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The experiment captured events occurring on attosecond timescales.
A groundbreaking experiment at SLAC National Accelerator Laboratory has allowed scientists to watch electrons move inside a molecule at an astonishingly small timescale.
The experiment reveals how the earliest stages of a chemical reaction unfold.

For decades, scientists have understood that electrons play a central role in chemical reactions. When chemical bonds break and new ones form, electrons are ultimately responsible for driving these transformations. But observing exactly what electrons do in the very first moments of a reaction has been extraordinarily difficult.

Now, researchers at the U.S. Department of Energy’s SLAC National Accelerator Laboratory have taken a major step toward solving that problem. Using an advanced X-ray laser, the team created a kind of molecular “movie” showing electron motion during the opening stages of a chemical reaction.
The experiment captured events occurring on attosecond timescales. An attosecond is one billionth of one billionth of a second—an almost unimaginably short period of time. Yet within these fleeting intervals, important changes inside a molecule can already begin.

“By improving these models and testing them against real-world experiments, we can better understand how electrons drive chemical reactions and, we hope one day, gain the ability to better predict and control those reactions,” said Taran Driver, SLAC lead scientist and author of the paper.  

Chemical reactions may appear instantaneous to us, but at the atomic level they involve a sequence of extremely rapid events. The researchers studied what happens when a molecule is suddenly stripped of one of its electrons. This process, known as impulsive ionization, places the remaining electrons into highly excited quantum states.
The team then used SLAC’s Linac Coherent Light Source (LCLS) X-ray free-electron laser to take snapshots of what happened next. The technique involved two precisely timed X-ray pulses. The first pulse triggered the process by removing an electron from the molecule. A second pulse arrived after a carefully controlled delay and allowed researchers to determine how the electrons had moved.

By changing the delay between the two X-ray flashes, scientists could effectively reconstruct a frame-by-frame sequence of molecular events. The experiment produced 10 timestamps during the first 10 femtoseconds of the reaction. A femtosecond is longer than an attosecond, but it is still only a millionth of a billionth of a second.

Together, these snapshots created something remarkably similar to a slow-motion movie of a chemical reaction. One of the most significant observations occurred within the first femtosecond.

After the initial X-ray pulse removed an electron, the molecule rapidly relaxed by ejecting another, lower-energy electron from one of its inner electron shells. Scientists call this process Coster-Kronig decay.

Although the phenomenon has been known theoretically, researchers had never before captured its evolution in real time on its natural timescale. This matters because the low-energy electrons produced during such processes can interact with surrounding molecules. In biological systems, for example, these electrons can contribute to radiation damage and can even play a role in breaking DNA strands.

Being able to observe the process directly gives scientists a much clearer picture of how energy moves through matter following high-energy radiation. The experiment revealed another remarkable phenomenon over the following few femtoseconds.

When the original electron was removed, it left behind what scientists describe as an electron hole—essentially a missing electron in the molecule. Instead of remaining in one place, this hole migrated through the molecule before another electron eventually filled it.

Researchers believe this motion was driven by quantum coherence, a phenomenon in which quantum states maintain a well-defined relationship with one another.

The observation is particularly important because this fleeting electronic motion may influence what happens later in the reaction, including the breaking and formation of chemical bonds.

In other words, the researchers were able to observe the electronic events that occur before conventional chemistry becomes visible at the level of changing molecular bonds. The experiment showed that after roughly 10 femtoseconds, the consequences of the earlier electron movements began to appear in the molecular structure.

Prabhat, an alumnus of the Indian Institute of Mass Communication, is a tech and defense journalist. While he enjoys writing on modern weapons and emerging tech, he has also reported on global politics and business. He has been previously associated with well-known media houses, including the International Business Times (Singapore Edition) and ANI.
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