Lights, Camera, Reaction!
Light absorption by molecules underpins countless processes, from photosynthesis in plants to electricity generation in organic solar cells. Within femtoseconds, such absorption can cause electrons to jump between energy levels, atoms to vibrate, and chemical bonds to rearrange. Observing such dynamics directly and in real time is vital for understanding and ultimately controlling light-induced molecular processes, but that task remains a great challenge. Now Dao Xiang at Shanghai Jiao Tong University and his colleagues have made a key step toward this goal [1]. They have demonstrated that ultrafast electron diffraction can facilitate direct, time-resolved tracking of electronic and nuclear motion during such reactions.
The researchers considered a benchmark reaction in which an ammonia molecule absorbs light and breaks into fragments. Earlier this year, a different team showed that the electronic motion during this breakup could be observed using ultrafast x-ray scattering (see Viewpoint: Watching Electron Dynamics Shape Chemical Reactions). After initiating the reaction using a laser pulse, Xiang and his colleagues instead fired a series of ultrashort, high-energy electron pulses at the molecule. The electrons scattered off the molecule and formed an evolving diffraction pattern on a detector.
By analyzing this diffraction pattern, the researchers were able to track the dynamics of the molecule’s outer electrons and hydrogen nuclei simultaneously through the entire reaction. This feat was made possible by the higher spatial resolution of ultrafast electron diffraction relative to ultrafast x-ray scattering and by the implementation of a new analysis technique that can disentangle the coupled electronic and nuclear motion. The researchers say that their approach could provide key insights into the interactions of light with a wide range of other molecules.
–Ryan Wilkinson
Ryan Wilkinson is a Corresponding Editor for Physics Magazine based in Durham, UK.
References
- T. Wang et al., “Probing valence electron and hydrogen dynamics using charge-pair imaging with ultrafast electron diffraction,” Phys. Rev. Lett. 135, 233002 (2025).