Opportunities for Two-Color Experiments in the Soft X-ray Regime at the European XFEL

Abstract

X-ray pump/X-ray probe applications are made possible at X-ray Free Electron Laser (XFEL) facilities by generating two X-ray pulses with different wavelengths and controllable temporal delay. In order to enable this capability at the European XFEL, an upgrade project to equip the soft X-ray SASE3 beamline with a magnetic chicane is underway. In the present paper we describe the status of the project, its scientific focus and expected performance, including start-to-end simulations of the photon beam transport up to the sample, as well as recent experimental results demonstrating two-color lasing at photon energies of 805 eV + 835 eV and 910 eV + 950 eV. Additionally, we discuss methods to analyze the spectral properties and the intensity of the generated radiation to provide on-line diagnostics for future user experiments.Appl. Sci. 2020, 10, 2728 2 of 20 short pulse duration and the high intensity of FEL pulses [5,6]. Recent examples of this type of studies at FELs are related to the dissociation dynamics of molecules, magnetic ordering of material, as well as charge transfer processes [7]. One major challenge in these experiments is related to the inherent temporal jitter of the two independent laser sources and to its precise characterization in order to actually take advantage of the short pulse duration. Until now, only experiments using a seeded FEL in combination with the optical seed laser could demonstrate high temporal resolution reaching the sub-femtosecond time scale [8].New developments at FEL sources have shown that it is also possible to generate two FEL pulses at different photon energies, which enable pump-probe experiments [9][10][11]. This method has two major advantages; first, the temporal jitter is drastically reduced, since both pulses are produced by the same electron bunch and, second, pump and probe pulses are both of high photon energies. This allows, for example, to selectively address different sites in a molecule or different subshells in an atom for inducing a process (pump) as well as for monitoring its evolution (probe). Very soon after the start of operation of the European XFEL discussions were initiated about the possibilities to realize such a two-color pump-probe scheme at the SASE3 Soft X-ray branch of the facility.Already the simplest way of generating two-color pulses at the SASE3 beamline of the European XFEL, in combination with the high-repetition rate capabilities of the facility is expected to enable novel exciting science at the two soft X-ray instruments Small Quantum Systems (SQS) [12,13] and Spectroscopy & Coherent Scattering (SCS) [12,14].In the following we will summarize the scope of the project (Section 2) and the scientific background for the two-color pump-probe (2CPP) set-up at the European XFEL with the help of concrete proof-of-principle experimental proposals (Section 3) followed by the results of simulations for pulse generation and transport (Section 4), while in Section 5 we show the results of recent experiments demonstrating the first two-color lasi...

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