Speedy nuclei do the twist
Ultrafast electron imaging captures never-before-seen nuclear motions in hydrocarbon molecules excited by light.
image: Researchers retrieved the time-varying molecular structure of photoexcited o-nitrophenol from ultrafast electron diffraction data using a genetic algorithm.
Credit: Image courtesy of J. P. F. Nunes (generated using Microsoft Copilot)
The Science
The way hydrocarbon molecules interact with light can affect the production of nitrous acid in the atmosphere, which contributes to pollution. In this study, researchers focused on the role of proton transfer in these interactions. Intramolecular proton transfers involve protons moving from one part of a molecule to another part of the molecule. The researchers used an ultrafast electron camera to image the motions of hydrocarbon molecules on scales ten thousand times smaller than the width of a human hair. This ultra-precise and ultrafast imaging technique, supported by advanced computations, reveals a proton transfer step followed by an out-of-plane twisting motion as key components of energy relaxation. Relaxation is the process by which the molecule moves from an excited, high-energy state to a lower energy ground state after absorbing light.
The Impact
Previous studies have proposed various ways that hydrocarbon molecules may relax after interacting with light. However, scientists lacked experimental data to verify which process occurs. This study identified a key relaxation pathway involving proton transfer and molecular “twisting.” This result lays the groundwork for studies of more complex molecules that scientists believe undergo similar interactions. It will also help researchers better understand how pollution forms.
Summary
The interactions between light and nitroaromatic hydrocarbon molecules have important implications for chemical processes in our atmosphere that can lead to smog and pollution. However, changes in molecular geometry due to interactions with light can be very difficult to measure because they occur at sub-Angstrom length scales (less than a tenth of a billionth of a meter) and femtosecond time scales (one millionth of a billionth of a second). The relativistic ultrafast electron diffraction (UED) instrument at the Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory provides the necessary spatial and time resolution to observe these ultrasmall and ultrafast motions. The LCLS is a Department of Energy (DOE) Office of Science light source user facility.
In this research, scientists used UED to observe the relaxation of photoexcited o-nitrophenol. Then, they used a genetic structure fitting algorithm to extract new information about small changes in the molecular shape from the UED data that were imperceptible in previous studies. Specifically, the experiment resolved the key processes in the relaxation of o-nitrophenol: proton transfer and deplanarization (i.e., a rotation of part of the molecule out of the molecular plane). Ab-initio multiple spawning simulations confirmed the experimental findings. The results provide new insights into proton transfer mediated relaxation and pave the way for studies of proton transfer in more complex systems.
Funding
This work was supported by the Department of Energy (DOE) Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. This research used resources of the Linac Coherent Light Source, a DOE Office of Science user facility.