English
The interaction between electrons and phonons guides the energy flow in photoexcited materials and controls the appearance of related phases. The tracking of electron or X-ray pulses in non-equilibrium structural dynamics reveals the strength of electron phonon interactions, decay channels of strong coupling modes, and the evolution of three-dimensional order. Despite these capabilities, the increasing correlation between anisotropic two-dimensional materials and functional heterostructures still requires techniques with single-layer sensitivity, particularly for obtaining out of plane phonon polarization. Here, we address the non-equilibrium phonon dynamics problem and quantify the contribution of excitons to the ordered parameters of the 1T-TiSe2 structure. To this end, we introduce ultrafast low-energy electron diffuse scattering and track phonon populations with strong momentum and flux dependence. Under the mediation of phonon phonon scattering, the accumulation of a few picoseconds near the region boundary precedes the slow generation of acoustic modes at the center of the region. These weakly coupled phonons have been shown to significantly delay the overall equilibrium of layered materials. In addition, we recorded the surface structure response of exciton condensed matter quenching extensively studied in materials, and determined that the contribution ratio of excitons and Peierls to the total lattice distortion in charge density wave phase is approximately 30:70. Surface sensitive methods supplement the toolbox of ultrafast structures and can further elucidate the effects of phonon scattering on many other phenomena in two-dimensional materials, such as the formation of interlayer excitons in twisted bilayers.
