By Owais AliReviewed by Lexie CornerMar 5 2024In a recent study published in Science, researchers have directly observed the real-time motion of electrons in water using intense X-Ray laser pulses. They achieved this by isolating the electronic response with precisely timed attosecond pulses, preventing molecular framework movement during experimentation.
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Monitoring the rapid movement of electrons in molecular systems following ionization or excitation has been a key objective in chemistry and physics. However, tracking ultrafast electron dynamics on the attosecond timescale (10-18 seconds) poses a significant challenge, as conventional laser-based spectroscopy is limited to femtosecond (10-15 s) resolution.
The X-Ray attosecond transient absorption spectroscopy (AX-ATAS), operating on the attosecond timescale, overcomes this limitation and enables the separation of electronic responses from slower nuclear motions that previously obscured observation.
"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," says Linda Young, Senior author of the study and Distinguished Fellow at the U.S. Department of Energy's Argonne National Laboratory.
All-X-Ray Attosecond Pump-Probe Spectroscopy
The researchers conducted these experiments at the Linac Coherent Light Source (LCLS) in the SLAC National Accelerator Laboratory.
LCLS generates extremely short (approximately 10-100 attosecond), intense X-Ray free electron laser (XFEL) pulses via acceleration of electron bunches to high energy, followed by rapid deceleration upon injection into arrays of alternating magnets called undulators. This forces the electrons to emit synchronized X-Ray photons that can be focused down to micron spots on a sample.
One significant innovation was the generation of precisely timed pairs of attosecond X-Ray pulses using a specialized double undulator configuration at LCLS. This enabled the researchers to implement an "all X-Ray" attosecond pump-probe spectroscopy scheme.
In this setup, the first pump pulse ionized a liquid water sample, while the second probe pulse measured the induced electronic structural changes 700 attoseconds later, minimizing distortion from nuclear dynamics. This configuration overcomes limitations associated with coupling ultrafast optical and X-Ray pulses for attosecond spectroscopy.
With advanced jet technology to flow a thin sheet of pure liquid water across the approximately 10 μm diameter XFEL beam focus, the stage was set to visualize electron processes in an aqueous system faster than ever.
Real-Time Tracking of High-Energy Electron States in Water
When the intense XFEL pump pulse removes (ionizes) a core 1s electron from a water molecule, the created hole can be filled rapidly by electrons descending from the outer valence orbitals on a sub-femtosecond timescale.
This electronic relaxation initiates a characteristic sequence of transient intermediate states with holes (unoccupied orbitals) in various valence locations. By tuning the XFEL probe pulse energy to core-level absorptions sensitive to these valence vacancies, the researchers could visually track the hole migration among valence states populated within the first few hundred attoseconds following the ionization of water.
The researchers also observed that the distinct double-peak feature of the 1b1 absorption signal (fingerprinting population of 1b1 valence holes), which vanishes within approximately 700 attoseconds after ionization, does not reflect two distinct structural motifs of liquid water at equilibrium—contrary to previous interpretations.
Instead, their findings demonstrate that this transient spectral signature arises from ultrafast distortion of the 1b1 orbital itself, induced by sub-femtosecond electron dynamics initiated by ionization.
Reshaping Our Understanding of Water Structure
This discovery has far-reaching implications as it resolves a decade-long controversy regarding the origin of the split 1b1 peak universally observed in X-Ray measurements of liquid water. Previously, the distinct low and high-energy components of this signal were alternatively interpreted as evidence for two different hydrogen-bonded structural motifs intrinsically existing in ambient water, in contrast to simpler models picturing a homogeneous network of molecules.
However, hydrogen atom motions occur on similar timescales of a few hundred femtoseconds, which could also cause the split peak shape without requiring distinct structural species. Previous experiments could not distinguish between these pictures.
The sub-femtosecond temporal resolution achievable with AX-ATAS allowed the researchers to unambiguously demonstrate that the dual 1b1 component arises from ultrafast dynamical distortion rather than intrinsically heterogeneous structural motifs. This represents a significant advancement in the field.
"Basically, what people were seeing in previous experiments was the blur caused by moving hydrogen atoms. We were able to eliminate that movement by doing all of our recording before the atoms had time to move," says Linda Young.
By exploiting the unprecedented capability of XFELs to outpace nuclear motion, this work illustrates how tracking purely electronic phenomena can paradoxically provide new insights into the arrangement of atoms at equilibrium.
Conclusion
The study establishes AX-ATAS as a groundbreaking technique for researching high-energy quantum states and electron dynamics activated by ionizing radiation. Initially tested on liquid water, the method holds potential for application in diverse systems, broadening insights into attosecond-scale electronic dynamics.
"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," explains Linda Young.
Ongoing AX-ATAS developments will broaden its application to diverse materials, enhancing precision and efficiency to reveal fundamental principles of electron dynamics and correlation during chemical transformations on an attosecond scale in complex systems.
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References and Further Reading
Li, S., et al. (2024). Attosecond-pump attosecond-probe x-ray spectroscopy of liquid water. Science. doi.org/10.1126/science.adn6059
Pacific Northwest National Laboratory. (2024). Scientists report first look at electrons moving in real-time in liquid water. [Online] Pacific Northwest National Laboratory. Available at: https://phys.org/news/2024-02-scientists-electrons-real-liquid.html
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