First-ever "freeze-frame" of electrons in water used the attosecond time scale

A recent study akin to the precision of stop-motion cinematography, scientists have achieved a monumental feat in the realm of chemistry and physics by capturing the swift movement of an electron, while simultaneously halting the motion of the larger atom it orbits within a liquid water sample.

This pioneering experiment unveils a novel perspective on the electronic structure of molecules in their liquid state, traversing time scales that were once deemed unreachable through conventional X-ray methods.

Time barriers: The attosecond breakthrough

The essence of this research lies in its ability to observe the immediate electronic reactions following the impact of an X-ray on a target. This observation is crucial for comprehending the ramifications of radiation exposure on various objects and, more importantly, on human health.

“The chemical reactions induced by radiation that we want to study are the result of the electronic response of the target that happens on the attosecond timescale,” said Linda Young, a senior author of the research and Distinguished Fellow at Argonne National Laboratory.

“Until now radiation chemists could only resolve events at the picosecond timescale, a million times slower than an attosecond. It’s kind of like saying ‘I was born and then I died.’ You’d like to know what happens in between. That’s what we are now able to do.”

This research was made possible through a collaborative effort involving scientists from several Department of Energy national laboratories and universities across the U.S. and Germany.

These researchers united their expertise in both experimental and theoretical domains to unravel the intricate processes that ensue when ionizing radiation from an X-ray source interacts with matter.

Concept to reality: The power of collaboration

One of the primary motivations behind this collective endeavor was to delve deeper into the complex chemistry induced by radiation, particularly concerning the long-term effects of ionizing radiation on the chemicals found in nuclear waste.

The project received support from the Interfacial Dynamics in Radioactive Environments and Materials (IDREAM) Energy Frontier Research Center, sponsored by the Department of Energy and based at Pacific Northwest National Laboratory (PNNL).

Carolyn Pearce, director of IDREAM EFRC and a chemist at PNNL, highlighted the integral role of collaboration in this achievement.

“Members of our early-career network participated in the experiment, and then joined our full experimental and theoretical teams to analyze and understand the data,” she stated. “We couldn’t have done this without the IDREAM partnerships.”

New lens on liquid life: The water experiment

The experimental technique employed, known as X-ray attosecond transient absorption spectroscopy in liquids, enables scientists to observe electrons being energized by X-rays and transitioning to an excited state before the atomic nucleus can react.

Liquid water was chosen for this experiment due to its fundamental importance in both biological and chemical processes.

This advancement not only resolves a longstanding scientific debate regarding the interpretation of X-ray signals in previous experiments but also opens new avenues in attosecond science — a field that has recently been recognized with the 2023 Nobel Prize in Physics.

Glimpse into atomic choreography

This domain of study requires the use of attosecond X-ray pulses, a resource available at only a few specialized facilities globally, including the Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory, where this experiment was conducted.

“Attosecond time-resolved experiments are one of the flagship R&D developments at the Linac Coherent Light Source,” said Ago Marinelli from the SLAC National Accelerator Laboratory.

Together with James Cryan, Marinelli led the development of the synchronized pair of X-ray attosecond pump/probe pulses that this experiment used.

“It’s exciting to see these developments being applied to new kinds of experiments and taking attosecond science into new directions,” Marinelli said.

Water and electrons launch a new era in attosecond science

The researchers see this study as the launchpad for an entirely new trajectory in attosecond science.

PNNL experimental chemists, Argonne and the University of Chicago physicists, SLAC’s X-ray spectroscopy experts and accelerator physicists, the University of Washington’s theoretical chemists, and attosecond science theorists from the Hamburg Centre for Ultrafast Imaging and the Center for Free-Electron Laser Science (CFEL) at Deutsches Elektronen-Synchrotron (DESY) in Hamburg, Germany, all collaborated to make this discovery.

Throughout the global pandemic, in 2021 and into 2022, the PNNL team employed techniques SLAC developed to spray an ultra-thin water sheet across the X-ray pump pulse path.

Emily Nienhuis, an early-career chemist at PNNL who initiated the project as a post-doctoral research associate, said, “We needed a nice, flat, thin sheet of water to focus the X-rays on. The LCLS developed this capability.”

Nienhuis also showed that PNNL could use this technique to study specific concentrated solutions critical to the IDREAM EFRC, which they will examine in the research’s next phase.

Deciphering the quantum code

After collecting the X-ray data, Xiaosong Li, a theoretical chemist, and Lixin Lu, a graduate student from the University of Washington, applied their expertise in interpreting X-ray signals to replicate the signals observed at SLAC.

The CFEL team, under the leadership of theoretician Robin Santra, modeled the liquid water’s response to attosecond X-rays, confirming that the signal was indeed within the attosecond timescale.

“Using the Hyak supercomputer at the University of Washington, we developed a cutting-edge computational chemistry technique that enabled detailed characterization of the transient high-energy quantum states in water,” said Li, the Larry R. Dalton Endowed Chair in Chemistry at the University of Washington and a Laboratory Fellow at PNNL.

“This methodological breakthrough yielded a pivotal advancement in the quantum-level understanding of ultrafast chemical transformation, with exceptional accuracy and atomic-level detail.”

Future of radiation research: From space to medicine

Principal Investigator Young conceptualized and oversaw the study, with on-site leadership by first author and postdoc Shuai Li.

Physicist Gilles Doumy from Argonne and Kai Li, a graduate student from the University of Chicago, were integral in conducting the experiments and analyzing the data.

The research team managed to capture a glimpse of electrons’ real-time movement in liquid water, offering a unique perspective while the rest of the world paused.

“The methodology we developed permits the study of the origin and evolution of reactive species produced by radiation-induced processes, such as encountered in space travel, cancer treatments, nuclear reactors and legacy waste,” said Young.

Future of electrons in attosecond motion

In summary, this incredible research into attosecond science represents a pivotal advancement, offering an unprecedented glimpse into the atomic and subatomic realms.

By capturing the swift movements of electrons in liquid water while effectively “freezing” the motion of their atomic nuclei, scientists have opened new doors for understanding the fundamental processes that govern chemical and physical reactions at the quantum level.

This collaborative effort, bridging the expertise of chemists, physicists, and theorists across the globe, resolves longstanding scientific debates and sets the stage for future innovations in fields ranging from radiation chemistry to medical treatments.

The methodology developed through this research promises to enhance our ability to study the immediate effects of radiation on matter, marking a significant leap forward in our quest to harness the potential of attosecond science for practical applications.

The full study was published in the journal Science.

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