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Physical review / A 104(5), 053102 (2021). doi:10.1103/PhysRevA.104.053102, The effects of postcollision interaction (PCI) in the time evolution of photoinduced Auger decay, assisted by a terahertz (THz) field, is theoretically investigated. We propose a time-dependent model which is based on the quantum-mechanical description of the photoinduced Auger process and quasiclassical description of the PCI. The suggested model is used for calculating the PCI distortion of the Auger spectrum at different temporal overlaps of the exciting extreme ultraviolet pulse and the streaking THz pulse., Published by Inst., Woodbury, NY

Journal of physics / B 54(8), 085601 (2021). doi:10.1088/1361-6455/abf154, Auger electron spectra were simulated in the presence of a terahertz streaking field to study post-collision interaction (PCI) effects in the time-evolution of photoinduced Auger decays. The PCI is characterized by the ratio of the spectral line width for Auger electron emission in opposite directions with respect to the THz-field. These calculations have been performed using the analytical semiclassical model developed by Bauch and Bonitz (2012 Physical Review A 85 053416). The results are shown for Ne(KLL), Ar(LMM), Kr(MNN) and Xe(NOO) Auger transitions whereby different possible experimental conditions were evaluated., Published by IOP Publ., Bristol

We present time-resolved ultraviolet-pump x-ray probe Auger spectra of 2-thiouracil. An ultraviolet induced shift towards higher kinetic energies is observed in the sulfur 2p Auger decay. The difference Auger spectra of pumped and unpumped molecules exhibit ultrafast dynamics in the shift amplitude, in which three phases can be recognized. In the first 100 fs, a shift towards higher kinetic energies is observed, followed by a 400 fs shift back to lower kinetic energies and a 1 ps shift again to higher kinetic energies. We use a simple Coulomb-model, aided by quantum chemical calculations of potential energy states, to deduce a C–S bond expansion within the first 100 fs. The bond elongation triggers internal conversion from the photoexcited S2 to the S1 state. Based on timescales, the subsequent dynamics can be interpreted in terms of S1 nuclear relaxation and S1-triplet internal conversion.

Physical review letters 118(1), 013002 (2017). doi:10.1103/PhysRevLett.118.013002, Intense, circularly polarized extreme-ultraviolet and near-infrared (NIR) laser pulses are combined to double ionize atomic helium via the oriented intermediate He+(3p) resonance state. Applying angle-resolved electron spectroscopy, we find a large photon helicity dependence of the spectrum and the angular distribution of the electrons ejected from the resonance by NIR multiphoton absorption. The measured circular dichroism is unexpectedly found to vary strongly as a function of the NIR intensity. The experimental data are well described by theoretical modeling and possible mechanisms are discussed., Published by APS, College Park, Md.

We report on soft X-ray scattering experiments on cryogenic hydrogen and simple metal samples. As a source of intense, ultrashort soft X-ray pulses we have used free-electron laser radiation at 92 eV photon energy from FLASH at DESY, Hamburg. X-ray pulses with energies up to 150 μJ and durations 15-50 fs provide interaction with the sample leading simultaneously to plasma formation and scattering. Experiments exploiting both of these interactions have been carried out, using the same experimental setup. Firstly, recording of soft X-ray inelastic scattering from near-solid density hydrogen plasmas at few electron volt temperatures confirms the feasibility of this diagnostics technique. Secondly, the soft X-ray excitation of few electron volt solid-density plasmas in bulk metal samples could be studied by recording soft X-ray line and continuum emission integrated over emission times from fs to ns. © 2009 Elsevier B.V.

New journal of physics 18(4), 043017 (2016). doi:10.1088/1367-2630/18/4/043017, The evolution of individual, large gas-phase xenon clusters, turned into a nanoplasma by a high power infrared laser pulse, is tracked from femtoseconds up to nanoseconds after laser excitation via coherent diffractive imaging, using ultra-short soft x-ray free electron laser pulses. A decline of scattering signal at high detection angles with increasing time delay indicates a softening of the cluster surface. Here we demonstrate, for the first time a representative speckle pattern of a new stage of cluster expansion for xenon clusters after a nanosecond irradiation. The analysis of the measured average speckle size and the envelope of the intensity distribution reveals a mean cluster size and length scale of internal density fluctuations. The measured diffraction patterns were reproduced by scattering simulations which assumed that the cluster expands with pronounced internal density fluctuations hundreds of picoseconds after excitation., Published by Dt. Physikalische Ges., [Bad Honnef]

The influence of prepulses on the conversion efficiency (CE) of laser radiation into 13 nm extreme ultraviolet (EUV) radiation, resulting from lithium-like oxygen ions from O20 μm water droplets, was investigated. The laser pulse durations in the experiment ranged from 200 fs to 120 ps. Applying prepulses preceding the main pulse at various delays of up to 11 ns, it was shown that the CE increases differently for each measured pulse duration. The strongest dependence on the introduction of a prepulse was observed for 2 ps laser pulses with a 20 mJ laser pulse energy. The EUV CE was improved by a factor of 15 by the introduction of a prepulse. Calculations on the atomic physics of oxygen ions and simulations of the laser–plasma interaction revealed the influence of the prepulse on the EUV yield.

