Influence of Photoemission Geometry on Timing and Efficiency in 4D Ultrafast Electron Microscopy

Broader adoption of 4D ultrafast electron microscopy (UEM) for the study of chemical, materials, and quantum systems is being driven by development of new instruments as well as continuous improvement and characterization of existing technologies. Perhaps owing to the still-high barrier to entry, the full range of capabilities of laser-driven 4D UEM instruments has yet to be established, particularly when operated at extremely low beam currents (~fA). Accordingly, with an eye on beam stability, we have conducted particle tracing simulations of unconventional off-axis photoemission geometries in a UEM equipped with a thermionic-emission gun. Specifically, we have explored the impact of experimentally adjustable parameters on the time-of-flight (TOF), the collection efficiency (CE), and the temporal width of ultrashort photoelectron packets. The adjustable parameters include the Wehnelt aperture diameter (D_W), the cathode set-back position (Z_tip), and the position of the femtosecond laser on the Wehnelt aperture surface relative to the optic axis (R_photo). Notable findings include significant sensitivity of TOF to D_W and Z_tip, as well as non-intuitive responses of CE and temporal width to varying R_photo. As a means to improve accessibility, practical implications and recommendations are emphasized wherever possible.