Parallel Broadband Femtosecond Reflection Spectroscopy at a Soft X-Ray Free-Electron Laser

Abstract

X-ray absorption spectroscopy (XAS) and the directly linked X-ray reflectivity near absorption edges yield a wealth of specific information on the electronic structure around the resonantly addressed element. Observing the dynamic response of complex materials to optical excitations in pump–probe experiments requires high sensitivity to small changes in the spectra which in turn necessitates the brilliance of free electron laser (FEL) pulses. However, due to the fluctuating spectral content of pulses generated by self-amplified spontaneous emission (SASE), FEL experiments often struggle to reach the full sensitivity and time-resolution that FELs can in principle enable. Here, we implement a setup which solves two common challenges in this type of spectroscopy using FELs: First, we achieve a high spectral resolution by using a spectrometer downstream of the sample instead of a monochromator upstream of the sample. Thus, the full FEL bandwidth contributes to the measurement at the same time, and the FEL pulse duration is not elongated by a monochromator. Second, the FEL beam is divided into identical copies by a transmission grating beam splitter so that two spectra from separate spots on the sample (or from the sample and known reference) can be recorded in-parallel with the same spectrometer, enabling a spectrally resolved intensity normalization of pulse fluctuations in pump–probe scenarios. We analyze the capabilities of this setup around the oxygen K- and nickel L-edges recorded with third harmonic radiation of the free electron laser in Hamburg (FLASH), demonstrating the capability for pump–probe measurements with sensitivity to reflectivity changes on the per mill level.

Shot noise limited soft x-ray absorption spectroscopy in solution at a SASE-FEL using a transmission grating beam splitter

“…In the case of a liquid flat jet, the thickness often systematically varies in the vertical direction, 19 requiring appropriate corrections. On the other hand, dispersion downstream of the sample 46 allows for a small focus using the entire source bandwidth on the sample, provided that the sample can withstand the fluence of the full beam. Such a method then requires that one dimension of the detector is used for spectral resolution, so that the number of illuminated pixels per energy bin is substantially reduced.…”

Photon-In/Photon-Out X-ray Free-Electron Laser Studies of Radiolysis

“…Similar capabilities are currently being considered at other facilities [85,86]. Advanced beamsplitter techniques that simultaneously provide sample and reference probe beams onto a single 2D-detector promise to further enhance the sensitivity for transient absorption studies [87,88]. The incorporation of RIXS further discriminates transient species with overlapping absorption resonances, and while statistically limited at present 120 Hz repetition rates, will become more routine at the MHz repetition rates soon expected.…”

Ultrafast manipulation of the NiO antiferromagnetic order via sub-gap optical excitation

“…The time-resolved X-ray reflectivity experiments were carried out using the MUSIX end-station at beamline FL24 at the free-electron laser, FLASH, in Hamburg. 27,28 Experiments were carried out in reflection geometry as illustrated in Fig. 2(a).…”