The rates and outcomes of virtually all photophysical and photochemical processes are determined by conical intersections. These are regions of degeneracy between electronic states on the nuclear landscape of molecules where electrons and nuclei evolve on comparable timescales and thus become strongly coupled, enabling radiationless relaxation channels upon optical excitation. Due to their ultrafast nature and vast complexity, monitoring conical intersections experimentally is an open challenge. We present a simulation study on the ultrafast photorelaxation of uracil, based on a quantum description of the nuclei. We demonstrate an additional window into conical intersections obtained by recording the transient wavepacket coherence during this passage with an X-ray free-electron laser pulse. Two major findings are reported. First, we find that the vibronic coherence at the conical intersection lives for several hundred femtoseconds and can be measured during this entire time. Second, the time-dependent energy-splitting landscape of the participating vibrational and electronic states is directly extracted from Wigner spectrograms of the signal. These offer a physical picture of the quantum conical intersection pathways through visualizing their transient vibronic coherence distributions. The path of a nuclear wavepacket in the vicinity of the conical intersection is directly mapped by the proposed experiment.

几乎所有光物理和光化学过程的速率和结果均由圆锥形交叉点确定。这些是分子核态上电子态之间的简并区域，在这些区域中，电子和核以可比较的时标演化并因此强耦合，从而在光激发时启用无辐射弛豫通道。由于其超快的特性和巨大的复杂性，通过实验监控圆锥形交叉口是一个开放的挑战。我们基于核的量子描述，提出了关于尿嘧啶超快速光松弛的模拟研究。我们演示了通过记录X射线自由电子激光脉冲在通过过程中的瞬态波包相干性而获得的圆锥形相交的附加窗口。报告了两个主要发现。第一，我们发现圆锥形相交处的振动相干寿命为数百飞秒，并且可以在整个时间内进行测量。其次，直接从信号的维格纳频谱图中提取参与的振动和电子状态随时间变化的能量分裂态势。这些通过可视化瞬态振动电子相干分布，提供了量子圆锥形相交路径的物理图像。所提出的实验直接绘制了圆锥形交叉点附近的核波包的路径。直接从信号的维格纳频谱图中提取出参与的振动和电子状态随时间变化的能量分裂态势。这些通过可视化瞬态振动电子相干分布，提供了量子圆锥形相交路径的物理图像。所提出的实验直接绘制了圆锥形交叉点附近的核波包的路径。直接从信号的维格纳频谱图中提取参与的振动和电子状态随时间变化的能量分裂态势。这些通过可视化瞬态振动电子相干分布提供了量子圆锥形相交路径的物理图像。所提出的实验直接绘制了圆锥形交叉点附近的核波包的路径。