Scientists report first glimpse of electrons moving in real time in liquid water

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

The results, reported in the journal Scienceopen a new window on the electronic structure of molecules in the liquid phase on a time scale previously inaccessible with X-rays. The new technique reveals the immediate electronic response when a target is struck by an X-ray, an important step in understanding the effects of radiation exposure on objects and people.

“The radiation-induced chemical reactions we want to study are the result of the electronic response of the target that occurs on the attosecond time scale,” said Linda Young, lead author of the research and distinguished member of the Laboratory. Argonne National.

“Until now, radiochemists could only resolve events on the scale of a picosecond, a million times slower than an attosecond. It’s a bit like saying ‘I was born and then I died. ‘ You would like to know what happens between the two. That’s what we can now do.

A multi-institutional group of scientists from several Department of Energy national laboratories and universities in the United States and Germany combined experiments and theory to reveal in real time the consequences when ionizing radiation from an X-ray source strikes the material.

Working on the time scales at which the action occurs will allow the research team to better understand the complex chemistry induced by radiation. Indeed, these researchers initially came together to develop the tools needed to understand the effect of prolonged exposure to ionizing radiation on chemicals found in nuclear waste.

“Our early career network members participated in the experiment and then joined our full experimental and theoretical teams to analyze and understand the data,” said Carolyn Pearce, IDREAM EFRC director and PNNL chemist. “We could not have achieved this without the IDREAM partnerships.”

From the Nobel Prize to the field

Subatomic particles move so quickly that capturing their actions requires a probe capable of measuring time in attoseconds, a span of time so short that there are more attoseconds per second than there have been seconds in the history of the universe.

The current investigation builds on the new science of attosecond physics, rewarded by the 2023 Nobel Prize in Physics. Attosecond X-ray pulses are available in only a handful of specialized facilities around the world. This research team conducted their experimental work at the Linac Coherent Light Source (LCLS), located at the SLAC National Accelerator Laboratory in Menlo Park, California, where the local team pioneered the development of lasers at free electrons at attosecond X-rays.

“The time-resolved attosecond experiments are one of the flagship R&D developments at the Linac Coherent Light Source,” said Ago Marinelli of the SLAC National Accelerator Laboratory, who with James Cryan led the development of the synchronized pair of attosecond X-ray pump/probe pulses used in this experiment. “It’s exciting to see these developments applied to new types of experiments and to take attosecond science in new directions.”

The technique developed in this study, all attosecond transient absorption spectroscopy of X-rays in liquids, allowed them to “monitor” electrons excited by the X-rays as they move toward an excited state, all before before the largest atomic nucleus has time to move. They chose liquid water as a test case for an experiment.

“We now have a tool that, in principle, allows you to track the movement of electrons and see newly ionized molecules as they form in real time,” said Young, also a professor in the Department of physics and James Franck. University of Chicago Institute.

These newly reported findings resolve a long-standing scientific debate over whether the X-ray signals observed in previous experiments are the result of different structural shapes, or “patterns,” in the dynamics of water atoms or of hydrogen. These experiments demonstrate conclusively that these signals are not evidence of two structural motifs in ambient liquid water.

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

From simple to complex reactions

The researchers view the current study as the start of an entirely new direction for attosecond science.

To make this discovery, PNNL experimental chemists teamed up with physicists from Argonne and the University of Chicago, X-ray spectroscopy specialists and accelerator physicists from SLAC, theoretical chemists from the University of Washington and attosecond science theorists from the Center for Ultrafast Imaging in Hamburg. the Center for Free Electron Laser Science (CFEL), Deutsches Elektronen-Synchrotron (DESY), Hamburg, Germany.

During the global pandemic, in 2021 and 2022, the PNNL team used techniques developed at SLAC to spray an ultra-thin sheet of pure water onto the X-ray pump’s pulse path.

“We needed a nice, flat, thin sheet of water where we could focus the X-rays,” said Emily Nienhuis, an early-career chemist at PNNL who started the project as a postdoctoral research associate. . “This capability was developed at LCLS.” At PNNL, Nienhuis demonstrated that this technique can also be used to study the specific concentrated solutions that are at the heart of the IDREAM EFRC and will be investigated in the next stage of the research.

From experience to theory

Once the radiological data was collected, theoretical chemist Xiaosong Li and graduate student Lixin Lu of the University of Washington applied their knowledge of interpreting radiological signals to reproduce the signals observed at SLAC. The CFEL team, led by theorist Robin Santra, modeled the response of liquid water to attosecond X-rays to verify that the observed signal was indeed confined to the attosecond time scale.

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

“This methodological breakthrough has resulted in a crucial advance in the quantum understanding of ultrafast chemical transformation, with exceptional precision and atomic-level detail.”

Principal investigator Young initiated the study and oversaw its execution, led onsite by first author and postdoctoral fellow Shuai Li. Physicist Gilles Doumy, also of Argonne, and graduate student Kai Li of the University of Chicago were part of the team that conducted the experiments and analyzed the data. The Argonne Center for Nanoscale Materials, a DOE Office of Science user facility, helped characterize the water sheet jet target.

Together, the research team was able to observe the real-time movement of electrons in liquid water while the rest of the world was still.

“The methodology we have developed makes it possible to study the origin and evolution of reactive species produced by radiation-induced processes, such as those encountered in space travel, cancer treatments, nuclear reactors and waste. alumni,” Young said.