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The journal of physical chemistry / A 125, 10138���10143 (2021). doi:10.1021/acs.jpca.1c06033, We performed a time-resolved spectroscopy experixment on the dissociation of oxygen molecules after the interactionwith intense extreme-ultraviolet (XUV) light from the free-electronlaser in Hamburg at Deutsches Elektronen-Synchrotron. Using anXUV-pump/XUV-probe transient-absorption geometry with asplit-and-delay unit, we observe the onset of electronic transitionsin the O$^{2+}$ cation near 50 eV photon energy, marking the end ofthe progression from a molecule to two isolated atoms. We observetwo different time scales of 290 �� 53 and 180 �� 76 fs for theemergence of different ionic transitions, indicating differentdissociation pathways taken by the departing oxygen atoms.With regard to the emerging opportunities of tuning the centralfrequencies of pump and probe pulses and of increasing the probe���pulse bandwidth, future pump���probe transient-absorptionexperiments are expected to provide a detailed view of the coupled nuclear and electronic dynamics during molecular dissociation, Published by Soc., Washington, DC

Physical review / X 11(3), 031001 (1-9) (2021). doi:10.1103/PhysRevX.11.031001, In this work, we use an extreme-ultraviolet (XUV) free-electron laser (FEL) to resonantly excite the I: 4$d_{5/2}–σ^∗$ transition of a gas-phase di-iodomethane (CH$_2$I$_2$) target. This site-specific excitation generates a 4$d$ core hole located at an iodine site, which leaves the molecule in a well-defined excited state. We subsequently measure the time-dependent absorption change of the molecule with the FEL probe spectrum centered on the same I: 4$_d$ resonance. Using ab initio calculations of absorption spectra of a transient isomerization pathway observed in earlier studies, our time-resolved measurements allow us to assign the timescales of the previously reported direct and indirect dissociation pathways. The presented method is thus sensitive to excited-state molecular geometries in a time-resolved manner, following a core-resonant site-specific trigger., Published by APS, College Park, Md.

The free-free opacity in plasmas is fundamental to our understanding of energy transport in stellar interiors and for inertial confinement fusion research. However, theoretical predictions in the challenging dense plasma regime are conflicting and there is a dearth of accurate experimental data to allow for direct model validation. Here we present time-resolved transmission measurements in solid-density Al heated by an XUV free-electron laser. We use a novel functional optimization approach to extract the temperature-dependent absorption coefficient directly from an oversampled pool of single-shot measurements, and find a pronounced enhancement of the opacity as the plasma is heated to temperatures of order of the Fermi energy. Plasma heating and opacity enhancement are observed on ultrafast timescales, within the duration of the femtosecond XUV pulse. We attribute further rises in the opacity on ps timescales to melt and the formation of warm dense matter.

Physical review letters 123(10), 103001 (2019). doi:10.1103/PhysRevLett.123.103001, We demonstrate time-resolved nonlinear extreme-ultraviolet absorption spectroscopy on multiply charged ions, here applied to the doubly charged neon ion, driven by a phase-locked sequence of two intense free-electron laser pulses. Absorption signatures of resonance lines due to $2p-3d$ bound–bound transitions between the spin-orbit multiplets $^3P_{0,1,2}$ and $^3D_{1,2,3}$ of the transiently produced doubly charged Ne$^{2+}$ ion are revealed, with time-dependent spectral changes over a time-delay range of (2.4±0.3) fs. Furthermore, we observe 10-meV-scale spectral shifts of these resonances owing to the ac Stark effect. We use a time-dependent quantum model to explain the observations by an enhanced coupling of the ionic quantum states with the partially coherent free-electron laser radiation when the phase-locked pump and probe pulses precisely overlap in time., Published by APS, College Park, Md.

