Single-Photoelectron Collection Efficiency in 4D Ultrafast Electron Microscopy

Wyatt A. Curtis, Simon Willis, David Flannigan

Research output: Contribution to journalArticlepeer-review

3 Scopus citations

Abstract

In femtosecond (fs) 4D ultrafast electron microscopy (UEM), a tradeoff is made between photoelectrons per packet and time resolution. One consequence of this can be longer-than-desirable acquisition times for low-density packets, and particularly for low repetition rates when complete photothermal dissipation is required. Thus, gaining an understanding of photoelectron trajectories in the gun region is important for identifying factors that limit collection efficiency (CE; fraction of photoelectrons that enter the illumination system). Here, we continue our work on the systematic study of photoelectron trajectories in the gun region of a Thermo Fisher/FEI Tecnai Femto UEM, focusing specifically on CE in the single-electron regime. Using General Particle Tracer, calculated field maps, and the exact architecture of the Tecnai Femto UEM, we simulated the effects of fs laser parameters and key gun elements on CE. The results indicate CE strongly depends upon the laser spot size on the source, the (unbiased) Wehnelt aperture diameter, and the incident photon energy. The CE dispersion with laser spot size is found to be strongly dependent on aperture diameter, being nearly dispersionless for the largest apertures. A gun crossover is also observed, with the beam-waist position being dependent on the aperture diameter, further illustrating that the Wehnelt aperture acts as a simple, fixed electrostatic lens in UEM mode. This work provides further insights into the operational aspects of fs 4D UEM.

Original languageEnglish (US)
Pages (from-to)14044-14054
Number of pages11
JournalPhysical Chemistry Chemical Physics
Volume24
Issue number22
DOIs
StatePublished - May 25 2022

Bibliographical note

Funding Information:
This material is based on work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award No. DE-SC0018204. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. DGE-1839286. This work was supported partially by the National Science Foundation through the University of Minnesota MRSEC under Award Number DMR-2011401. Acknowledgement is made to the Donors of the American Chemical Society Petroleum Research Fund for partial support of this research under Award No. 60584-ND10. We thank Dr Erik Kieft of Thermo Fisher Scientific for assistance with modeling the FEI Tecnai Femto architecture and for ensuring accurate electrostatic field maps were generated.

Publisher Copyright:
© 2022 The Royal Society of Chemistry

MRSEC Support

  • Partial

PubMed: MeSH publication types

  • Journal Article

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