For the first time, scientists have used ultrafast X-ray flashes to take a direct image of a single electron as it moved during a chemical reaction.
In the new study, published Aug. 20 in the journal Physical Review Letters, the researchers accomplished this incredible feat by imaging how a valence electron — an electron in the outer shell of an atom — moved when an ammonia molecule broke apart.
For decades, scientists have used ultrafast X-ray scattering to image atoms and their chemical reactions. The scattering uses supershort bursts of X-rays to freeze tiny, fast-moving molecules in action. X-rays have the perfect wavelength range for capturing details at the atomic scale, which is why they’re ideal for imaging molecules.
However, X-rays interact strongly only with core electrons near the atom’s nucleus. Valence electrons — the outermost electrons in an atom and the ones actually responsible for the chemical reactions — were hidden.
“We wanted to take pictures of the actual electrons that are driving that motion,” Ian Gabalski, a physics doctoral student and lead author of the study, told Live Science.
If scientists can understand how valence electrons move during chemical reactions, it could help them design better drugs, cleaner chemical processes, and more efficient materials, Gabalski said.
To get started, the team needed to find the right molecule. It turned out to be ammonia.
“Ammonia is kind of special,” Gabalski said. “Because it has mostly light atoms, there aren’t a lot of core electrons to drown out the signal from the outer ones. So we had a shot at actually seeing that valence electron.”
The experiment was conducted at the SLAC National Accelerator Laboratory’s Linac Coherent Light Source, a facility that produces intense, short X-ray pulses. First, the team gave the ammonia molecule a tiny jolt of ultraviolet light, which made one of the electrons “jump” to a higher energy level. Electrons in molecules usually stay in low-energy states, and if they are pushed to a higher one, it triggers a chemical reaction. Then, with the X-ray beam, the researchers recorded how the electron’s “cloud” shifted as the molecule began to break apart.
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In quantum physics, electrons aren’t seen as tiny balls orbiting the nucleus. Instead, they exist as probability clouds, “where higher density means you’re more likely to see the electron,” Gabalski explained. These clouds are also known as orbitals, and each one has a distinct shape depending on the energy and position of the electron.
To map this electron cloud, the team ran quantum mechanical simulations to calculate the molecule’s electronic structure. “So now this program that we use for these kinds of calculations goes and it figures out where the electrons are filling up those orbitals around the molecule,” Gabalski said.
The X-rays themselves act like waves, and when they pass through the electron’s probability cloud, they scatter in different directions. “But then those X-rays can go and interfere with each other,” Gabalski said. By measuring this interference pattern, the team reconstructed an image of the electron’s orbital and saw how the electron moved during the reaction.
They compared the results to two theoretical models: one that included valence electron motion, and one that didn’t. The data matched the first model, confirming that they had captured the electron’s rearrangement in action.
The researchers hope to adapt the system for use in more complex, 3D environments that better mimic real tissues. That would move it closer to applications in regenerative medicine, such as growing or repairing tissue on demand.