Intense femtosecond optical laser pulses drive material phase transitions via kinetic reactions otherwise hidden in equilibrium measurements. This stimulates interest in uncovering reaction dynamics of individual atoms prompted by photoexcited electrons. However, our understanding on ultrafast atomic dynamics has been limited by the challenges in resolving irreversible processes at the relevant space–time resolution. By establishing single-pulse time-resolved diffraction imaging technique using an X-ray free-electron laser, we overcome this to directly observe the intermediate processes accompanied during the nonequilibrium phase transitions. Recent investigation of exotic kinetic transitions forbidden in thermodynamic equilibrium condition will be introduced to highlight the potential available with the diffractive imaging scheme.
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