A research team led by Argonne National Laboratory physicists has isolated the energetic movement of an electron while ‘freezing’ the motion of the much larger atom it orbits in a sample of liquid water.
Shuai et al. used a synchronized attosecond X-ray pulse pair (pictured pink and green here) from an X-ray free electron laser to study the energetic response of electrons (gold) in liquid water on attosecond time scale, while the hydrogen (white) and oxygen (red) atoms are ‘frozen’ in time. Image credit: Nathan Johnson, Pacific Northwest National Laboratory.
“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 study’s senior author Professor Linda Young, a researcher at Argonne National Laboratory.
Professor Young and her colleagues combined experiments and theory to reveal in real-time the consequences when ionizing radiation from an X-ray source hits matter.
Working on the time scales where the action happens will allow them to understand complex radiation-induced chemistry more deeply.
Indeed, the researchers initially came together to develop the tools needed to understand the effect of prolonged exposure to ionizing radiation on the chemicals found in nuclear waste.
“Attosecond time-resolved experiments are one of the flagship R&D developments at the Linac Coherent Light Source,” said study’s co-author Dr. Ago Marinelli, a researcher at the SLAC National Accelerator Laborator.
“It’s exciting to see these developments being applied to new kinds of experiments and taking attosecond science into new directions.”
The technique developed by the scientists — all X-ray attosecond transient absorption spectroscopy in liquids — allowed them to ‘watch’ electrons energized by X-rays as they move into an excited state, all before the bulkier atomic nucleus has time to move.
“We now have a tool where, in principle, you can follow the movement of electrons and see newly ionized molecules as they’re formed in real-time,” Professor Young said.
The findings resolve a long-standing scientific debate about whether X-ray signals seen in previous experiments are the result of different structural shapes, or motifs, of water or hydrogen atom dynamics.
These experiments demonstrate conclusively that those signals are not 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,” Professor Young explained.
“We were able to eliminate that movement by doing all of our recording before the atoms had time to move.”
To make the discovery, the authors used techniques developed at SLAC to spray an ultra-thin sheet of pure water across the X-ray pump pulse path.
“We needed a nice, flat, thin sheet of water where we could focus the X-rays,” said study’s co-author Dr. Emily Nienhuis, a chemist at Pacific Northwest National Laboratory.
Once the X-ray data had been collected, the researchers applied their knowledge of interpreting the X-ray signals to reproduce the signals observed at SLAC.
They modeled 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, we developed a cutting-edge computational chemistry technique that enabled detailed characterization of the transient high-energy quantum states in water,” said study’s co-author Xiaosong Li, a researcher at the University of Washington and Pacific Northwest National Laboratory.
“This methodological breakthrough yielded a pivotal advancement in the quantum-level understanding of ultrafast chemical transformation, with exceptional accuracy and atomic-level detail.”
Together, the team got a peek at the real-time motion of electrons in liquid water.
“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,” Professor Young said.
The team’s results were published in the journal Science.
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L. Shuai et al. 2024. Attosecond-pump attosecond-probe x-ray spectroscopy of liquid water. Science, published online February 15, 2024; doi: 10.1126/science.adn6059
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