The stochastic nature of the self-amplified spontaneous emission (SASE) process in hard x-ray free electron laser (FEL) photon pulse generation often requires online spectral monitoring on a pulse-to-pulse basis. Experimenters and machine operators use the spectral information to better understand their data and guide the experiment, or to optimize the machine parameters through feedbacks to achieve a desired stability. Even experiments that use monochromators benefit from online spectral measurements as it allows the FEL operators to use machine feedbacks on the generated spectra so that the light would not jitter too far out of the monochromator’s acceptance bandwidth.
Online spectrometers have existed for several years in several facilities like LCLS, SACLA, and SwissFEL, and have mostly been used to characterize hard x-rays at photon energies typically above 4 keV. This abstract presents ongoing work on the development of the online tender x-ray spectrometer (TXS)], meant to cover the energy range between 2000 and 4000 eV and fill a gap for online spectral measurements at SwissFEL.
X-ray absorption spectroscopy (XAS) is a widely used technique for determining the electronic structure of matter. In contrast to X-ray photoelectron spectroscopy (XPS), XAS makes use of photons only, and therefore suffers less from absorption of the probe beam, i.e., electrons or photons, respectively. This is true for hard X-rays probing, e.g., the Kedges of d-metals in metal hydrides (albeit with limited chemical information). Soft X-rays, which are suited to analyze the electronic structure of hydrogen in solids, have a limited absorption length in gases. Photons with energies of less than 50 eV (“hydrogen K-edge” <;20 eV) are absorbed in less than 1 mm at ambient pressure, which is needed for technical hydrides. Recently, we developed a membrane-based approach to study materials exposed to high hydrogen “pressures” while keeping analysis chamber under high vacuum - thus effectively achieving high pressure XPS analysis. In this paper, we demonstrate that the membrane approach originally designed for XPS can be equally well used for XAS. We show first results on the electronic structure of hydrogen in Pd-Ag alloy as measured by in situ XAS using a laboratory extreme ultraviolet (EUV) source.
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