We present an extreme ultraviolet (EUV) microscope using a Schwarzschild objective which is optimized for single-shot sub-micrometer imaging of laser-plasma targets. The microscope has been designed and constructed for imaging the scattering from an EUV-heated solid-density hydrogen jet. Imaging of a cryogenic hydrogen target was demonstrated using single pulses of the free-electron laser in Hamburg (FLASH) free-electron laser at a wavelength of 13.5 nm. In a single exposure, we observe a hydrogen jet with ice fragments with a spatial resolution in the sub-micrometer range. In situ EUV imaging is expected to enable novel experimental capabilities for warm dense matter studies of micrometer-sized samples in laser-plasma experiments.

Structural dynamics 5(5), 054501 (2018). doi:10.1063/1.5042847, We present a general experimental concept for jitter-free pump and probe experiments at free electron lasers. By generating pump and probe pulse from one and the same X-ray pulse using an optical split-and-delay unit, we obtain a temporal resolution that is limited only by the X-ray pulse lengths. In a two-color X-ray pump and X-ray probe experiment with sub 70 fs temporal resolution, we selectively probe the response of orbital and charge degree of freedom in the prototypical functional oxide magnetite after photoexcitation. We find electronic order to be quenched on a time scale of (30 ± 30) fs and hence most likely faster than what is to be expected for any lattice dynamics. Our experimental result hints to the formation of a short lived transient state with decoupled electronic and lattice degree of freedom in magnetite. The excitation and relaxation mechanism for X-ray pumping is discussed within a simple model leading to the conclusion that within the first 10 fs the original photoexcitation decays into low-energy electronic excitations comparable to what is achieved by optical pump pulse excitation. Our findings show on which time scales dynamical decoupling of degrees of freedom in functional oxides can be expected and how to probe this selectively with soft X-ray pulses. Results can be expected to provide crucial information for theories for ultrafast behavior of materials and help to develop concepts for novel switching devices., Published by AIP Publishing LLC, Melville, NY

We use a Mach-Zehnder type autocorrelator to split and delay XUV pulses from the FLASH soft X-ray laser for triggering and subsequently probing the explosion of aerosolised sugar balls. FLASH was running at 182 eV photon energy with pulses of 70 fs duration. The delay between the pump-probe pulses was varied between zero and 5 ps, and the pulses were focused to reach peak intensities above 1016 W/cm2 with an off-axis parabola. The direct pulse triggered the explosion of single aerosolised sucrose nano-particles, while the delayed pulse probed the exploding structure. The ejected ions were measured by ion time of flight spectrometry, and the particle sizes were measured by coherent diffractive imaging. The results show that sucrose particles of 560-1000 nm diameter retain their size for about 500 fs following the first exposure. Significant sample expansion happens between 500 fs and 1 ps. We present simulations to support these observations.

We investigate subpicosecond dynamics of warm dense hydrogen at the XUV free-electron laser facility (FLASH) at DESY (Hamburg). Ultrafast impulsive electron heating is initiated by a â‰300-fs short x-ray burst of 92-eV photon energy. A second pulse probes the sample via x-ray scattering at jitter-free variable time delay. We show that the initial molecular structure dissociates within (0.9±0.2) ps, allowing us to infer the energy transfer rate between electrons and ions. We evaluate Saha and Thomas-Fermi ionization models in radiation hydrodynamics simulations, predicting plasma parameters that are subsequently used to calculate the static structure factor. A conductivity model for partially ionized plasma is validated by two-temperature density-functional theory coupled to molecular dynamic simulations and agrees with the experimental data. Our results provide important insights and the needed experimental data on transport properties of dense plasmas. © 2014 American Physical Society.

