The chaotic nature of x-ray free-electron-laser pulses is a major bottleneck that has limited the joint temporal and spectral resolution of spectroscopic measurements. We show how to use the stochastic x-ray field statistics to overcome this difficulty through correlation signals averaged over independent pulse realizations. No control is required over the spectral phase of the pulse, enabling immediate application of existing, noisy x-ray free-electron-laser pulses. The proposed stimulated Raman technique provides the combined broad observation bandwidth and high time-frequency resolution needed for the observation of elementary molecular events. A model is used to simulate chaotic free-electron-laser pulses and calculate their correlation properties. The resulting joint temporal and spectral resolution is exemplified for a molecular model system with time-dependent frequencies and for the RNA base uracil passing through a conical intersection. Ultrafast coherences, which constitute a direct signature of the nonadiabatic dynamics, are resolved. The detail and depth of physical information accessed by the proposed stochastic signal are virtually identical to those obtained by phase-controlled pulses.

中文翻译：

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具有随机自由电子激光脉冲的受激X射线拉曼信号的高时间和光谱分辨率

X射线自由电子激光脉冲的混沌性质是一个主要瓶颈，它限制了光谱测量的联合时间和光谱分辨率。我们展示了如何使用随机X射线场统计数据通过在独立脉冲实现上平均的相关信号来克服此困难。不需要控制脉冲的频谱相位，从而可以立即应用现有的，嘈杂的X射线自由电子激光脉冲。所提出的受激拉曼技术提供了观察分子基本事件所需的宽广的观察带宽和高时频分辨率的组合。使用模型来模拟混沌自由电子激光脉冲并计算其相关特性。所得到的联合时间和光谱分辨率以具有随时间变化的频率的分子模型系统和通过圆锥形交叉点的RNA基础尿嘧啶为例。解决了构成非绝热动力学的直接特征的超快相干性。所提出的随机信号访问的物理信息的细节和深度实际上与相位控制脉冲获得的信息相同。