Speaker
Description
Ultrafast electron diffraction (UED) [1-3] is a powerful tool for tracking the nuclear dynamics of photo-induced gas-phase reactions in real-time with picometre spatial and 150-240-fs temporal resolution [4,5]. However, time-resolving rapidly evolving photo-induced processes, such as photoisomerization [6] in vision (<350-fs predicted total timescale), remains challenging due to space-charge-induced pulse broadening in high-charge electron pulses required for existing low-repetition-rate (≤1-kHz) gas-phase UED setups.
We present a <100-keV UED instrument operating at high repetition rates (30-100+ kHz) that utilizes direct electron detection. Initial results demonstrate the capability to measure time-resolved electron scattering signals with single-electron, temporally uncompressed pulses at 30 kHz [7]. This was possible by using significantly lower bunch charges but at high repetition rate, while also measuring the primary unscattered electron beam which is used to normalise all scattering data, leading to a 10-100 factor improvement in signal-to-noise ratio of detected signals [7]. In a temporally uncompressed mode, we were able to investigate electron-phonon dynamics in aluminium using 10^2 electrons/pulse with sub-400-fs resolution [7].
Recent upgrades to our instrumentation and laser system (CARBIDE, 80-W, 2-mJ) have enabled THz electron streaking at 40-100 kHz, allowing precise temporal characterization of electron pulse compression using a radiofrequency (RF) microwave cavity [8]. We demonstrate 30-fold compression of an electron pulse containing >10^4 electrons, reducing a ~3,000 fs pulse to ~100 fs, making our setup one of the brightest in Europe. Moreover, combining this compression capability with high repetition rate operation (up to 2-MHz) and direct detection makes our set-up one of the highest electron flux UED setups in the world. This combination uniquely positions it for high-sensitivity gas-phase UED measurements while retaining the flexibility to study solid-state thin films.
[1] K. Amini and J. Biegert, Adv. At. Mol. Opt. Phys., Chapter 3., 2020
[2] M. Centurion et al., Annu. Rev. Phys. Chem. 73, 21 (2022).
[3] H. Ihee et al., Science 291, 458 (2001).
[4] J. Yang et al., Science 368, 885 (2020).
[5] Y. Xiong et al., Phys. Rev. Research 2, 043064 (2020).
[6] J. K. Yu et al., J. Am. Chem. Soc. 142, 20680 (2020).
[7] F. R. Diaz, Structural Dynamics 11, 054302, (2024).
[8] T. van Oudheusden et al., Phys. Rev. Lett. 105, 264801, (2010).