For the European x-ray free electron laser (XFEL) a split-and-delay unit based on geometrical wavefront beam splitting and multilayer mirrors is built which covers the range of photon energies from 5 keV up to 20 keV. Maximum delays between $\mathrm{\ensuremath{\Delta}}\ensuremath{\tau}=\ifmmode\pm\else\textpm\fi{}2.5\text{ }\text{ }\mathrm{ps}$ at $h\ensuremath{\nu}=20\text{ }\mathrm{keV}$ and up to $\mathrm{\ensuremath{\Delta}}\ensuremath{\tau}=\ifmmode\pm\else\textpm\fi{}23\text{ }\text{ }\mathrm{ps}$ at $h\ensuremath{\nu}=5\text{ }\text{ }\mathrm{keV}$ will be possible. Time-dependent wave-optics simulations have been performed by means of Synchrotron Radiation Workshop software for XFEL pulses at $h\ensuremath{\nu}=5\text{ }\text{ }\mathrm{keV}$. The XFEL radiation was simulated using results of time-dependent simulations applying the self-amplified spontaneous emission code FAST. Main features of the optical layout, including diffraction on the beam splitter edge and optics imperfections measured with a nanometer optic component measuring machine slope measuring profiler, were taken into account. The impact of these effects on the characterization of the temporal properties of XFEL pulses is analyzed. An approach based on fast Fourier transformation allows for the evaluation of the temporal coherence despite large wavefront distortions caused by the optics imperfections. In this way, the fringes resulting from time-dependent two-beam interference can be filtered and evaluated yielding a coherence time of ${\ensuremath{\tau}}_{c}=0.187\text{ }\text{ }\mathrm{fs}$ (HWHM) for real, nonperfect mirrors, while for ideal mirrors a coherence time of ${\ensuremath{\tau}}_{c}=0.191\text{ }\text{ }\mathrm{fs}$ (HWHM) is expected.

Journal of physics / B 46(16), 164005 (2013). doi:10.1088/0953-4075/46/16/164005, For time-resolved extreme ultraviolet (XUV) pump–XUV probe experiments at the Free electron LASer in Hamburg (FLASH), a split-and-delay unit (SDU) has been built. It is implemented in beamline BL2 which provides a focal spot size of about 20 μm diameter in the experiment. The beam is divided geometrically into two paths which can be delayed from −3 to +15 ps with respect to each other. The transmission up to 200 eV photon energy is above 35% in one beam path and 74% in the other. The latter transmits the XUV beam again from 305 to 570 eV (>1% transmission). Thus almost the whole spectral range at FLASH is covered by the SDU with reasonable transmission, including the option to transport high-energy third harmonic radiation in one of the beam paths. Both beam paths are realigned into the original direction of the radiation at the end of the SDU. Thus the utilization of the divided as well as the original beam is enabled by simply moving the optical elements of the SDU into or out of the beam. Using the SDU, the coherence length and the average pulse duration at FLASH was determined to be 0.9–1.8 μm, depending on the wavelength, and about 30 fs, respectively, for the specific electron bunch parameters., Published by IOP Publ., Bristol

Physical review / A 80(2), 4 (2009). doi:10.1103/PhysRevA.80.025402, The pulse duration of soft x-ray free-electron laser (FEL) radiation is directly measured by time-resolved observation of doubly charged helium ions at 51.8 eV. A wave front splitting autocorrelator produces two correlated FEL pulses with a resolution of better than a femtosecond. In the interesting intensity range from 10$_{13}$ to 10$_{16]$ W/cm$_2$ direct and sequential double ionization contribute to the ion yield which has significant influence on the correlation width, being a general feature at high photon energies. Here, a duration of τL=(29±5) fs is derived for the soft x-ray pulses at FLASH., Published by APS, College Park, Md.

The temporal coherence properties of soft x-ray free electron laser pulses at FLASH are measured at 23.9 nm by interfering two time-delayed partial beams directly on a CCD camera. The partial beams are obtained by wave front beam splitting in an autocorrelator operating at photon energies from h nu = 30 to 200 eV. At zero delay a visibility of (0.63+/- 0.04) is measured. The delay of one partial beam reveals a coherence time of 6 fs at 23.9 nm. The visibility further displays a non-monotonic decay, which can be rationalized by the presence of multiple pulse structure.