DC photoelectron gun parameters for ultrafast electron microscopy

We present a characterization of the performance of an ultrashort laser pulse driven DC photoelectron gun based on the thermionic emission gun design of Togawa et al. [Togawa, K., Shintake, T., Inagaki, T., Onoe, K. & Tanaka, T. (2007). Phys Rev Spec Top-AC 10, 020703]. The gun design intrinsically provides adequate optical access and accommodates the generation of approximately 1 mm2 electron beams while contributing negligible divergent effects at the anode aperture. Both single-photon (with up to 20,000 electrons/pulse) and two-photon photoemission are observed from Ta and Cu(100) photocathodes driven by the harmonics (approximately 4 ps pulses at 261 nm and approximately 200 fs pulses at 532 nm, respectively) of a high-power femtosecond Yb:KGW laser. The results, including the dependence of the photoemission efficiency on the polarization state of the drive laser radiation, are consistent with expectations. The implications of these observations and other physical limitations for the development of a dynamic transmission electron microscope with sub-1 nm.ps space-time resolution are discussed.