Abstract:
Ultrafast electron diffraction (UED)[1] is a powerful tool that can monitor the nuclear dynamics of photo-induced gas-phase reactions in real-time with picometre and <250-fs spatiotemporal resolution. However, the temporal resolution of state-of-the-art UED setups (<1-kHz) is insufficient to time-resolve rapidly evolving photo-induced reactions (e.g., <350-fs predicted timescale of photoisomerization which plays a crucial role in vision) due to the severe space-charge dispersion experienced in electron pulses containing 104 to 105 electrons.
Here, we present a new 30-kHz, 100-keV UED setup employing direct electron detection[2] (see Fig. 1a) that will be capable of performing time-resolved measurements of photochemical reactions in gas-phase molecules with <100-fs temporal resolution. This is made possible by operating below the severe space-charge dispersion regime using electron pulses containing very few electrons (<102) but with sufficient electron flux (>106 electrons/s). Latest results from our pump-probe UED instrument are presented (see Fig. 1c-f) with details of the current implementation of radiofrequency-compressed electron pulses. We show that operating with 102 electrons provides very good electron beam properties (transverse coherence length of 8.9 nm, FWHM diameter at sample of 100-µm), with General Particle Tracer simulations (GPT) predicting that the FWHM electron pulse duration of uncompressed (~300-fs; Fig. 1b) and compressed (~35-fs; Fig. 1c) electrons are suitable for the studying a wide-range of photochemical reactions, some of which were so far too fast to be studied with UED. Temporal compression of electrons is achieved using a radiofrequency (RF) field. We have corrected the RF-laser timing jitter of 380-fs to 13.5-fs. Work is on-going on characterising electron compression. In the near future, we will upgrade our set-up for gas-phase studies and we will implement a new, 40-kHz OPA laser system (80-W, 2-mJ).
[1] K. Amini, A. Rouzée, M. J. J. Vrakking, Royal Society of Chemistry, 25 (2023).
[2] F. R. Diaz, M. Mero, K. Amini, Structural Dynamics 11, 054302 (2024).