Multi-photon processes in atoms and molecules: The Small Quantum Systems (SQS) Instrument at the European XFEL

The Small Quantum Systems (SQS) scientific instrument at the European XFEL is dedicated to investigations on atoms, molecules, clusters and nanoparticles. The instrument is located at the SASE3 soft X-ray undulator, which produces intense FEL radiation in the photon energy range between 250 eV and 3000 eV. In the first months of operation the accelerator was delivering up to 2000 pulses per second to the SQS experimental station with pulse energies between 1 and 5 mJ. These characteristics in combination with a pulse duration of about 25 fs provide an ideal basis for numerous investigations involving site-selective core excitations and focusing on non-linear phenomena and the dynamics of ultrafast processes.

Three experimental stations, AQS (Atomic-like Quantum Systems), NQS (Nano-size Quantum Systems) and REMI (REaction MIcroscope), which are optimized for different samples and different type of experiments, were successfully brought into operation and are available for the user community. First scientific investigations were concentrating on studies of multi-photon processes [1-4] using high-resolution electron, ion and electron-ion coincidence spectroscopy as well as on studies of larger systems using coherent diffraction imaging. The experiments covered processes of (resonant) multiple ionization, above threshold ionization (ATI), double core hole (DCH) formation and stimulated X-ray Raman scattering (SXRS) in atoms as well as x-ray induced dissociation dynamics of small molecules, and highlighted the excellent performances of the instrument.

In the talk, after the presentation of the SQS instrument some of the recent scientific results will be discussed. Finally, the latest upgrades related to pump-probe experiments combining the soft X-ray pulses either with a synchronized optical laser or with another X-ray pulse of different photon energy will be described.

[1] G. Kastirke et al., Phys. Rev. X 10, 021052 (2020)
[2] U. Eichmann et al., Science 369, 1630 (2020)
[3] G. Kastirke et al., Phys. Rev. Lett. 125, 163201 (2020)
[4] T. Mazza et al., Phys. Rev. X 10, 041056 (2020)