A missing flash of light revealed a molecular secret
Scientists have found a way to see ultrafast molecular interactions inside liquids using an extreme laser technique once thought impossible for fluids. When they mixed nearly identical chemicals, one combination behaved strangely—producing less light and erasing a single harmonic signal altogether. Simulations revealed that a subtle molecular “handshake” was interfering with electron motion. The discovery shows that liquids can briefly organize in ways that dramatically change how electrons behave.
Liquids and solutions may seem simple, but at the molecular level they are constantly in motion. When sugar dissolves in water, for example, each sugar molecule is quickly surrounded by shifting clusters of water molecules. Inside living cells, the situation becomes even more intricate. Tiny liquid droplets ferry proteins or RNA and help organize many of the cell's chemical reactions.
Despite their central role in biology and chemistry, liquids have long resisted close inspection. Unlike solids, they have no fixed structure, and the most important interactions between dissolved molecules and their surroundings happen at extreme speeds. These ultrafast events, where chemistry truly unfolds, have largely remained out of reach for scientists.
A New Way to See Ultrafast Chemistry in Liquids
Researchers from Ohio State University and Louisiana State University have now demonstrated that high-harmonic spectroscopy (HHS) can expose hidden molecular structures inside liquids. This nonlinear optical technique is capable of tracking electron motion on attosecond timescales. The work, published in PNAS, shows that HHS can directly probe solute-solvent interactions in liquid solutions, something that had not been possible before.
HHS uses extremely short laser pulses to momentarily pull electrons away from molecules. When those electrons snap back, they emit light that carries detailed information about how electrons and even atomic nuclei move. These snapshots occur on timescales far faster than conventional methods can resolve. Traditional optical spectroscopy has been widely used to study liquids because it is gentle and easy to interpret, but it operates much more slowly. HHS, on the other hand, reaches into the extreme-ultraviolet range and can resolve events lasting just an attosecond, a billionth of a billionth of a second.
Overcoming the Challenges of Studying Liquids
Until now, HHS experiments were mostly limited to gases and solids, where conditions are easier to control. Liquids present two major obstacles. They absorb much of the harmonic light that is produced, and their constantly moving molecules make the resulting signals difficult to analyze.
To address these issues, the OSU-LSU team developed an ultrathin liquid "sheet" that allows more of the emitted light to escape. Using this approach, they showed for the first time that HHS can capture rapid molecular dynamics and subtle structural changes in liquids.
A Surprising Result from Simple Liquid Mixtures
With this new setup, the researchers tested how HHS behaves in straightforward liquid mixtures. They shined intense mid-infrared laser light on methanol combined with small amounts of halobenzenes. These molecules are nearly identical, differing only by a single atom: fluorine, chlorine, bromine, or iodine. Halobenzenes produce strong harmonic signals that stand out clearly, while methanol provides a relatively clean background. The expectation was that even when present in low concentrations, the halobenzene signal would dominate.