Femtosecond x-ray and electron diffraction hold promise to image the evolving structures of single molecules. We present a unified quantum-electrodynamical formulation of diffraction signals, based on the exact many-body nuclear + electronic wavefunction that can be extracted from quantum chemistry simulations. This gives a framework for analyzing various approximate molecular dynamics simulations. We show that the complete description of ultrafast diffraction signals contains interesting contributions involving mixed elastic and inelastic scattered photons that are usually masked by other larger contributions and are neglected. These terms include overlaps of nuclear wavepackets between different electronic states that provide an electronic decoherence mechanism and are important for the time-resolved imaging of conical intersections.

中文翻译：

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超快分子X射线和电子衍射中电子和核相干的特征

飞秒X射线和电子衍射有望对单个分子的演化结构进行成像。我们基于可从量子化学模拟中提取的精确多体核+电子波函数，给出了衍射信号的统一量子电动力学公式。这提供了用于分析各种近似分子动力学模拟的框架。我们表明，超快衍射信号的完整描述包含有趣的贡献，其中涉及混合的弹性和非弹性散射光子，这些光子通常被其他较大的贡献掩盖并且被忽略。这些术语包括不同电子状态之间的核波包的重叠，这些重叠提供了电子退相干机制，对于圆锥形交叉点的时间分辨成像很重要。