Louis F. DiMauro, professor of physics at Ohio State University, leads a research effort that is challenging long-standing assumptions about how liquids behave at the smallest scales. In a recent study conducted with collaborators at Louisiana State University, his team demonstrated that ultrafast laser techniques can reveal detailed information about molecular structure in liquid solutions, including interactions that occur on timescales far too short for conventional measurements. The work focuses on how electrons move through liquids and how those motions are shaped by local chemical environments.
Moore, E., Giri, S., Koutsogiannis, A., Alavi, T., McCracken, G., Lopata, K., Herbert, J. M., Gaarde, M. B., & DiMauro, L. F. (2025). Solvation-induced local structure in liquids probed by high-harmonic spectroscopy. Proceedings of the National Academy of Sciences, 122(48). https://doi.org/10.1073/pnas.2514825122
Liquids are often treated as disordered systems whose microscopic details average out over time. In reality, molecules in a liquid are constantly rearranging, forming brief associations and responding to their surroundings. These fast, local interactions play a critical role in chemistry and biology, but they are difficult to observe directly. The challenge lies in the fact that the most relevant processes occur on attosecond timescales, while many experimental tools operate orders of magnitude more slowly.
Louis F. DiMauro, professor of physics at Ohio State University stated,
“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.”
To access this regime, the researchers used high-harmonic spectroscopy, a nonlinear optical technique that tracks electron motion by driving electrons away from a molecule with an intense laser pulse and then observing the light emitted when the electrons return. This emitted light contains harmonics of the original laser frequency and reflects the dynamics of the electron during its brief excursion. High-harmonic spectroscopy has been widely applied in gases and solids, but extending it to liquids has proven difficult due to strong absorption and structural disorder.
The team addressed these limitations by generating an ultrathin sheet of liquid, thin enough to allow the emitted high-energy light to escape before being absorbed. This approach made it possible to collect clear harmonic spectra from liquid samples. With this setup, the researchers investigated mixtures of methanol with small concentrations of halobenzenes, a family of molecules that differ only in the halogen atom they contain. These systems were chosen to minimize chemical complexity while allowing subtle interaction effects to stand out.
For most of the mixtures, the harmonic signals behaved as expected, closely resembling a simple combination of the signals from the individual components. One case, however, showed a striking deviation. When fluorobenzene was mixed with methanol, the overall harmonic signal was reduced, and one specific harmonic frequency disappeared entirely. This selective suppression indicated destructive interference in the harmonic generation process and suggested that a particular molecular interaction was strongly influencing electron motion.
To understand the origin of this effect, the experimental results were paired with detailed molecular dynamics simulations. These simulations revealed that fluorobenzene interacts with methanol differently than the other halobenzenes. The highly electronegative fluorine atom tends to align with the hydroxyl group of methanol, creating a more ordered local solvation structure. This arrangement alters the electronic environment experienced by an electron during the laser interaction.
Further theoretical modeling showed that the electron density associated with the fluorine atom effectively acts as a scattering barrier. As electrons accelerate under the laser field, this barrier disrupts their trajectories and interferes with their recombination, leading to both a reduction in overall harmonic yield and the complete suppression of a single harmonic. The calculations also indicated that the effect is highly sensitive to the precise location of this barrier, meaning the missing harmonic encodes information about the local liquid structure.
The findings suggest that high-harmonic spectroscopy can serve as a probe of short-range order in liquids, rather than merely averaging over their disorder. This has implications beyond the specific chemical system studied. Many processes relevant to chemistry, biology, and materials science take place in liquid environments and involve electrons with comparable energies, including radiation damage and charge transport in soft matter.
Although the technique is still at an early stage, the study shows that combining ultrafast experiments with advanced simulations can uncover structural details in liquids that were previously inaccessible. Rather than being a limitation, the disappearance of a harmonic becomes a measurable signal tied directly to molecular organization. As experimental methods and theoretical models continue to improve, this approach could offer engineers and scientists a new way to study complex liquid systems with both temporal and structural precision.
Adrian graduated with a Masters Degree (1st Class Honours) in Chemical Engineering from Chester University along with Harris. His master’s research aimed to develop a standardadised clean water oxygenation transfer procedure to test bubble diffusers that are currently used in the wastewater industry commercial market. He has also undergone placments in both US and China primarely focused within the R&D department and is an associate member of the Institute of Chemical Engineers (IChemE).
