Reassessing the Practical Resolution Limits in Compact Ultrafast Electron Diffraction

Ultrafast Electron Diffraction (UED) is a powerful tool for probing atomic-scale dynamics with femtosecond temporal resolution and ångström-level spatial precision. It enables direct observation of structural changes in non-equilibrium systems, including photoinduced transformations, phase transitions, and chemical reactions. While large-scale MeV facilities have advanced UED, the growing demand for accessible, laboratory-scale instruments highlights the need for compact solutions. We present a compact electrostatic UED instrument developed in our laboratory. Operating at a cathode voltage of 100 kV, the source produces ~200 fs (full width at half maximum, FWMH) electron pulses in a low areal density regime (< 0.1 electrons μm⁻2), primarily limited by the duration of the driving ultraviolet pulse. Simple estimations and N-particle tracer simulations indicate that, with ultrashort laser excitation and enhanced high-voltage conditioning to boost the extraction field, the temporal response could approach the 50-fs threshold. Combined with multi-kilohertz repetition rates, this balance of pulse duration and average beam brightness ensures both ultrafast performance and robust diffraction signals. Initial static and dynamic diffraction experiments, performed at ~0.05 electrons μm⁻2 and 6 kHz, validate the instrument’s baseline performance and sensitivity. These results demonstrate the potential for capturing ultrafast structural dynamics within a compact, laboratory-scale platform.