. In
essence, they captured a chemical reaction in its most critical and elusive
act.
Chemical reactions are responsible for
literally everything: breathing, cooking, digesting, creating energy,
pharmaceuticals, and household products like soap. So, understanding how they
work at a fundamental level could help researchers design combinations the
world has never seen. With an almost infinite number of new combinations
possible, these new molecules could have endless applications from more
efficient energy production to new materials like mold-proof walls and even
better building blocks for quantum computers.
In her previous work, Ni used colder and
colder temperatures to work this chemical magic: forging molecules from atoms
that would otherwise never react. Cooled to such extremes, atoms and molecules
slow to a quantum crawl, their lowest possible energy state. There, Ni can
manipulate molecular interactions with utmost precision. But even she could
only see the start of her reactions: two molecules go in, but then what? What
happened in the middle and the end was a black hole only theories could try to
explain.
Chemical reactions occur in just
millionths of a billionth of a second, better known in the scientific world as
femtoseconds. Even today's most sophisticated technology can't capture
something so short-lived, though some come close. In the last twenty years,
scientists have used ultra-fast lasers like fast-action cameras, snapping rapid
images of reactions as they occur. But they can't capture the whole picture.
"Most of the time," Ni said, "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."
Until now.
Ni's ultracold temperatures force
reactions to a comparatively numbed speed. "Because [the molecules] are so
cold," Ni said, "now we kind of have a bottleneck effect." When
she and her team reacted two potassium rubidium molecules -- chosen for their
pliability -- the ultracold temperatures forced the molecules to linger in the
intermediate stage for microseconds. Microseconds -- mere millionths of a
second -- may seem short, but that's millions of times longer than usual and
long enough for Ni and her team to investigate the phase when bonds break and
form, in essence, how one molecule turns into another.
With this intimate vision, Ni said she
and her team can test theories that predict what happens in a reaction's black
hole to confirm if they got it right. Then, her team 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.
Already, the team is exploring what else
they can learn in their ultracold test bed. Next, for example, they could
manipulate the reactants, exciting them before they react to see how their
heightened energy impacts the outcome. Or, 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 said. "Now, with
this apparatus, we can think about this. Without this technique, without this
paper, we cannot even think about this."
1
No comments:
Post a Comment