Practical Electron Microscopy and Database

In the method of laser-based pulsed electron beam generation for EMs, a photocathode replaces the traditional FEG. The generation of pulsed electron beams relies on the interaction between a short-duration laser pulse and a photocathode. That is, the laser-based photocathode method directly generates pulsed electron beams by using laser pulses to release electrons from a photocathode via the photoelectric effect. The laser-based pulsed electron beam generation method is primarily used in ultrafast electron microscopy (UEM). When the laser strikes the photocathode, electrons are emitted via the photoelectric effect. The energy from the laser photons excites electrons in the material, allowing them to overcome the work function of the photocathode and be ejected as free electrons. The duration of the laser pulse, typically in the femtosecond range, defines the timing of electron emission, resulting in the generation of electron packets rather than a continuous beam. By adjusting the repetition rate of the laser, the temporal spacing between these electron packets can be precisely controlled, providing high temporal resolution and allowing for the study of ultrafast dynamics. This method offers a distinct advantage over conventional continuous emission from field emission guns, as it enables a controlled and tunable pulsed electron beam, reducing specimen damage and enhancing temporal resolution in applications such as UEM. 

Figure 2637 shows the overview of the fs laser-based approach of pulsed-beam generation in TEM.

This technique enables time-resolved studies of dynamic processes at the atomic and molecular levels. By generating electron pulses in the femtosecond range, UEM allows researchers to capture ultrafast events, such as atomic motion, phase transitions, or reaction mechanisms, that occur on extremely short timescales:

[1] David J. Flannigan, Elisah J. VandenBussche, Pulsed-beam transmission electron microscopy and radiation damage, Micron, 172, 103501, 2023.
[2] VandenBussche, E. J.; Flannigan, D. J. Reducing Radiation Damage in Soft Matter with Femtosecond- Timed Single-Electron Packets. Nano Lett. 19, 6687-6694. 10.1021/acs.nanolett.9b03074, 2019.