Multiple-core-hole resonance spectroscopy with ultraintense X-ray pulses.

Understanding the interaction of intense, femtosecond X-ray pulses with heavy atoms is crucial for gaining insights into the structure and dynamics of matter. One key aspect of nonlinear light-matter interaction was, so far, not studied systematically at free-electron lasers-its dependence on the photon energy. Here, we use resonant ion spectroscopy to map out the transient electronic structures occurring during the complex charge-up pathways of xenon. Massively hollow atoms featuring up to six simultaneous core holes determine the spectra at specific photon energies and charge states. We also illustrate how different X-ray pulse parameters, which are usually intertwined, can be partially disentangled. The extraction of resonance spectra is facilitated by the possibility of working with a constant number of photons per X-ray pulse at all photon energies and the fact that the ion yields become independent of the peak fluence beyond a saturation point. Our study lays the groundwork for spectroscopic investigations of transient atomic species in exotic, multiple-core-hole states that have not been explored previously.

Harmonic radiation contribution and X-ray transmission at the Small Quantum Systems instrument of European XFEL.

Transmission measurements of the soft X-ray beamline to the Small Quantum Systems (SQS) scientific instrument at the SASE3 undulator of European XFEL are presented. Measurements are reported for a wide range of photon energies (650 eV to 2400 eV), using X-ray gas monitors as well as a bolometric radiometer. The results are in good agreement with simulations for the beam transport and show a transmission of up to 80% over the whole photon energy range. The contribution of second- and third-harmonic radiation of the soft X-ray undulator is determined at selected photon energies by performing transmission measurements using a gas absorber to provide variable attenuation of the incoming photon flux. A comparison of the results with semi-analytic calculations for the generation of free-electron laser pulses in the SASE3 undulator reveals an influence of apertures along the beam transport on the exact harmonic content to be accounted for at the experiment. The second-harmonic content is measured to be in the range of 0.1% to 0.3%, while the third-harmonic contributed a few percent to the SASE3 emission. For experiments at the SQS instrument, these numbers can be reduced through specific selections of the mirror reflection angles.

The beam transport system for the Small Quantum Systems instrument at the European XFEL: optical layout and first commissioning results.

The Small Quantum Systems instrument is one of the six operating instruments of the European XFEL, dedicated to the atomic, molecular and cluster physics communities. The instrument started its user operation at the end of 2018 after a commissioning phase. The design and characterization of the beam transport system are described here. The X-ray optical components of the beamline are detailed, and the beamline performances, transmission and focusing capabilities are reported. It is shown that the X-ray beam can be effectively focused as predicted by ray-tracing simulations. The impact of non-ideal X-ray source conditions on the focusing performances is discussed.

High-resolution electron time-of-flight spectrometers for angle-resolved measurements at the SQS Instrument at the European XFEL.

A set of electron time-of-flight spectrometers for high-resolution angle-resolved spectroscopy was developed for the Small Quantum Systems (SQS) instrument at the SASE3 soft X-ray branch of the European XFEL. The resolving power of this spectrometer design is demonstrated to exceed 10 000 (E/ΔE), using the well known Ne 1s-13p resonant Auger spectrum measured at a photon energy of 867.11 eV at a third-generation synchrotron radiation source. At the European XFEL, a width of ∼0.5 eV full width at half-maximum for a kinetic energy of 800 eV was demonstrated. It is expected that this linewidth can be reached over a broad range of kinetic energies. An array of these spectrometers, with different angular orientations, is tailored for the Atomic-like Quantum Systems endstation for high-resolution angle-resolved spectroscopy of gaseous samples.

A localized view on molecular dissociation via electron-ion partial covariance.

Inner-shell photoelectron spectroscopy provides an element-specific probe of molecular structure, as core-electron binding energies are sensitive to the chemical environment. Short-wavelength femtosecond light sources, such as Free-Electron Lasers (FELs), even enable time-resolved site-specific investigations of molecular photochemistry. Here, we study the ultraviolet photodissociation of the prototypical chiral molecule 1-iodo-2-methylbutane, probed by extreme-ultraviolet (XUV) pulses from the Free-electron LASer in Hamburg (FLASH) through the ultrafast evolution of the iodine 4d binding energy. Methodologically, we employ electron-ion partial covariance imaging as a technique to isolate otherwise elusive features in a two-dimensional photoelectron spectrum arising from different photofragmentation pathways. The experimental and theoretical results for the time-resolved electron spectra of the 4d3/2 and 4d5/2 atomic and molecular levels that are disentangled by this method provide a key step towards studying structural and chemical changes from a specific spectator site.

Timing and X-ray pulse characterization at the Small Quantum Systems instrument of the European X-ray Free Electron Laser.

This contribution presents the initial characterization of the pump-probe performance at the Small Quantum Systems (SQS) instrument of the European X-ray Free Electron Laser. It is demonstrated that time-resolved experiments can be performed by measuring the X-ray/optical cross-correlation exploiting the laser-assisted Auger decay in neon. Applying time-of-arrival corrections based on simultaneous spectral encoding measurements allow us to significantly improve the temporal resolution of this experiment. These results pave the way for ultrafast pump-probe investigations of gaseous media at the SQS instrument combining intense and tunable soft X-rays with versatile optical laser capabilities.

