Ultrafast Laser Technique Reveals Molecular Interactions in Liquids

Scientists from Ohio State University and Louisiana State University demonstrated a new method for observing ultrafast molecular behavior in liquids using high-harmonic spectroscopy with attosecond laser pulses. This research provides a way to directly study how molecules interact in solution at extremely short timescales.

The team addressed two main obstacles in applying high-harmonic spectroscopy to liquids. They used an ultrathin sheet of liquid to minimize absorption and allow harmonic light to escape, overcoming both strong absorption and rapid molecular motion that typically hinder such measurements.

Experiments focused on mixtures of methanol and various halobenzenes. Among the tested combinations, only the pairing of fluorobenzene and methanol resulted in a marked decrease in emitted light and the complete suppression of a specific harmonic signal.

Simulations conducted by the researchers indicated that fluorobenzene and methanol form a structured solvation arrangement through hydrogen bonding, which they described as a 'molecular handshake.' This interaction was not observed with other halobenzenes in the study.

What the numbers show

• Attosecond-scale laser pulses were used for measurements
• Findings published in PNAS in 2025
• Fluorobenzene–methanol mixture showed full suppression of a specific harmonic

Theoretical models developed as part of the study showed that the unique solvation structure between fluorobenzene and methanol creates an electron-scattering barrier. This barrier disrupts the process of harmonic generation, accounting for both the missing harmonic and the reduced overall light emission seen in the experiments.

The work represents the first time that solution-phase high-harmonic generation has been used to reveal local solute-solvent interactions in liquids. This approach allows researchers to probe the dynamics of molecules in liquid environments with unprecedented temporal resolution.

The results were published in the Proceedings of the National Academy of Sciences in 2025. The publication details both the experimental techniques and the theoretical analysis that led to these findings.

By demonstrating that high-harmonic spectroscopy can provide insights into the structure and dynamics of molecules in liquids, the study opens new possibilities for investigating chemical processes in solution. The method may be applied to a range of systems where understanding ultrafast molecular interactions is important.

* This article is based on publicly available information at the time of writing.