Mode-dependent phonon relaxation in time-resolved electron diffraction pattern simulations

Time-resolved pump-probe experiments offer unique possibilities for studying ultrafast processes; however, simulation tools for interpreting phonon dynamics in electron diffraction patterns at the sub-picosecond scale remain limited. We introduce the frozen trajectory excitation (FTE) method of exciting phonons beyond their thermal equilibrium population by modifying a molecular dynamics trajectory in the (q→,ω)-space, and couple it with a new approach to ensemble sampling that extends frozen phonon multislice simulations into the time domain. In this approach, phonons with a certain natural frequency (ω) and located within an arbitrarily selected range of phonon wave vector (q→) are first selectively excited within a single molecular dynamics trajectory. Subsequently, several parallel relaxation runs are started at random points, and snapshots from these trajectories serve as inputs for multislice simulations at defined time delays. We apply this framework to fcc Ni with relaxation time resolution of 10fs. The simulations reveal multi-phonon scattering processes as well as strong mode dependence in phonon relaxation, highlighting the importance of considering phonon-specific behavior in ultrafast dynamics. Our results show that mode-dependent relaxation leaves measurable signatures in diffraction patterns, providing predictive guidance for future time-resolved TEM studies.