Development of a high bunch charge and high transverse coherence ultrafast electron source for structural studies of large-periodicity materials

Abstract

We developed an ultrafast time-resolved electron diffractometer with a tungsten probe photocathode. High transverse coherence and short-pulsed electrons were generated from a tungsten tip with a diameter of 1 μm. The transverse coherence length and duration of the electron pulses accelerated to 45 keV were experimentally and numerically characterized under high electron density conditions. The electron diffraction patterns from multi-walled carbon nanotubes with outer diameters of 3–10 nm demonstrated the transverse coherence length of the electron pulses at ∼10 nm. The optical pump (wavelength: 515 nm) and electron diffraction probe measurements on a 2H–MoTe2 thin film demonstrate a decrease in the intensity of electron diffraction spots due to thermal effects. According to the pump–probe measurements, the electron pulse duration is determined to be in the range of 6–13 ps, evolving as the power 1/3 of the number of electrons in a pulse (250–2300 electrons/pulse at the sample position). This dependence, which differs from pancake-shaped electron pulses emitted from conventional flat metal photocathodes, arises from the hemispherical propagation of electrons emitted from the tungsten tip. The experimental characterization of the electron pulses agreed with the results of the particle-tracking calculations. The developed setup with a high bunch charge and high transverse coherence ultrashort pulsed electron source is suitable for understanding photoinduced structural dynamics in large-periodicity materials in chemistry and biology. The insights obtained in this study should also contribute to the development of point electron sources in ultrafast time-resolved diffractometers as well as ultrafast transmission or scanning electron microscopes.