Interactions among and between electrons and phonons steer the energy flow in photo-excited materials and govern the emergence of correlated phases. The strength of electron–phonon interactions, decay channels of strongly coupled modes and the evolution of three-dimensional order are revealed by electron or X-ray pulses tracking non-equilibrium structural dynamics. Despite such capabilities, the growing relevance of inherently anisotropic two-dimensional materials and functional heterostructures still calls for techniques with monolayer sensitivity and, specifically, access to out-of-plane phonon polarizations. Here, we resolve non-equilibrium phonon dynamics and quantify the excitonic contribution to the structural order parameter in 1T-TiSe₂. To this end, we introduce ultrafast low-energy electron diffuse scattering and trace strongly momentum- and fluence-dependent phonon populations. Mediated by phonon–phonon scattering, a few-picosecond build-up near the zone boundary precedes a far slower generation of zone-centre acoustic modes. These weakly coupled phonons are shown to substantially delay overall equilibration in layered materials. Moreover, we record the surface structural response to a quench of the material’s widely investigated exciton condensate, identifying an approximate 30:70 ratio of excitonic versus Peierls contributions to the total lattice distortion in the charge density wave phase. The surface-sensitive approach complements the ultrafast structural toolbox and may further elucidate the impact of phonon scattering in numerous other phenomena within two-dimensional materials, such as the formation of interlayer excitons in twisted bilayers.

电子和声子之间的相互作用引导光激发材料中的能量流并控制相关相的出现。跟踪非平衡结构动力学的电子或 X 射线脉冲揭示了电子-声子相互作用的强度、强耦合模式的衰变通道以及三维序的演化。尽管具有这些能力，本质上各向异性二维材料和功能异质结构的相关性日益增长，仍然需要具有单层灵敏度的技术，特别是获得面外声子偏振的技术。在这里，我们解决了非平衡声子动力学问题，并量化了激子对 1T-TiSe ₂结构有序参数的贡献。为此，我们引入超快低能电子漫散射，并追踪强烈动量和注量依赖的声子群体。在声子-声子散射的介导下，区域边界附近几皮秒的积累先于区域中心声学模式的缓慢生成。这些弱耦合声子被证明会大大延迟层状材料的整体平衡。此外，我们记录了材料广泛研究的激子凝聚体淬灭的表面结构响应，确定了激子与 Peierls 对电荷密度波相中总晶格畸变的贡献比例约为 30:70。表面敏感方法补充了超快结构工具箱，并可以进一步阐明声子散射对二维材料中许多其他现象的影响，例如扭曲双层中层间激子的形成。