The experimental set-up

Contact: michael.meyer@xfel.eu

 

The Small Quantum Systems (SQS) instru­ment at European XFEL is dedicated to the investigation of non-linear phenomena, strong field effects and pho­toinduced ionization and dissociation dynamics of gas phase samples, such as atoms, molecules, ions, nanoparticles and small bio-molecules.

The SQS instrument is installed behind the SASE3 soft X-ray undulator enabling experiments in the pho­ton energy range from about 250 eV (i.e. well below the C 1s threshold) up to about 3000 eV (i.e. close to the Ar 1s threshold). The undulator delivers ultra-short, intense X-ray pulses of up to 10 mJ pulse en­ergy and 20-30 fs pulse duration to the instrument. A set of highly polished focusing mirrors enable a spot size of about 1 micron in the interaction region, which corresponds to several 10¹⁸ W/cm² and provides an ideal basis for studies of nonlinear phenomena. Bending mirrors allow us to move the focus, thereby offering the installation of two experiment chambers at the same time at the instrument. The instrument supports three main experiment stations:

- the Atomic-like Quantum Systems (AQS) station for spectroscopic studies on atoms and small molecules, equipped with high-resolution electron and ion spectrometers and a 1D-imaging soft X-ray fluores­cence spectrometer.

- the Nano-size Quantum Systems (NQS) station for spectroscopic and coherent diffraction experi­ments on larger systems such as clusters, nano-par­ticles and bio-molecules (1 MPixel imaging detectors for low (pnCCD) and high (DSSC) rep. rate experi­ments.

- the Reaction Microscope (REMI) for coincidence experiments on small molecular systems, enabling the measurement of momenta of all dissociation products, electrons and ions.

 

 

Layout of the SQS scientific instrument.

 

Besides investigations of nonlinear processes, the di­rect observation of intra-molecular processes such as structural changes, and forming or breaking of in­dividual bonds can be addressed at SQS. The short pulse duration, for example, of about 20-30 fs (or even less upon special preparation), offers a perfect ground for the corresponding studies (generally fol­lowing a pump-probe type excitation scheme). In ad­dition to spectroscopic techniques, the complete analysis of all particles pro­duced in the process, i.e. of all electrons and ions, with a reaction microscope demonstrates the unique power of the REMI end­station [1]. Being able to extract the momenta of all fragmentation products in coincidence allows the re­construction of the molecular structure in the moment of dissociation. The first experiments at SQS using the REMI endstation took advantage of the high in­tensity of the FEL pulses and have opened up this field of dynamical studies at SQS.

Schematic representation of the REMI set-up.

 

Imaging the molecular structure at any moment of a chemical reaction is one of the central goals for the complete understanding of the process. X-ray diffrac­tion imaging has become a key technology for com­paratively large samples, such as proteins and nano­particles for example, but is completely inadequate for smaller molecules. For these samples, Coulomb Explosion Imaging (CEI) has provided excellent re­sults and is particularly suited for XFELs. Multi-pho­ton processes are used to charge up all atoms of a molecule and – due to the Coulomb repulsion of the generated ions – trigger its complete fragmentation. By measuring the emission directions and velocities of all fragments, the molecular geometry can be inferred, as shown for the aromatic molecule iodopryd­ine (C₅H₄IN) [2].

 

[1] G. Kastirke et al., Phys. Rev. X 10, 021052 (2020)
[2] R. Boll et al., Nature Physics 18, 423 (2022)

 

Homepage of the SQS instrument