Researchers at Ohio State University and Kansas State University have captured the first-ever images of atoms moving in a molecule. Shown here is molecular nitrogen. The researchers used an ultrafast laser to knock one electron from the molecule, and recorded the diffraction pattern that was created when the electron scattered off the molecule. The image highlights any changes the molecule went through during the time between laser pulses: one quadrillionth of a second. The constituent atoms’ movement is shown as a measure of increasing angular momentum, on a scale from dark blue to pink, with pink showing the region of greatest momentum. Image: Cosmin Blaga, Ohio State University |
Using a new ultrafast camera, researchers have recorded the first real-time
image of two atoms vibrating in a molecule.
Key to the experiment, which appears in Nature, is the researchers’
use of the energy of a molecule’s own electron as a kind of “flash bulb” to
illuminate the molecular motion.
The team used ultrafast laser pulses to knock one electron out of its
natural orbit in a molecule. The electron then fell back toward the molecule
scattered off of it, analogous to the way a flash of light scatters around an
object, or a water ripple scatters in a pond.
Principal investigator Louis DiMauro of Ohio State University said that the feat marks a
first step toward not only observing chemical reactions, but also controlling them
on an atomic scale.
“Through these experiments, we realized that we can control the quantum
trajectory of the electron when it comes back to the molecule, by adjusting the
laser that launches it,” said DiMauro, who is a professor of physics at Ohio State. “The next step will be to see if we can steer the electron in just the right
way to actually control a chemical reaction.”
A standard technique for imaging a still object involves shooting the object
with an electron beam—bombarding it with millions of electrons per second. The
researchers’ new single-electron quantum approach allowed them to image rapid
molecular motion, based on theoretical developments by the paper’s coauthors at
Kansas State University.
A technique called laser-induced electron diffraction (LIED) is commonly
used in surface science to study solid materials. Here, the researchers used it
to study the movement of atoms in a single molecule.
The molecules they chose to study were simple ones: nitrogen and oxygen.
Nitrogen and oxygen are common atmospheric gases, and scientists already know
every detail of their structure, so these two very basic molecules made a good
test case for the LIED method.
In each case, the researchers hit the molecule with laser light pulses of 50
femtoseconds. They were able to knock a single electron out of the outer shell
of the molecule and detect the scattered signal of the electron as it recollided
with the molecule.
DiMauro and Ohio
State postdoctoral
researcher Cosmin Blaga likened the scattered electron signal to the
diffraction pattern that light forms when it passes through slits. Given only
the diffraction pattern, scientists can reconstruct the size and shape of the
slits. In this case, given the diffraction pattern of the electron, the
physicists reconstructed the size and shape of the molecule—that is, the
locations of the constituent atoms’ nuclei.
The key, explained Blaga, is that during the brief span of time between when
the electron is knocked out of the molecule and when it re-collides, the atoms
in the molecules have moved. The LIED method can capture this movement, “similar to making a movie of the quantum world,” he added.
Beyond its potential for controlling chemical reactions, the technique
offers a new tool to study the structure and dynamics of matter, he said. “Ultimately, we want to really understand how chemical reactions take place. So,
long-term, there would be applications in materials science and even chemical
manufacturing.”
“You could use this to study individual atoms,” DiMauro added, “but the
greater impact to science will come when we can study reactions between more
complex molecules. Looking at two atoms—that’s a long way from studying a more
interesting molecule like a protein.”
