First-ever atomic freeze-frame of liquid water

Scientists used a synchronized attosecond x-ray pulse pair (shown in pink and green) from an X-ray free electron laser to study the energetic response of electrons (gold) in liquid water on attosecond time scales, while the hydrogen (white) and oxygen (red) atoms are ‘frozen’ in time.Nathan Johnson/Pacific Northwest National Laboratory

In an experiment akin to stop-motion photography, an international team of scientists has isolated the energetic movement of an electron in a sample of liquid water – while “freezing” the motion of the much larger atom it orbits.

The finding reveals the immediate response of an electron when hit with an X-ray, an essential step in understanding the effects of radiation exposure on objects and people. The results, published Feb. 15 in the journal Science, provide a new window into the electronic structure of molecules in the liquid phase on a timescale previously unattainable with X-rays.

“What happens to an atom when it is struck by ionizing radiation, like an X-ray? Seeing the earliest stages of this process has long been a missing piece in understanding how radiation affects matter,” said co-senior author Xiaosong Li, the Larry R. Dalton Endowed Chair in Chemistry at the University of Washington and a laboratory fellow at the Pacific Northwest National Laboratory. “This new technique for the first time shows us that missing piece and opens the door to seeing the steps where so much complex – and interesting – chemistry occurs!”

Li co-led the team behind this breakthrough with co-senior authors Linda Young, a distinguished fellow at Argonne National Laboratory and professor at the University of Chicago, and Robin Santra, professor at the German Electron Synchrotron and the University of Hamburg. The team received critical funding and support from the Interfacial Dynamics in Radioactive Environments and Materials (IDREAM) partnership, a Department of Energy center headquartered at PNNL.

The collaboration used a combination of experiments and theoretical insights to see in real time what happens when ionizing radiation from an X-ray source hits matter. Revealing these moments is not as simple as snapping a photo. Subatomic particles move so fast that capturing their actions requires using a probe that can measure time in attoseconds. There are more attoseconds in a second than there have been seconds in the history of the universe.

“Until now radiation chemists could only resolve events at the picosecond timescale, a million times slower than an attosecond,” said Young. “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.”

The team – under the guidance of Young and co-lead author Shuai Li, a postdoctoral researcher at Argonne – set out to develop a whole new experimental approach to achieve attosecond resolution using X-rays. Attosecond X-ray pulses are only available in a handful of specialized facilities worldwide, so the team partnered with scientists at the SLAC National Accelerator Laboratory in California to use the facility’s Linac Coherent Light Source for developing attosecond X-ray free-electron lasers, with key input from scientists at PNNL.

To record the movement of electrons excited by X-ray radiation, scientists create a thin sheet of liquid water – approximately 1 cm wide – as a target for the X-ray beam.Emily Nienhuis/Pacific Northwest National Laboratory

The resulting technique, AX-ATAS – or all X-ray attosecond transient absorption spectroscopy – employed two delicate X-ray pulses: One to “excite” its target matter and one to probe how the excited matter responded. This approach would theoretically allow the scientists to “watch” electrons energized by X-rays as they move into an excited state, all before the bulkier atomic nucleus has time to move. They chose the liquid water as their test case for an experiment.

“And on our first experiment, it worked!” said Li. “But the signal we picked up in the data was ‘convoluted.’ It turns out that, in this transient snapshot, we were probing so many quantum states that we had to develop a completely new computational analysis method to understand the data.”

Quantum mechanical principles underlie the behavior of all matter, but its signatures are often hidden in experiments like these. But, using AX-ATAS at the attosecond timescale, the scientists were picking up quantum-level details – and needed new methods to make sense of the data.

To that end, Li, a theoretical chemist, worked with co-lead author Lixin Lu – who conducted this research as a UW doctoral student in chemistry and is now a postdoctoral researcher at Stanford University – to reproduce the signals observed at SLAC. The German Electron Synchrotron-based team under Santra and co-lead author Swarnendu Bhattacharyya, a postdoctoral researcher, modelled the liquid water response to attosecond X-rays to verify that the observed signal was indeed confined to 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, who is also UW Associate Vice Provost for research cyberinfrastructure. “This methodological breakthrough yielded a pivotal advancement in the quantum-level understanding of ultrafast chemical transformation, with exceptional accuracy and atomic-level detail.”

The team’s analysis resolved a long-standing scientific debate about whether X-ray signals seen in previous experiments are the result of hydrogen atom dynamics or different structural “motifs” of water. The experiments showed no evidence for two structural motifs in ambient liquid water.

“Basically, what people were seeing in previous experiments was the blur caused by moving hydrogen atoms,” said Young. “We were able to eliminate that movement by doing all of our recording before the atoms had time to move.”

The current investigation builds upon the new science of attosecond physics, recognized with the 2023 Nobel Prize in Physics. Working at the attosecond timescale will allow the researchers to understand complex radiation-induced chemistry at a fundamental level. This team initially came together to develop tools to understand the effect of prolonged exposure to ionizing radiation on the chemicals found in nuclear waste.

The researchers envision the current study as the beginning of a whole new direction for attosecond science.

“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.

Co-authors on the paper are Carolyn Pearce of PNNL and Washington State University; Kai Li of the University of Chicago and Argonne; Emily Nienhuis of PNNL; Giles Doumy and R.D. Schaller at Argonne; Ludger Inhester of the German Electron Synchrotron and the Hamburg Centre for Ultrafast Imaging; and S. Moeller, M.F. Lin, G. Dakovski, D.J. Hoffman, D. Garratt, Kirk Larsen, J.D. Koralek, C.Y. Hampton, D. Cesar, Joseph Duris, Z. Zhang, Nicholas Sudar, James Cryan and A. Marinelli at SLAC. The research was funded by the U.S. Department of Energy, the German Research Foundation and the German Electron Synchrotron.

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