Ultrafast Electron Diffraction Shows Electron Dynamics During Bond Breaking
For many years, it was not known how chemistry works at the molecular-level, but recently scientists have been able to see electrons in action as molecules break apart. While chemistry books show how reactions start and end, they rarely show what happens in the middle. Seeing this is important because chemistry is controlled by quantum rules, which govern how electrons and atoms move.
Until now, watching these tiny movements directly was thought to be impossible because electrons and atoms move extremely fast. But a team at Shanghai Jiao Tong University has made a breakthrough. Using a very advanced technique called ultrafast electron diffraction (UED), they were able to watch both electrons and atomic nuclei move in real space as a molecule broke apart.
“Studying electron movements is very important for understanding fundamental physics and for research in materials and chemistry,” said Dao Xiang, the lead author of the study.
Tracking Electrons and Atoms
Normal diffraction techniques can track heavy atoms easily, but they struggle to detect the subtle shifts in electrons, which are the particles that drive chemical changes. To solve this, the researchers improved UED to an unprecedented level of precision in both space and time.
They studied ammonia (NH₃), a simple molecule with interesting electron behavior. First, they hit the ammonia molecules with a short laser pulse. This excited an electron on the nitrogen atom, moving it into a different orbital. This caused the molecule to bend like an umbrella, and one hydrogen atom to start separating from the nitrogen.
Next, the team sent an ultrafast pulse of high-energy electrons at the excited ammonia. These electrons scattered off both the atomic nuclei and the surrounding electrons. The resulting pattern, captured on a detector, contained hidden information about the location of electrons and atoms at each moment.
To make sense of this data, the researchers used a method called charge pair distribution function (CPDF) analysis. This allowed them to see three interactions at once: nucleus-to-nucleus, electron-to-nucleus, and electron-to-electron. This let them map how electrons moved and how hydrogen atoms shifted at the same time.
Their setup was sensitive enough to track hydrogen motion, which is usually very difficult because hydrogen scatters electrons weakly and moves extremely fast. The result was the first real-time, real-space picture of how electrons and atoms respond as ammonia breaks apart, something previous methods could not achieve.
Why This Matters
This study is a big step forward for chemistry and physics. It shows that ultrafast electron diffractionUED can detect changes in valence electrons, the ones responsible for chemical reactions, moving beyond old models that treated atoms as static. This could help scientists better understand reaction mechanisms, energy transfer, and quantum effects in molecules.
The technique has challenges, though, because electron signals are subtle and can be hidden by heavier atoms. The team plans to refine their method and apply it to more complex molecules in the future.
“Our next goal is to use this method on other molecular systems, showing that electron diffraction can reveal how valence electrons rearrange even in complex organic molecules,” Xiang said.
