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We determine the instrument response of an ultrafast electron microscope equipped with a conventional thermionic electron gun and absent modifications beyond the optical ports. Using flat, graphite-encircled LaB6 cathodes, we image space-charge effects as a function of photoelectron-packet population and find that an applied Wehnelt bias has a negligible effect on the threshold levels (>103 electrons per pulse) but does appear to suppress blurring at the upper limits (∼105 electrons). Using plasma lensing, we determine the instrument-response time for 700-fs laser pulses and find that single-electron packets are laser limited (1 ps), while broadening occurs well below the space-charge limit.
Bibliographical noteFunding Information:
This work was supported partially by the National Science Foundation through the University of Minnesota MRSEC under Award Number DMR-1420013 and partially by the Arnold and Mabel Beckman Foundation through a Beckman Young Investigator Award. The funding sources had no involvement in the study design; in the collection, analysis, and interpretation of the data; in the writing of the report; and in the decision to submit the article for publication.
How much support was provided by MRSEC?
Reporting period for MRSEC
- Period 3