The probing of coherent lattice vibrations in solids has conventionally been carried out using time-resolved transient optical spectroscopy, with which only the relative oscillation amplitude can be obtained. Using time-resolved X-ray techniques, absolute electron–phonon coupling strength could be extracted. However, the complexity of such an experiment renders it impossible to be carried out in conventional laboratories. Here we demonstrate that the electron–phonon, anharmonic phonon–phonon coupling and their relaxation dynamics can be probed in real time using high-harmonic spectroscopy. Our technique is background-free and has extreme sensitivity directly in the energy domain. In combination with the optical deformation potential calculated from density functional perturbation theory and the absolute energy modulation depth, our measurement reveals the maximum displacement of neighbouring oxygen atoms in α-quartz crystal to tens of picometres in real space. By employing a straightforward and robust time-windowed Gabor analysis for the phonon-modulated high-harmonic spectrum, we successfully observe channel-resolved four-phonon scattering processes in such highly nonlinear interactions. Our work opens a new realm for the accurate measurement of coherent phonons and their scattering dynamics, which allows for potential benchmarking ab initio calculations in solids.

固体中相干晶格振动的探测通常使用时间分辨瞬态光谱进行，仅能获得相对振荡幅度。使用时间分辨X射线技术，可以提取绝对电子声子耦合强度。然而，此类实验的复杂性使其不可能在传统实验室中进行。在这里，我们证明可以使用高次谐波光谱实时探测电子-声子、非简谐声子-声子耦合及其弛豫动力学。我们的技术是无背景的，并且直接在能量域中具有极高的灵敏度。结合密度泛函微扰理论计算的光学形变势和绝对能量调制深度，我们的测量揭示了α-石英晶体中相邻氧原子在真实空间中的最大位移为数十皮米。通过对声子调制高次谐波谱采用简单且鲁棒的时窗伽伯分析，我们成功地观察到了这种高度非线性相互作用中通道分辨的四声子散射过程。我们的工作为相干声子及其散射动力学的精确测量开辟了新领域，从而可以在固体中进行潜在的基准从头计算。