Researchers from Ohio State University and Louisiana State University have pioneered a technique to observe ultrafast molecular interactions in liquids using high-harmonic spectroscopy. In a surprising experiment with fluorobenzene and methanol, they discovered a subtle hydrogen bond that suppresses light emission. This breakthrough, published in PNAS, opens new windows into liquid dynamics essential for chemistry and biology.
Liquids play a crucial role in biological and chemical processes, yet their molecular behaviors have been hard to observe due to constant motion and ultrafast interactions. Traditional methods like optical spectroscopy are too slow to capture these events, which occur on attosecond timescales—a billionth of a billionth of a second.
A team from Ohio State University (OSU) and Louisiana State University (LSU) has changed that by adapting high-harmonic spectroscopy (HHS), a nonlinear optical technique previously limited to gases and solids. HHS employs intense, short laser pulses to ionize molecules, releasing electrons that recombine and emit light revealing electron and nuclear movements. To overcome liquids' challenges—such as light absorption and signal complexity—the researchers created an ultrathin liquid sheet that lets more harmonic light escape for detection.
Testing simple mixtures, they combined methanol with halobenzenes, molecules differing only in one halogen atom: fluorine, chlorine, bromine, or iodine. Most mixtures produced expected harmonic signals, blending the components' emissions. However, the fluorobenzene-methanol solution behaved differently, yielding less light overall and completely suppressing one harmonic.
"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."
Simulations explained this as a 'molecular handshake'—a hydrogen bond between fluorobenzene's fluorine and methanol's oxygen-hydrogen group, driven by fluorine's electronegativity. This organized structure creates an electron-scattering barrier, interfering with harmonic generation. "We found that the PhF-methanol mixture is subtly different from the others," noted John Herbert, professor of chemistry at OSU. The LSU team confirmed this via time-dependent Schrödinger equation models, showing the barrier's position affects suppression patterns, providing insights into local solvation structures.
"We were excited to be able to combine results from experiment and theory, across physics, chemistry, and optics, to learn something new about electron dynamics in the complex liquid environment," said Mette Gaarde, Boyd Professor of Physics at LSU.
This advance could illuminate processes in cells, radiation damage, and materials, with HHS now sensitive to solute-solvent interactions. Funded by the DOE and NSF, the study appears in Proceedings of the National Academy of Sciences (2025).
