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Abstract 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 (DW), the cathode set‐back position (Ztip), and the position of the femtosecond laser on the Wehnelt aperture surface relative to the optic axis (Rphoto). Notable findings include significant sensitivity of TOF toDWandZtip, as well as non‐intuitive responses of CE and temporal width to varyingRphoto. As a means to improve accessibility, practical implications and recommendations are emphasized wherever possible.
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Delineation of the impact on temporal behaviors of off-axis photoemission in an ultrafast electron microscope
Efforts to push the spatiotemporal imaging-resolution limits of femtosecond laser-driven ultrafast electron microscopes (UEMs) to the combined angstrom–fs range will benefit from stable sources capable of generating high bunch charges. Recent demonstrations of unconventional off-axis photoemitting geometries are promising, but connections to the observed onset of structural dynamics are yet to be established. Here we use the in-situ photoexcitation of coherent phonons to quantify the relative time-of-flight (r-TOF) of photoelectron packets generated from the Ni Wehnelt aperture and from a Ta cathode set-back from the aperture plane. We further support the UEM experiments with particle-tracing simulations of the precise electron-gun architecture and photoemitting geometries. In this way, we measure discernible shifts in electron-packet TOF of tens of picoseconds for the two photoemitting surfaces. These shifts arise from the impact that the Wehnelt-aperture off-axis orientation has on the electron-momentum distribution, which modifies both the collection efficiency and the temporal-packet distribution relative to on-axis emission. Future needs are identified; we expect this and other developments in UEM electron-gun configuration to expand the range of material phenomena that can be directly imaged on scales commensurate with fundamental structural dynamics.
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- Award ID(s):
- 2011401
- PAR ID:
- 10590016
- Publisher / Repository:
- Rev. Sci. Instrum.
- Date Published:
- Journal Name:
- Review of Scientific Instruments
- Volume:
- 95
- Issue:
- 9
- ISSN:
- 0034-6748
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1063/5.0222993
