Understanding the dynamics of electron-phonon and phonon-phonon interactions is important to unravel the complex behavior of materials subject to ultrafast laser excitation. We report the results of studying these interactions in femtosecond laser-excited tungsten (W) using the technique of ultrafast electron diffuse scattering (UEDS). By tracking changes of diffuse scattering signal over time, we resolve the dynamics of phonon populations across the Brillouin zone in W. Our results shed light on both electron-phonon and phonon-phonon coupling dynamics in W [Mo et al. Science Advances, in press (2024)]. This paper outlines the fundamental principle behind the UEDS technique, provides a brief overview of the experimental setup, and presents selected results of time-resolved diffuse scattering patterns.
In X-ray Free-Electron Lasers (FELs), intense and coherent pulses are generated via amplification of the undulator radiation from micro-bunched electron pulses. The initial radiation is spontaneous and intrinsically stochastic, thus causing shot-to-shot fluctuations in the intensity, pointing, and spatiotemporal profile of the X-ray beam. In this work, we use deep neural networks to investigate the fluctuations in X-ray beam profiles, thereby obtaining statistical information on the lasing process. A supervised model was built to classify X-ray images, and an unsupervised one to study the distribution of beam profiles. We have found that round-shaped profiles appear more often with increasing monochromator bandwidth, suggesting that some round-shaped images can be superpositions of higher-order modes. Our results also suggest that the X-ray beam continues to evolve past the FEL saturation length towards a round-shaped beam profile.
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