Hard X-ray spectroscopy is an element specific probe of electronic state, but signals are weak and require intense light to study low concentration samples. Free electron laser facilities offer the highest intensity X-rays of any available light source. The light produced at such facilities is stochastic, with spikey, broadband spectra that change drastically from shot to shot. Here, using aqueous ferrocyanide, we show that the resonant X-ray emission (RXES) spectrum can be inferred by correlating for each shot the fluorescence intensity from the sample with spectra of the fluctuating, self-amplified spontaneous emission (SASE) source. We obtain resolved narrow and chemically rich information in core-to-valence transitions of the pre-edge region at the Fe K-edge. Our approach avoids monochromatization, provides higher photon flux to the sample, and allows non-resonant signals like elastic scattering to be simultaneously recorded. The spectra obtained match well with spectra measured using a monochromator. We also show that inaccurate measurements of the stochastic light spectra reduce the measurement efficiency of our approach.

硬X射线光谱是电子态的元素特异性探针，但信号较弱，需要强光才能研究低浓度样品。自由电子激光设备可提供任何可用光源中最高强度的 X 射线。这些设施产生的光是随机的，具有尖峰、宽带光谱，每次拍摄都会发生巨大变化。在这里，我们使用亚铁氰化物水溶液，表明可以通过将每次发射的样品荧光强度与波动的自放大自发发射（SASE）源的光谱相关联来推断共振 X 射线发射（RXES）光谱。我们在 Fe K 边缘的前边缘区域的核到价态跃迁中获得了解析的狭窄且化学丰富的信息。我们的方法避免单色化，为样品提供更高的光子通量，并允许同时记录弹性散射等非共振信号。获得的光谱与使用单色仪测量的光谱非常匹配。我们还表明，随机光谱的不准确测量降低了我们方法的测量效率。