Resonant X-ray emission spectroscopy from broadband stochastic pulses at an X-ray free electron laser

Franklin D Fuller,Jan Kern Jan Kern,Tetsuo Katayama,Tsukasa Takanashi,Daehyun You,Makina Yabashi,Kiyoshi Ueda,Anton Loukianov,Yiwen Li,Thomas Fransson,Tsu-Chien Weng,Vittal K Yachandra,Roberto Alonso-Mori,Uwe Bergmann,Philippe Wernet,Junko Yano

doi:10.1038/s42004-021-00512-3

Abstract

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.

Highlights

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
The prospect of using spontaneous emission (SASE) light for resonant experiments is exciting because the bandwidth mismatch between the monochromator and source is avoided, increasing the number of photons that can be delivered to the sample for each X-ray Free Electron Lasers (XFELs) shot by up to 100 times
Reconstruction with around 2.5 times more shots and we see that this gives an improved agreement between monochromatic and stochastic measurements, mainly in the vicinity of 7.120 keV. Both monochromatic and stochastic measurements exhibit a RIXS shift of the pre-edge peak in Fig. 1a, where the peak appears at slightly lower emission energy than the emission does for higher incident photon energies

Summary

Introduction

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. Kayser et al.[10] demonstrated for the first time that hard X-ray absorption spectra can be experimentally obtained by correlation of X-ray emission with measurements of the stochastic XFEL pulse spectrum They were able to reconstruct Fe K-edge absorption of Fe2O3 nanoparticles. In combined time resolved scattering/spectroscopy experiments, the high intensity afforded by SASE pulses is crucial to the experimental viability of the scattering signal, so only spectroscopies which can be performed with a SASE beam like X-ray emission have been paired with them to date Such emission/scattering experiments have been used[14,15,16,17] to observe concurrent atomic motions via scattering and localized valence electronic state evolution via emission spectroscopy. Shorter X-ray pulses are needed to probe attosecond processes that are fundamental to electron-light interaction and electron-electron correlation

Methods

Results

Conclusion