X-ray diffraction is routinely used for structure determination of stationary molecular samples. Modern X-ray photon sources, e.g., from free-electron lasers, enable us to add temporal resolution to these scattering events, thereby providing a movie of atomic motions. We simulate and decipher the various contributions to the X-ray diffraction pattern for the femtosecond isomerization of azobenzene, a textbook photochemical process. A wealth of information is encoded besides real-time monitoring of the molecular charge density for the cis to trans isomerization. In particular, vibronic coherences emerge at the conical intersection, contributing to the total diffraction signal by mixed elastic and inelastic photon scattering. They cause distinct phase modulations in momentum space, which directly reflect the real-space phase modulation of the electronic transition density during the nonadiabatic passage. To overcome the masking by the intense elastic scattering contributions from the electronic populations in the total diffraction signal, we discuss how this information can be retrieved, e.g., by employing very hard X-rays to record large scattering momentum transfers.

X 射线衍射通常用于固定分子样品的结构测定。现代 X 射线光子源，例如来自自由电子激光器，使我们能够为这些散射事件增加时间分辨率，从而提供原子运动的电影。我们模拟并破译了对偶氮苯飞秒异构化（教科书光化学过程）的 X 射线衍射图谱的各种贡献。除了实时监测顺式到反式的分子电荷密度外，还编码了大量信息异构化。特别是，振动相干出现在锥形交叉点，通过混合弹性和非弹性光子散射对总衍射信号做出贡献。它们在动量空间中引起明显的相位调制，直接反映了非绝热通道期间电子跃迁密度的真实空间相位调制。为了克服总衍射信号中电子群体的强烈弹性散射贡献的掩蔽，我们讨论了如何检索这些信息，例如，通过使用非常硬的 X 射线来记录大的散射动量转移。