Photon shot-noise limited transient absorption soft X-ray spectroscopy at the European XFEL

TL;DR

This paper introduces a specialized soft X-ray absorption spectroscopy setup at EuXFEL that achieves near photon shot-noise limited detection through innovative beam-splitting and high-speed imaging, enabling detailed out-of-equilibrium material studies.

Contribution

The paper presents a novel experimental setup combining a beam-splitting zone plate and high-speed detector to perform shot-by-shot normalized soft X-ray absorption measurements at EuXFEL.

Findings

Achieved photon shot-noise limited detection sensitivity.

Enabled real-time shot-by-shot analysis of transient absorption.

Demonstrated the setup's capability for out-of-equilibrium dynamics studies.

Abstract

Femtosecond transient soft X-ray Absorption Spectroscopy (XAS) is a very promising technique that can be employed at X-ray Free Electron Lasers (FELs) to investigate out-of-equilibrium dynamics for material and energy research. Here we present a dedicated setup for soft X-rays available at the Spectroscopy & Coherent Scattering (SCS) instrument at the European X-ray Free Electron Laser (EuXFEL). It consists of a beam-splitting off-axis zone plate (BOZ) used in transmission to create three copies of the incoming beam, which are used to measure the transmitted intensity through the excited and unexcited sample, as well as to monitor the incoming intensity. Since these three intensity signals are detected shot-by-shot and simultaneously, this setup allows normalized shot-by-shot analysis of the transmission. For photon detection, the DSSC imaging detector, which is capable of recording up to 800 images at 4.5 MHz frame rate during the FEL burst, is employed and allows approaching the photon shot-noise limit. We review the setup and its capabilities, as well as the online and offline analysis tools provided to users.