Electron diffraction method tracks molecules in solutions

Understanding ultrafast photochemical reactions, including those occurring in our eyes, requires resolving their structural dynamics. Electron scattering, which boasts high structural sensitivity and time resolution, can reveal how molecules move over time, but has not been compatible with liquid samples.

Ledbetter et al. created a method called liquid ultrafast electron diffraction (LUED) by combining two techniques, namely mega-electron volt ultrafast electron diffraction (MeV-UED) and liquid sheet jets that can produce sub-micrometer liquid droplets under vacuum.

The authors demonstrated the method by measuring the photodissociation reaction of triiodide in water, which was initiated by a laser pulse. The difference in the electron diffraction signals, which were distinguishable from the surrounding signals of water molecules, corresponded to changes in the distance between the iodide atoms.

They examined the reaction over two timescales. On the femtosecond timescale, the data showed signs of dissociation and the authors were able to estimate the average dissociation speed for the triiodide. On the picosecond timescale, they compared the data to molecular dynamics simulations and were able to determine that most of the initially dissociated species rapidly recombined within the first picosecond after dissociation.

“The results are exciting because they show for the first time that LUED is a viable method for understanding molecular structural dynamics in solution environments, complementary to more established x-ray solution scattering methods,” said author Kathryn Ledbetter.

Improvements to the MeV-UED instrument at SLAC National Accelerator Laboratory will allow the study of more systems, including those with lighter atoms and higher complexity.

Source: “Photodissociation of aqueous I₃^- observed with liquid-phase ultrafast mega-electron-volt electron diffraction,” by K. Ledbetter, E. Biasin, J. P. F. Nunes, M. Centurion, K. J. Gaffney, M. Kozina, M. F. Lin, X. Shen, J. Yang, X. J. Wang, T. J. A. Wolf, and A. A. Cordones, Structural Dynamics (2020). The article can be accessed at https://doi.org/10.1063/4.0000051 .