Scientists capture the first moments of chemical change after molecular ionisation

Using one of the world’s most advanced X-ray lasers, an international team of researchers has observed electron dynamics on attosecond timescales.

Researchers have observed the molecular response to sudden ionisation with unprecedented temporal resolution, revealing how electrons move and how their quantum states decay during the first few femtoseconds after a molecule loses an electron.

Using pairs of attosecond X-ray pulses, the team, which includes researchers from Imperial’s Department of Physics, tracked the behaviour of an ionised molecule from less than 300 attoseconds after ionisation to around 10 femtoseconds later.

Published today in Nature Physics, the findings provide one of the clearest views yet of the electronic processes that precede chemical change.

Capturing the earliest stages of chemistry

When a molecule suddenly loses an electron, the remaining electrons do not simply settle into a new arrangement. Instead, they are thrown into a highly excited quantum state, triggering ultrafast processes that can ultimately influence how chemical bonds break and form.

Understanding how these fast electronic rearrangements unfold has been a longstanding challenge.

Similar processes occur whenever molecules are exposed to high-energy radiation, including X-rays, energetic electrons and cosmic rays, but until now scientists have lacked the tools to directly observe the initial electron dynamics that drive these changes.

To observe these processes directly, the researchers used a pair of attosecond X-ray pulses from the Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory. Electrons move on attosecond timescales, making such ultrashort pulses essential for capturing the molecule’s response almost immediately after ionisation.

The first pulse removed an electron from an aminophenol molecule, while the second pulse probed how the resulting electronic state evolved over time.

The measurements revealed three distinct stages in the molecule’s response. First, some highly excited electronic states decayed within less than a femtosecond through a process known as Coster-Kronig decay, in which excess energy causes the molecule to rapidly eject a second electron.

Researchers then observed coherent electron motion as the “hole” left behind by the missing electron moved across the molecule. Finally, after several femtoseconds, the atoms themselves began to respond, causing these electronic oscillations to gradually fade. This marked the point at which electronic and atomic motion became coupled, a key step in the process that eventually drives chemical change.  

Tracking electron motion in real time

The findings provide a rare glimpse of the ultrafast electronic processes that unfold before molecular transformations begin.

By comparing their measurements with advanced simulations, the researchers were able to identify the different processes contributing to the evolving signal and track coherent electron motion within the molecule.  

The study also revealed differences between experimental observations and theoretical predictions, providing valuable data for improving future models of molecular behaviour.

Together, these observations provide new insights into the fundamental processes that occur when molecules are suddenly ionised.

As a result, the work has implications for fields ranging from astrochemistry and atmospheric science to studies of radiation-driven processes in biological molecules.

Imperial researchers played a key role in the study. The methodology behind the measurement was first proposed and demonstrated by Professor Jon Marangos and Professor Vitali Averbukh.  

Professor Averbukh and Dr Marco Ruberti developed one of the core theoretical tools used to understand the results, while Professor Marangos and Dr Oliver Alexander developed experimental and data-analysis methods that were crucial to the measurements.

Looking ahead

The work forms part of a long-running collaboration between Imperial researchers and scientists at SLAC to develop attosecond X-ray techniques capable of observing matter on its natural timescale.

The measurements were carried out using the LCLS X-ray free-electron laser, one of only a handful of facilities worldwide capable of producing the ultrashort pulses needed for these experiments.

Since the data for the current study were collected, the facility has been upgraded to operate at much higher pulse rates, allowing scientists to gather significantly larger datasets and explore increasingly complex systems.

Researchers are already extending the technique to other molecules, including amino acids, to build a deeper understanding of how sudden electronic excitation drives molecular dynamics.

By revealing what happens in the first few femtoseconds after ionisation, the team hopes to provide new insights into the fundamental mechanisms through which electronic motion shapes the behaviour of matter.

Dr Oliver Alexander said, "This work allows us to understand nature on the natural timescale of quantum mechanics and how those quantum processes shape the world we observe.

“These measurements lay the foundation for future studies of increasingly complex systems, helping us understand how molecular structure influences behaviour on attosecond timescales. Ultimately, this could shed light on how matter responds to high-energy radiation in environments ranging from biological systems to interstellar space."