Science enabled by MeV-UED and future opportunities

X-ray Free Electron Laser (XFEL) revolutionized ultrafast science. One of the enabling technologies for XFEL is high-brightness electron source based on the photocathode RF gun. The MeV electrons produced by the photocathode RF gun also made it feasible for MeV ultrafast electron diffraction [1-2]. MeV ultrafast electron diffraction (MeV-UED) is a new paradigm of the ultrafast electron scattering because of its stronger diffraction and higher sensitivity to the structure dynamics [2]. Furthermore, MeV electrons experience less multiple-scattering and possess “real” flat Ewald-sphere, which make it feasible quantitatively interpreting the physical observables and validating theoretic and simulation models. MeV-UED is an “ideal” tool to image nonequilibrium and transient quantum states using total scattering technique. Science enabled by MeV-UED is broad and transformative, it includes imaging fundamental photochemical processes [3-4], hydrogen bond dynamics in liquid water [5], capturing light-induced transient states of quantum materials [6-8], tracking charge & energy transport across nano-scale heterostructures [9-10], and imaging phase transformation under extreme conditions [11-12].

Simultaneous imaging of electronic and nuclear structural dynamics [13], together with ultrafast electron diffuse scattering [14], has emerged as a next frontier for ultrafast electron scattering. The capability to simultaneously visualize electronic and nuclear dynamics provides a powerful approach for investigating electron–nuclear interactions in materials and chemical processes. Meanwhile, ultrafast electron diffuse scattering enables direct tracking of energy flow among elementary excitations and their mutual couplings in quantum materials.

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, “Femtosecond time-resolved MeV electron diffraction”, New Journal of Physics 17 (6), 063004 (2014).

3. J. Yang et al, “Imaging CF3I conical intersection and photodissociation dynamics with ultrafast electron diffraction”,

Science 361, 64 (2018).

4. T. J. A. Wolf et al,” The photochemical ring-opening of 1,3-cyclohexadiene imaged by ultrafast electron

diffraction”, Nature Chemistry 11, pages 504–509 (2019).

5. J. Yang et al., ”Direct observation of ultrafast hydrogen bond strengthening in liquid water”, Nature 596, 531–535

(2021).

6. E. J. Sie et al, ”An ultrafast symmetry switch in a Weyl semimetal”, Nature 565,61–66(2019).

7. A. Kogar et al,” Light-induced charge density wave in LaTe3”, Nat. Phys.16, 159 (2019).

8. A. Zong et al,”Spin-mediated shear oscillators in a van der Waals antiferromagnet”,

Nature 620, pages 988–993 (2023).

9. D. Luo et al, "Twist-Angle-Dependent Ultrafast Charge Transfer in MoS2-Graphene van der Waals

Heterostructures", Nano Lett. 2021, 21, 19, 8051–8057.

10. A. Sood et al, "Bidirectional phonon emission in two-dimensional heterostructures triggered by ultrafast charge

transfer", Nat. Nanotechnol.18, 29–35 (2023).

11. M. Mo et al, "Heterogeneous to homogeneous melting transition visualized with ultrafast electron

diffraction", Science 360, 1451 (2018).

12. M. Mo et al, "Ultrafast visualization of incipient plasticity in dynamically compressed matter", Nat. Commun. 13,

(2022).

13. J. Yang et al, "Simultaneous observation of nuclear and electronic dynamics by ultrafast electron

diffraction", Science 368, 885 (2020).

14. T. Chase et al, "Ultrafast electron diffraction from non-equilibrium phonons in femtosecond laser heated Au

films", Appl. Phys. Lett. 108, 41909 (2016).

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