Real-space imaging of ultrafast electron dynamics and subsequent crystallographic dynamics via photo-induced phase transition

Photo-induced phase transition (PIPT) involves substantial changes in both electronic and crystallographic structures caused by electron excitations. Because PIPT is ultrafast and exhibits inhomogeneous behaviour, its fundamental processes, in particular how the ultrafast dynamics of photoexcited electrons trigger lattice deformation and the time scales involved, remain poorly understood. In this study, the dynamics of the photoexcited electrons and the subsequent volume change of organic charge transfer complexes are directly visualised simultaneously in the time and energy domains using photoemission electron microscopy with a femtosecond pulsed laser as the excitation source. The results show that an anisotropic volume change of a few percent is initiated following the accumulation of electrons in unoccupied states over approximately a picosecond.