X-ray absorption near-edge structure (XANES) spectroscopy provides element specificity and is a powerful experimental method to probe local unoccupied electronic structures. In the soft x-ray regime, it is especially well suited for the study of 3d-metals and light elements such as nitrogen. Recent developments in vacuum-compatible liquid flat jets have facilitated soft x-ray transmission spectroscopy on molecules in solution, providing information on valence charge distributions of heteroatoms and metal centers. Here, we demonstrate XANES spectroscopy of molecules in solution at the nitrogen K-edge, performed at FLASH, the Free-Electron Laser (FEL) in Hamburg. A split-beam referencing scheme optimally characterizes the strong shot-to-shot fluctuations intrinsic to the process of self-amplified spontaneous emission on which most FELs are based. Due to this normalization, a sensitivity of 1% relative transmission change is achieved, limited by fundamental photon shot noise. The effective FEL bandwidth is increased by streaking the electron energy over the FEL pulse train to measure a wider spectral window without changing FEL parameters. We propose modifications to the experimental setup with the potential of improving the instrument sensitivity by two orders of magnitude, thereby exploiting the high peak fluence of FELs to enable unprecedented sensitivity for femtosecond XANES spectroscopy on liquids in the soft x-ray spectral region.

中文翻译：

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使用透射光栅分束器在SASE-FEL溶液中限制散粒噪声的软X射线吸收光谱

X射线吸收近边缘结构（XANES）光谱提供元素特异性，并且是探测局部未占据的电子结构的有力实验方法。在软X射线状态下，它特别适合研究3d金属和轻元素（如氮）。真空兼容液体平面射流的最新发展促进了溶液中分子的软X射线透射光谱学，提供了有关杂原子和金属中心的价态电荷分布的信息。在这里，我们展示了氮K溶液中分子的XANES光谱-edge，在汉堡的FLASH（自由电子激光器（FEL））上执行。分割光束参考方案最佳地描述了大多数FEL所基于的自放大自发发射过程固有的强烈的逐次波动特征。由于这种归一化，受到基本光子散粒噪声的限制，实现了1％的相对透射率变化的灵敏度。有效的FEL带宽通过在FEL脉冲序列上增加电子能量来增加，从而在不更改FEL参数的情况下测量更宽的光谱窗口而增加。我们建议对实验装置进行修改，以将仪器灵敏度提高两个数量级，从而利用FEL的高峰通量来实现飞秒XANES光谱对软X射线光谱区域中液体的灵敏性。