Thermionic Emission Dynamics of Ultrafast Electron Sources

Ultrafast electron sources, which enable high spatiotemporal resolution in time-resolved electron microscopy and scanning probe microscopy, are receiving increased attention. The most widely used method for achieving ultrafast electron sources involves irradiating metal tips by ultrashort laser pulses, causing electron beam emission via the photoelectric effect (including photon-driven quantum or field-driven classical emission). However, the thermionic electron emission process due to the heating effect of ultrashort lasers, particularly its dynamic aspects, has been rarely addressed in previous studies. In this paper, we improved the signal-to-noise ratio of two-pulse correlation measurement on the tip electron emission by nearly two orders of magnitude using delay time modulation method. This allowed us to obtain information on the temperature evolution of hot electrons and phonons in a non-equilibrium state and to extract characteristic time scales for electron-phonon and phonon-phonon scattering. Our findings indicate that the thermionic electron emission, unlike the instantaneous photoelectric effect, causes electron emission to lag behind the laser pulse by tens of picoseconds, thus significantly affecting the ultrafast dynamics detection of samples. Furthermore, such a lagging effect was found to be sensitive to the local structure of the metal tip, offering new insights into the improved design of ultrafast electron sources.