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Abstract:
In this thesis we combine femtosecond electron diffuse scattering experiments and ab intio calculations to study the non-equilibrium lattice dynamics in a thin film of photo-excited MoS2. By simultaneously analyzing the elastic and diffuse scattering signals, we reveal the non-thermal character of lattice dynamics and obtain a detailed momentum-resolved view on the electron-phonon and phonon-phonon coupling over a wide range of the Brillouin zone. We also explore the extent to which branch-resolution can be extracted from the data. We show that our ability to retrieve energy resolution is inherently limited by multicollinearity of the branch-resolved structure factors and multi-phonon scattering. These effects, observed in MoS2, are expected to play significant roles in most materials of similar complexity. We find that partial energy-resolution can nevertheless be achieved by reducing the number of phonon branches by grouping them into effective branches. Overall, this work contributes to a deeper understanding of the recently developed time-resolved diffuse scattering methods. This understanding is necessary in view of reaching the ultimate goal of a phonon mode-resolved view of lattice dynamics in materials.
