[en] We have measured the relative photoionization cross section for the formation of Li²⁺ ions between 148 and 161 eV photon energy with higher photon-energy resolution than in previous Li²⁺ studies. This energy region is characterized by double and triple excitations that lead to strong enhancements in the Li²⁺ cross section. As a result, the double-to-single photoionization ratio shows a dramatic resonance structure not seen before. We have determined the resonance positions and widths using Fano-profile fits to the Li²⁺ data and compare them to previously published values and a calculated Li²⁺ cross-section curve

[en] Using monochromatized synchrotron light and the ion time-of-flight method to detect the singly and doubly charged sodium and potassium ions, we have investigated the relative double-photoionization cross-section ratios of Na from 52.4 to 150 eV and of K from 36.5 to 170 eV. We found that both followed a scaling law previously observed for Li and Be, which describes the photon energy dependence of that ratio and allows one to estimate its absolute value. We have also obtained the relative single- and double-photoionization cross sections for Na and K

[en] Due to the stochastic nature of the Self Amplifying Spontaneous Emission (SASE) process and the resulting pulse-to-pulse fluctuations of the Free Electron Laser (FEL) photon energies, experimenters working with FELs need to get real-time feedback about the photon properties for their experiments. Investigations of narrow atomic or molecular resonances, phase transitions, or any other kind of effect heavily dependent on photon energy would need to know the precise FEL photon energy for each individual photon bunch. Furthermore, any spectrometer developed to deliver the information of these properties should not significantly interfere or degrade the FEL beam. Therefore, the group at the Free Electron Laser in Hamburg (FLASH) has developed an online photoionization spectrometer that uses ion time of flight (I-TOF) measurement methods on noble gases to measure the photon energy of each pulse. This paper presents the first test results for the viability of this online photoionization spectrometer (OPS).

[en] We examined the relative ionization and fragmentation probabilities of C₆₀ after excitation by monochromatized synchrotron radiation in the energy range of 18-280 eV. The energy dependence of the relative cross sections of C₆₀^q+ (q=1-4) and the associated fragments C_60-2n^q+ (n=1-6 for q=2,3 and n=1,2 for q=4) are analyzed. We present the ionization and appearance energies for those ions and fragments as well as the partial cross sections for the C₆₀ ions

[en] We have measured the doubly excited states of ground-state magnesium below the Mg⁺(3p) threshold using highly monochromatized synchrotron radiation. We have observed a 3s² → 3pnd Rydberg series that has not been detected before. We compare our high-resolution measurement with previously obtained experimental and theoretical resonance positions, widths and quantum defects when available

[en] The ESB instrument at the SwissFEL ARAMIS hard X-ray free electron laser is designed to perform pump-probe experiments in condensed matter and material science employing photon-in and photon-out techniques. It includes a femtosecond optical laser system to generate a variety of pump beams, a X-ray optical scheme to tailor the X-ray probe beam, shot-to-shot diagnostics to monitor the X-ray intensity and arrival time, and two endstations operated at a single focus position that include multi-purpose sample environments and 2D pixel detectors for data collection.

[en] FLASH, the Free-electron LASer in Hamburg, is a worldwide unique source for extremely bright ultra-short laser-like pulses tunable in a wide spectral range in the extreme ultraviolet and soft x-ray region (Ackermann et al 2007 Nat. Photonics 1 336-42). To fully exploit the features of this new generation of light sources, a user facility with efficient radiation transport to the experimental area and novel online photon diagnostics capable of characterizing the unique parameters of the FLASH radiation has been built. It serves a broad user community active in many scientific fields ranging from atomic and molecular physics to plasma and solid state physics as well as chemistry and biology. A special focus is placed on the exploitation of the ultra-short FLASH pulses using pump-probe techniques. Thus, the facility is equipped with optical and THz sources synchronized to FLASH. This paper gives a detailed overview of the FLASH user facility.

[en] Exceptional behavior of light-matter interaction in the extreme ultraviolet is demonstrated. The photoionization of different rare gases was compared at the free-electron laser in Hamburg, FLASH, by applying ion spectroscopy at the wavelength of 13.7 nm and irradiance levels of thousands of terawatts per square centimeter. In the case of xenon, the degree of nonlinear photoionization was found to be significantly higher than for neon, argon, and krypton. This target specific behavior cannot be explained by the standard theories developed for optical strong-field phenomena. We suspect that the collective giant 4d resonance of xenon is the driving force behind the effect that arises in this spectral range.

[en] We have observed the simultaneous inner-shell absorption of two extreme-ultraviolet photons by a Xe atom in an experiment performed at the short-wavelength free electron laser facility FLASH. Photoelectron spectroscopy permitted us to unambiguously identify a feature resulting from the ionization of a single electron of the 4d subshell of Xe by two photons each of energy (93±1) eV. The feature's intensity has a quadratic dependence on the pulse energy. The results are discussed and interpreted within the framework of recent results of ion spectroscopy experiments of Xe obtained at ultrahigh irradiance in the extreme-ultraviolet regime.

[en] We use extremely intense, ultrashort soft X-Ray pulses generated by the LCLS X-Ray Free Electron Laser to investigate the production of molecular double core-hole states by sequential two-photon X-Ray absorption. The effect of critical LCLS parameters such as the number of photons per pulse, the pulse duration, and the focal spot size on the photoelectron and Auger spectra is modeled in detail and the results of these simulations are used as an aid in the interpretation of the experimental spectra obtained. The emphasis is on double core-hole formation in small polyatomic molecules such as CO₂.