Abstract:  The X-ray Free-electron laser (XFEL) has transformed the ultrafast science. XFEL technology also enabled mega-electron-volt ultrafast electron diffraction (MeV-UED) [1]. MeV-UED had broad impact on ultrafast science due to its capability of following dynamics on femtoseconds scale with the high spatial resolution and sensitivity [2]. The science enabled by MeV-UED includes imaging fundamental photochemical processes [3-4] and hydrogen bond dynamics in liquid water[5]; capturing light-induced transient states of quantum materials [6-7]. Furthermore, MeV electrons experience less multiple-scattering, and possess “real” flat Ewald-sphere; MeV-UED is an “ideal” tool to explore both structure and dynamics using total scattering technique. Recently, MeV-UED opened a new frontier of ultrafast electron scattering - simultaneously imaging electronic and nuclear structure dynamics [8- 10]. This provides a unique tool detangling electron-nuclear coupling, one of fundamental processes in material and chemical science.

1. X.J. Wang et al., Proceedings of the 2003 Particle Accelerator Conference, 2003, pp. 420-422 Vol.1, doi: 10.1109/PAC.2003.1288940.
2. P Zhu et al., New Journal of Physics 17 (6), 063004 (2014). 
3. J. Yang et al., Science 361, 64 (2018). 
4. T. J. A. Wolf et al., Nat. Chem. (2019).
5. J. Yang et al., Nature 596, 531–535 (2021). 
6. E. J. Sie et al., Nature 565,61–66(2019). 
7. A. Kogar et al., Nat. Phys.16, 159 (2019). 
8. J. Yang et al., "Simultaneous observation of nuclear and electronic dynamics by ultrafast electron diffraction", Science 368, 885 (2020). 
9. J. Li et al, "Concurrent probing of electron-lattice dephasing induced by photoexcitation in 1T -TaSeTe using ultrafast electron diffraction", Phys. Rev. B 101, 100304 (2020). 
10. E. G. Champenois et al., "Femtosecond Electronic and Hydrogen Structural Dynamics in Ammonia Imaged with Ultrafast Electron Diffraction", Phys. Rev. Lett. 131, (2023).

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