In a groundbreaking study, researchers from Ohio State University and Louisiana State University have unveiled a new method to observe ultrafast chemical processes in liquids. Using high-harmonic spectroscopy (HHS), a nonlinear optical technique, scientists can now track electron motion on attosecond timescales, revealing hidden molecular structures within liquid solutions. This discovery, published in the Proceedings of the National Academy of Sciences (PNAS), marks a significant advancement in understanding solute-solvent interactions that were previously inaccessible.
Liquids, despite their apparent simplicity, are complex at the molecular level, with molecules in constant motion. When substances like sugar dissolve in water, each molecule is surrounded by shifting clusters of water molecules. Inside living cells, tiny liquid droplets play a crucial role in organizing chemical reactions. However, due to their lack of fixed structure and the rapid pace of molecular interactions, liquids have long eluded detailed scientific examination.
A New Way to See Ultrafast Chemistry in Liquids
HHS offers a new lens through which scientists can observe these fleeting interactions. By utilizing extremely short laser pulses, HHS temporarily displaces electrons from molecules. As these electrons return, they emit light that contains detailed information about the movement of electrons and atomic nuclei. Unlike traditional optical spectroscopy, which operates at slower speeds, HHS reaches into the extreme-ultraviolet range, capturing events that last just an attosecond—a billionth of a billionth of a second.
Until now, HHS experiments were primarily conducted on gases and solids, where conditions are more easily controlled. Liquids present unique challenges, such as absorbing much of the harmonic light produced and the difficulty in analyzing signals due to the constant motion of molecules. To overcome these hurdles, the research team developed an ultrathin liquid “sheet” that allows more emitted light to escape, enabling the capture of rapid molecular dynamics and subtle structural changes.
A Surprising Result from Simple Liquid Mixtures
The researchers tested their new setup on simple liquid mixtures, shining intense mid-infrared laser light on methanol combined with small amounts of halobenzenes. These molecules, nearly identical except for a single atom—fluorine, chlorine, bromine, or iodine—produce strong harmonic signals against methanol’s clean background. Surprisingly, the fluorobenzene (PhF)-methanol solution yielded unexpected results.
“We were really surprised to see that the PhF-methanol solution gave completely different results from the other solutions,” said Lou DiMauro, Edward E. and Sylvia Hagenlocker Professor of Physics at OSU. “Not only was the mixture-yield much lower than for each liquid on its own, we also found that one harmonic was completely suppressed.”
This selective loss of light, akin to silencing a single note in a spectrum, indicated a specific molecular interaction affecting electron motion. To understand this phenomenon, the OSU theory team conducted large-scale molecular dynamics simulations.
Simulations Reveal a Molecular Handshake
John Herbert, professor of chemistry and leader of the theory effort, explained that the PhF-methanol mixture is unique due to the electronegativity of the fluorine atom, which promotes a “molecular handshake” or hydrogen bond with methanol’s O-H end. This results in a more organized solvation structure compared to other halobenzenes.
The LSU theory group further explored whether this arrangement could explain the experimental findings. Mette Gaarde, Boyd Professor of Physics, noted that the electron density around fluorine atoms created an additional barrier for accelerating electrons, disrupting the harmonic generation process. This scattering barrier accounted for the missing harmonic and reduced overall light output.
“We also learned that the suppression was very sensitive to the location of the barrier—this means that the detail of the harmonic suppression carries information about the local structure that was formed during the solvation process,” added Sucharita Giri, postdoctoral researcher at LSU.
Why This Discovery Matters
The implications of this discovery are vast. Many critical chemical and biological processes occur in liquid environments, and understanding electron scattering in dense liquids could have significant impacts on fields like chemistry, biology, and materials science. The energies of these electrons are similar to those responsible for radiation damage, offering potential insights into mitigating such effects.
“Our results demonstrate that solution-phase high-harmonic generation can be sensitive to the particular solute-solvent interactions and therefore to the local liquid environment. We are excited for the future of this field,” remarked DiMauro.
As researchers continue to refine HHS techniques, they anticipate gaining even more detailed views of how liquids respond to ultrafast laser pulses. Key contributors to this study include Eric Moore, Andreas Koutsogiannis, Tahereh Alavi, and Greg McCracken from OSU, and Kenneth Lopata from LSU. The research was funded by the DOE Office of Science, Basic Energy Sciences, and the National Science Foundation.
