Researchers from Ohio State University and Louisiana State University have demonstrated the application of high-harmonic spectroscopy (HHS) to analyze molecular structures and dynamics within liquid solutions. This development addresses long-standing challenges in observing ultrafast chemical processes in liquids, enabling the direct probing of solute-solvent interactions on attosecond timescales. The findings, published in PNAS, utilized an ultrathin liquid sheet to overcome experimental hurdles previously limiting HHS to gases and solids.
Challenges in Liquid Research
Liquids and solutions are characterized by continuous molecular motion and a lack of fixed structure, making the observation of ultrafast chemical events difficult. Despite their significance in biological processes, such as protein and RNA transport within living cells, and in general chemistry, the speed of molecular interactions in liquids has historically posed a challenge for scientific study. Conventional optical spectroscopy, while widely used for liquids due to its gentle nature and ease of interpretation, operates at slower speeds.
Advancements with High-Harmonic Spectroscopy
The research team successfully applied high-harmonic spectroscopy (HHS), a nonlinear optical technique, to analyze molecular structures within liquid environments. HHS tracks electron motion at attosecond timescales, which are a billionth of a billionth of a second. The technique involves using extremely short laser pulses to temporarily displace electrons from molecules. As these electrons return, they emit light that contains information about the movement of both electrons and atomic nuclei. Operating in the extreme-ultraviolet range, HHS can resolve events at speeds faster than those resolvable by traditional methods. This application marks the first time HHS has directly probed solute-solvent interactions in liquid solutions.
Overcoming Experimental Barriers
Previously, HHS experiments were primarily conducted in gases and solids, where experimental conditions are more controllable. Liquids present two main challenges for HHS: they absorb a significant portion of the generated harmonic light, and the continuous movement of their molecules complicates signal analysis. To mitigate these issues, the OSU-LSU team developed an ultrathin liquid sheet. This innovation allowed more emitted light to escape, enabling the capture of rapid molecular dynamics and subtle structural changes in liquids.
Experimental Observations and Theoretical Interpretations
Using the new experimental setup, researchers investigated HHS behavior in liquid mixtures by applying intense mid-infrared laser light to methanol combined with small amounts of halobenzenes (fluorine, chlorine, bromine, or iodine). Halobenzenes were chosen for their distinct harmonic signals, while methanol provided a relatively neutral background. While most mixtures produced harmonic emission resembling a blend of the two liquids, the fluorobenzene (PhF) in methanol solution yielded different results. This particular mixture produced a significantly lower harmonic yield than either liquid individually, and one harmonic signal was entirely suppressed. This suppression was identified as destructive interference, indicating an interaction near the emitters.
To understand these observations, the Ohio State theory team conducted molecular dynamics simulations. Their analysis indicated that the PhF-methanol mixture formed a more organized solvation structure compared to other halobenzenes. This was attributed to the electronegativity of the fluorine atom facilitating a hydrogen bond with the O-H end of methanol. Subsequently, the Louisiana State University theory group explored if this arrangement could account for the experimental findings. They hypothesized that the electron density around the fluorine atoms created an additional barrier that affected accelerating electrons, thereby disrupting the harmonic generation process. A model based on the time-dependent Schrödinger equation supported this hypothesis, explaining both the missing harmonic and the reduced light output. The sensitivity of this suppression to the barrier's location suggests it provides information about the local structure formed during the solvation process.
This research involved a collaboration between experimental and theoretical groups specializing in physics, chemistry, and optics, contributing to the understanding of electron dynamics in liquid environments.
Significance and Future Outlook
While further research is anticipated to fully explore the capabilities of HHS in liquids, the initial results suggest its potential. Many chemical and biological processes occur in liquid environments, and the electron energies involved are comparable to those that can cause radiation damage. A clearer understanding of electron scattering in dense liquids could have implications across chemistry, biology, and materials science. The findings indicate that solution-phase high-harmonic generation is sensitive to specific solute-solvent interactions and the local liquid environment. Continued advancements in both experiments and simulations are expected to broaden the application of this technique, offering more detailed insights into how liquids respond to ultrafast laser pulses.
Key contributors to this study included Eric Moore, Andreas Koutsogiannis, Tahereh Alavi, and Greg McCracken from Ohio State University, and Kenneth Lopata from Louisiana State University. Funding for this research was provided by the DOE Office of Science, Basic Energy Sciences, and the National Science Foundation.