Ultracold mystery: Solved
By manipulating ultracold molecules mid-chemical reaction, researchers
crack a molecular disappearing act

Date:
July 20, 2020
Source:
Harvard University
Summary:
Last December, researchers designed technology that could achieve
the lowest temperature chemical reactions and then broke and formed
the coldest bonds in the history of molecular coupling. Now, though
reactions are considered too fast to measure, they determined
the exact lifespan of their intermediate -- the space between
reactants and products -- and solved the mystery of why some
ultracold molecules simply disappear.



FULL STORY ==========================================================================
In a famous parable, three blind men encounter an elephant for the
first time.

Each touches a part -- the trunk, ear, or side -- and concludes the
creature is a thick snake, fan, or wall. This elephant, said Kang-Kuen
Ni, is like the quantum world. Scientists can only explore a cell of
this vast, unknown creature at a time. Now, Ni has revealed a few more
to explore.


==========================================================================
It all started last December, when she and her team completed a new
apparatus that could achieve the lowest temperature chemical reactions
of any currently available technology and then broke and formed the
coldest bonds in the history of molecular coupling. But their ultracold reactions also unexpectedly slowed the reaction to a sluggish speed,
gifting the researchers with a real-time glimpse of what happens during
a chemical transformation. Now, though reactions are considered too
fast to measure, Ni not only determined the lifetime of that reaction,
she solved an ultracold mystery in the process.

With ultracold chemistry, Ni, the Morris Kahn associate professor of
chemistry and chemical biology and of physics, and her team cooled two potassium-rubidium molecules to just above absolute zero and found the "intermediate," the space where reactants transform into products,
lived for about 360 nanoseconds (still billionths of a second, but long enough). "It's not the reactant. It's not the product. It's something in-between," Ni said. Watching that transformation, like touching the
side of an elephant, can tell her something new about how molecules,
the foundation of everything, work.

But they didn't just watch.

"This thing lives so long that now we can actually mess around with
it... with light," said Yu Liu, a graduate student in the Graduate
School of Arts and Sciences and first author on their study published
in Nature Physics. "Typical complexes, like those in a room-temperature reaction, you wouldn't be able to do much with because they dissociate
into products so quickly." Like Star Trek tractor beams, lasers can
trap and manipulate molecules. In ultracold physics, this is the go-to
method to capture and control atoms, observe them in their quantum ground
state or force them to react. But when scientists moved from manipulating
atoms to messing with molecules, something strange happened: molecules
started to disappear from view.



========================================================================== "They prepared these molecules, hoping to realize many of the applications
that they promise -- building quantum computers, for example -- but
instead what they see is loss," Liu said.

Alkali atoms, like the potassium and rubidium Ni and her team study,
are easy to cool down in the ultracold realm. In 1997, scientists won
a Nobel Prize in Physics for cooling and trapping alkali atoms in laser
light. But molecules are wonkier than atoms: They aren't just a spherical
thing sitting there, said Liu, they can rotate and vibrate. When trapped together in the laser light, the gas molecules bumped against each other
as expected, but some simply disappeared.

Scientists speculated that the molecular loss resulted from reactions --
two molecules bumped together and, instead of heading off in different directions, they transformed into new species. But how? "What we found
in this paper answers that question," Liu said. "The very thing that you
use to confine the molecule is killing the molecule." In other words,
it's the light's fault.

When Liu and Ni used lasers to manipulate that intermediate complex --
the middle of their chemical reaction -- they discovered the light
forced the molecules off their typical reaction path and into a new
one. A pair of molecules, stuck together as an intermediate complex,
can get "photo-excited" instead of following their traditional path,
Liu said. Alkali molecules are particularly susceptible because of how
long they live in their intermediate complex.



========================================================================== "Basically, if you want to eliminate loss," Liu said, "you've got
to turn off the light. You've got to find another way to trap these
things." Magnets, for example, or electric fields can trap molecules,
too. "But these are all technically demanding," said Liu. Light is
just simpler.

Next, Ni wants to see where these complexes go when they
disappear. Certain wavelengths of light (like the infrared the team
used to excite their potassium-rubidium molecules) can create different reaction paths -- but no one knows which wavelengths send molecules into
which new formations.

They also plan to explore what the complex looks like at various stages
of transformation. "To probe its structure," Liu said, "we can vary the frequency of the light and see how the degree of excitation varies. From
there, we can figure out where the energy levels of this thing are, which informs on its quantum mechanical construct." "We hope this will serve
as a model system," Ni said, an example for how researchers can explore
other low temperature reactions that don't involve potassium and rubidium.

"This reaction is, like many other chemical reactions, sort of a universe
in its own," said Liu. With each new observation, the team reveals a
tiny piece of the giant quantum elephant. Since there are an infinite
number of chemical reactions in the known universe, there's still a long,
long way to go.


========================================================================== Story Source: Materials provided by Harvard_University. Original written
by Caitlin McDermott-Murphy. Note: Content may be edited for style
and length.


========================================================================== Journal Reference:
1. Yu Liu, Ming-Guang Hu, Matthew A. Nichols, David D. Grimes,
Tijs Karman,
Hua Guo, Kang-Kuen Ni. Photo-excitation of long-lived transient
intermediates in ultracold reactions. Nature Physics, 2020; DOI:
10.1038/ s41567-020-0968-8 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200720112331.htm

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