Abstract
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.
Highlights
X-ray absorption near-edge structure (XANES) spectroscopy is one of the most common methods of x-ray spectroscopy, providing detailed information on local electronic structures: the position and magnitude of spectral features reports on oxidation states, spin configurations, and chemical bonds with element selectivity for light elements such as carbon, nitrogen, and oxygen as well as the important 3d transition metals.[1]
As this method can enable very high sensitivity, versions of it have been demonstrated at SPring-8 Angstrom Compact free electron LAser (SACLA)[43,44] Linac Coherent Light Source (LCLS),[45] and free-electron laser in Hamburg (FLASH),[22,46] showing considerably improved normalization of Spontaneous Emission (SASE) fluctuations compared to previous normalization schemes
Implementing the same principle of split-beam normalization, we present the acquisition of XANES spectra from femtosecond Free-Electron Laser (FEL) pulses with a sensitivity limited by photon counting statistics in the soft x-ray range on a liquid sample
Summary
X-ray absorption near-edge structure (XANES) spectroscopy is one of the most common methods of x-ray spectroscopy, providing detailed information on local electronic structures: the position and magnitude of spectral features reports on oxidation states, spin configurations, and chemical bonds with element selectivity for light elements such as carbon, nitrogen, and oxygen as well as the important 3d transition metals.[1]. We further lay out potential improvements to the optical setup, which will increase the effective photon flux by up to four orders of magnitude, increasing the shot-noise limit on sensitivity by about two orders of magnitude for a similar measurement duration. With these improvements, we expect that such experiments at soft x-ray FELs with superconducting accelerators will enable unique sensitivity for ultrafast absorption studies
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