High-intensity ultrashort pulses at extreme ultraviolet (XUV) and x-ray photon energies, delivered by state-of-the-art free-electron lasers (FELs), are revolutionizing the field of ultrafast ...spectroscopy. For crossing the next frontiers of research, precise, reliable and practical photonic tools for the spectro-temporal characterization of the pulses are becoming steadily more important. Here, we experimentally demonstrate a technique for the direct measurement of the frequency chirp of extreme-ultraviolet free-electron laser pulses based on fundamental nonlinear optics. It is implemented in XUV-only pump-probe transient-absorption geometry and provides in-situ information on the time-energy structure of FEL pulses. Using a rate-equation model for the time-dependent absorbance changes of an ionized neon target, we show how the frequency chirp can be directly extracted and quantified from measured data. Since the method does not rely on an additional external field, we expect a widespread implementation at FELs benefiting multiple science fields by in-situ on-target measurement and optimization of FEL-pulse properties.
The diversity of nanoparticle shapes generated by condensation from gaseous matter reflects the fundamental competition between thermodynamic equilibration and the persistence of metastable ...configurations during growth. In the kinetically limited regime, intermediate geometries that are favoured only in early formation stages can be imprinted in the finally observed ensemble of differently structured specimens. Here we demonstrate that single-shot wide-angle scattering of femtosecond soft X-ray free-electron laser pulses allows three-dimensional characterization of the resulting metastable nanoparticle structures. For individual free silver particles, which can be considered frozen in space for the duration of photon exposure, both shape and orientation are uncovered from measured scattering images. We identify regular shapes, including species with fivefold symmetry and surprisingly large aspect ratio up to particle radii of the order of 100 nm. Our approach includes scattering effects beyond Born's approximation and is remarkably efficient-opening up new routes in ultrafast nanophysics and free-electron laser science.
A split‐and‐delay unit for the extreme ultraviolet and soft X‐ray spectral regions has been built which enables time‐resolved experiments at beamlines FL23 and FL24 at the Free‐electron LASer in ...Hamburg (FLASH). Geometric wavefront splitting at a sharp edge of a beam splitting mirror is applied to split the incoming soft X‐ray pulse into two beams. Ni and Pt coatings at grazing incidence angles have been chosen in order to cover the whole spectral range of FLASH2 and beyond, up to hν = 1800 eV. In the variable beam path with a grazing incidence angle of ϑd = 1.8°, the total transmission (T) ranges are of the order of 0.48 < T < 0.84 for hν < 100 eV and T > 0.50 for 100 eV < hν < 650 eV with the Ni coating, and T > 0.06 for hν < 1800 eV for the Pt coating. For a fixed beam path with a grazing incidence angle of ϑf = 1.3°, a transmission of T > 0.61 with the Ni coating and T > 0.23 with a Pt coating is achieved. Soft X‐ray pump/soft X‐ray probe experiments are possible within a delay range of −5 ps < Δt < +18 ps with a nominal time resolution of tr = 66 as and a measured timing jitter of tj = 121 ± 2 as. First experiments with the split‐and‐delay unit determined the averaged coherence time of FLASH2 to be τc = 1.75 fs at λ = 8 nm, measured at a purposely reduced coherence of the free‐electron laser.
The properties of the recently installed split‐and‐delay unit at beamlines FL23 and FL24 at FLASH2 are presented. Its operational range, performance parameters and results of a first experiment are described.
We studied the nanoplasma formation and explosion dynamics of single large xenon clusters in ultrashort, intense x-ray free-electron laser pulses via ion spectroscopy. The simultaneous measurement of ...single-shot diffraction images enabled a single-cluster analysis that is free from any averaging over the cluster size and laser intensity distributions. The measured charge state-resolved ion energy spectra show narrow distributions with peak positions that scale linearly with final ion charge state. These two distinct signatures are attributed to highly efficient recombination that eventually leads to the dominant formation of neutral atoms in the cluster. The measured mean ion energies exceed the value expected without recombination by more than an order of magnitude, indicating that the energy release resulting from electron-ion recombination constitutes a previously unnoticed nanoplasma heating process. This conclusion is supported by results from semiclassical molecular dynamics simulations.
In this work, we use an extreme-ultraviolet (XUV) free-electron laser (FEL) to resonantly excite theI:4d5/2–σ*transition of a gas-phase di-iodomethane (CH2I2) target. This site-specific excitation ...generates a4dcore 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 sameI:4dresonance. 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.
We explore time-resolved Coulomb explosion induced by intense, extreme ultraviolet (XUV)
femtosecond pulses from a free-electron laser as a method to image photo-induced molecular
dynamics in two ...molecules, iodomethane and 2,6-difluoroiodobenzene. At an excitation
wavelength of 267 nm, the dominant reaction pathway in both molecules is neutral
dissociation via cleavage of the carbon–iodine bond. This allows investigating the
influence of the molecular environment on the absorption of an intense, femtosecond XUV
pulse and the subsequent Coulomb explosion process. We find that the XUV probe pulse
induces local inner-shell ionization of atomic iodine in dissociating iodomethane, in
contrast to non-selective ionization of all photofragments in difluoroiodobenzene. The
results reveal evidence of electron transfer from methyl and phenyl moieties to a multiply
charged iodine ion. In addition, indications for ultrafast charge rearrangement on the
phenyl radical are found, suggesting that time-resolved Coulomb explosion imaging is
sensitive to the localization of charge in extended molecules.
We have recorded the diffraction patterns from individual xenon clusters irradiated with intense extreme ultraviolet pulses to investigate the influence of light-induced electronic changes on the ...scattering response. The clusters were irradiated with short wavelength pulses in the wavelength regime of different 4d inner-shell resonances of neutral and ionic xenon, resulting in distinctly different optical properties from areas in the clusters with lower or higher charge states. The data show the emergence of a transient structure with a spatial extension of tens of nanometers within the otherwise homogeneous sample. Simulations indicate that ionization and nanoplasma formation result in a light-induced outer shell in the cluster with a strongly altered refractive index. The presented resonant scattering approach enables imaging of ultrafast electron dynamics on their natural timescale.
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