Observing Phonon Dynamics with Shot-to-Shot Acquisition Ultrafast Electron Diffraction

Ultrafast electron diffraction (UED) uses short-wavelength electrons to reveal structural dynamics triggered by ultrashort laser pulses in materials. Recently, UED has been extended to observe the changes of diffuse scattering [1] brought about by the non-thermal phonon population dynamics, induced by the electron-phonon coupling after electron excitation. Because the diffuse scattering is very weak, very high experimental sensitivity is required.<br/>We present ultrafast electron diffuse scattering achieved by 20 kHz shot-to-shot acquisition in a tabletop UED system [2]. The shot-to-shot acquisition system is synchronized to a chopper in the pump arm, allowing every individual measurement to be normalized to a reference under identical conditions. This leads to excellent signal-to-noise, enabling the measurement of the transient modification of diffuse scattering.<br/>We apply this technique to study of phonon dynamics in graphite. Ultrafast time and momentum-resolved electron energy loss (tr-q-EELS) measurements [3] measured the same system but with access to energy information as well momentum. tr-q-EELS revealed plasmon dynamics in graphite, which theoretical analysis linked to the population of phonons modes via inter- and intravalley scattering that depend on the pump wavelength. We use UED to investigate this hypothesis by directly by observing the formation of phonons in real time. By comparing the results of the two experiments we can reveal the full picture of the phonon dynamics in graphite.<br/><br/>[1] M. J. Stern et al, <i>Physical Review Letters, </i>2018.<br/>[2] R. Claude et al, <i>In Preparation.</i><br/>[3] F. Barantani et al, <i>In Preparation.</i>