Site-specific interrogation of an ionic chiral fragment during photolysis using an X-ray free-electron laser.

Short-wavelength free-electron lasers with their ultrashort pulses at high intensities have originated new approaches for tracking molecular dynamics from the vista of specific sites. X-ray pump X-ray probe schemes even allow to address individual atomic constituents with a 'trigger'-event that preludes the subsequent molecular dynamics while being able to selectively probe the evolving structure with a time-delayed second X-ray pulse. Here, we use a linearly polarized X-ray photon to trigger the photolysis of a prototypical chiral molecule, namely trifluoromethyloxirane (C3H3F3O), at the fluorine K-edge at around 700 eV. The created fluorine-containing fragments are then probed by a second, circularly polarized X-ray pulse of higher photon energy in order to investigate the chemically shifted inner-shell electrons of the ionic mother-fragment for their stereochemical sensitivity. We experimentally demonstrate and theoretically support how two-color X-ray pump X-ray probe experiments with polarization control enable XFELs as tools for chiral recognition.

Double Core-Hole Generation in O_{2} Molecules Using an X-Ray Free-Electron Laser: Molecular-Frame Photoelectron Angular Distributions.

We report on a multiparticle coincidence experiment performed at the European X-ray Free-Electron Laser at the Small Quantum Systems instrument using a COLTRIMS reaction microscope. By measuring two ions and two electrons in coincidence, we investigate double core-hole generation in O_{2} molecules in the gas phase. Single-site and two-site double core holes have been identified and their molecular-frame electron angular distributions have been obtained for a breakup of the oxygen molecule into two doubly charged ions. The measured distributions are compared to results of calculations performed within the frozen- and relaxed-core Hartree-Fock approximations.

Metrology characterization of ultraprecise bendable mirrors for the European XFEL: from offsite calibration to installation and commissioning.

The European XFEL requires long and ultraflat X-ray mirrors of high precision for the beam offset and distribution system [Altarelli et al. (2006), XFEL Technical Design Report, DESY 2006-097. DESY, Hamburg, Germany]. A general specification of the beam transport mirrors is a length of up to 950 mm and an optical surface with a deviation from a perfectly flat surface of <30 nm peak-to-valley and a figure error of <2 nm peak-to-valley. From a production point of view, such a mirror cannot be easily fabricated so, in each beamline, it is foreseen to have at least one mirror with bending capabilities. In this way, it is possible to correct the residual divergence of the beam in order to focus it in the correct position with high accuracy and repeatability. This is practically implemented using a mechanical bender in which the mirror is mounted and bent through a motorized actuator. One such system was characterized in the metrology lab using a large-aperture Fizeau interferometer and a capacitive sensor. It was then installed in the beamline and calibrated again using the X-ray beam. Here, the procedure is described and the two different methods are compared, stressing the differences and the possible explanations and improvements.

Monitoring of selected pharmaceuticals in surface waters of Croatia.

Sulfonamides, macrolides, torasemide, fumagillin, and chloramphenicol were simultaneously analyzed in surface water samples by using solid-phase extraction (SPE) and reversed-phase (RP) liquid chromatography-electrospray tandem mass spectrometry (LC-ESI-MS/MS). In the pre-concentration and clean-up process, the pH value of samples and volume of the solvent for extraction of analytes from cartridge were optimized. Extraction recoveries were high with values in the range from 62 to 115%. Limits of quantification (LoQ) were in the range from 0.02 to 0.2 µg L-1. Repeatability of the method was evaluated at LoQ and expressed as relative standard deviation (RSD). Calculated RSDs were low with values in the range from 2.4 to 14.5%. The method was successfully applied for analysis of surface water real samples. Samples were collected along the rivers in Croatia on four sampling sites in 2012 in Danube catchment areas, 19 sampling sites in Danube and Adriatic catchment areas in 2013, and another 19 places in 2014. Altogether, 20 target compounds were analyzed in 148 water samples and detected in 31 samples in range (0.1-5.3) µg L-1 or in 20.1% of samples. The most frequent and highest concentrations were detected for macrolide antibiotics. This is the first attempt of such monitoring in surface waters in Croatia.

Calibration and optimization of an x-ray bendable mirror using displacement-measuring sensors.

We propose a method to control and to adjust in a closed-loop a bendable x-ray mirror using displacement-measuring devices. For this purpose, the usage of capacitive and interferometric sensors is investigated and compared. We installed the sensors in a bender setup and used them to continuously measure the position and shape of the mirror in the lab. The sensors are vacuum-compatible such that the same concept can also be applied in final conditions. The measurement is used to keep the calibration of the system and to create a closed-loop control compensating for external influences: in a demonstration measurement, using a 950 mm long bendable mirror, the mirror sagitta is kept stable inside a range of 10 nm Peak-To-Valley (P-V).