Synopsis Intense, circularly polarized extreme-ultraviolet (XUV) and near-infrared (NIR) laser pulses are combined to double-ionize atomic helium via the oriented intermediate He+ (3p) resonance state. A strong and NIR-intensity dependent circular dichroism is accounted to a helicity dependent AC-Stark shift.

The ionization and fragmentation dynamics of iodine molecules (I2) are traced using very intense (∼1014 W cm−2) ultra-short (∼60 fs) light pulses with 87 eV photons of the Free-electron LASer at Hamburg (FLASH) in combination with a synchronized femtosecond optical laser. Within a pump–probe scheme the IR pulse initiates a molecular fragmentation and then, after an adjustable time delay, the system is exposed to an intense FEL pulse. This way we follow the creation of highly-charged molecular fragments as a function of time, and probe the dynamics of multi-photon absorption during the transition from a molecule to individual atoms.

The charge rearrangement in dissociating ${\mathrm{I}}_{2}^{n+}$ molecules is measured as a function of the internuclear distance $R$ using extreme ultraviolet pulses delivered by the free-electron laser in Hamburg. Within an extreme ultraviolet pump-probe scheme, the first pulse initiates dissociation by multiply ionizing ${\mathrm{I}}_{2}$, and the delayed probe pulse further ionizes one of the two fragments at a given time, thus triggering charge rearrangement at a well-defined $R$. The electron transfer between the fragments is monitored by analyzing the delay-dependent ion kinetic energies and charge states. The experimental results are in very good agreement with predictions of the classical over-the-barrier model demonstrating its validity in a thus far unexplored quasimolecular regime relevant for free-electron laser, plasma, and chemistry applications.

We report on a fully angular resolved spectrum of hard x-rays emitted from a laser produced plasma, which is generated by a laser intensity of 5 x 10 18 W/cm 2 on a tantalum surface. MeV x-rays were utilized to generate photoneutrons via photofission of beryllium.

The dissociation dynamics induced by a 100 fs, 400 nm laser pulse in a rotationally cold Br-2 sample was characterized by Coulomb explosion imaging (CEI) using a time-delayed extreme ultra-violet (XUV) FEL pulse, obtained from the Free electron LASer in Hamburg (FLASH). The momentum distribution of atomic fragments resulting from the 400 nm-induced dissociation was measured with a velocity map imaging spectrometer and used to monitor the internuclear distance as the molecule dissociated. By employing the simultaneously recorded in-house timing electro-optical sampling data, the time resolution of the final results could be improved to 300 fs, compared to the inherent 500 fs time-jitter of the FEL pulse. Before dissociation, the Br-2 molecules were transiently 'fixed in space' using laser-induced alignment. In addition, similar alignment techniques were used on CO2 molecules to allow the measurement of the photoelectron angular distribution (PAD) directly in the molecular frame (MF). Our results on MFPADs in aligned CO2 molecules, together with our investigation of the dissociation dynamics of the Br-2 molecules with CEI, show that information about the evolving molecular structure and electronic geometry can be retrieved from such experiments, therefore paving the way towards the study of complex non-adiabatic dynamics in molecules through XUV time-resolved photoion and photoelectron spectroscopy.

We present results of single-shot resonant magnetic scattering experiments of $\mathrm{Co}/\mathrm{Pt}$ multilayer systems using 100 fs long ultraintense pulses from an extreme ultraviolet (XUV) free-electron laser. An x-ray-induced breakdown of the resonant magnetic scattering channel during the pulse duration is observed at fluences of $5\text{ }\text{ }\mathrm{J}/{\mathrm{cm}}^{2}$. Simultaneously, the speckle contrast of the high-fluence scattering pattern is significantly reduced. We performed simulations of the nonequilibrium evolution of the $\mathrm{Co}/\mathrm{Pt}$ multilayer system during the XUV pulse duration. We find that the electronic state of the sample is strongly perturbed during the first few femtoseconds of exposure leading to an ultrafast quenching of the resonant magnetic scattering mechanism.

Single-shot time-of-flight spectra for Coulomb explosion of N-2 and I-2 molecules have been recorded at the Free Electron LASer in Hamburg (FLASH) and have been analysed using a partial covariance mapping technique. The partial covariance analysis unravels a detailed picture of all significant Coulomb explosion pathways, extending up to the N4+-N5+ channel for nitrogen and up to the I8+-I9+ channel for iodine. The observation of the latter channel is unexpected if only sequential ionization processes from the ground state ions are considered. The maximum kinetic energy release extracted from the covariance maps for each dissociation channel shows that Coulomb explosion of nitrogen molecules proceeds much faster than that of the iodine. The N-2 ionization dynamics is modelled using classical trajectory simulations in good agreement with the outcome of the experiments. The results suggest that covariance mapping of the Coulomb explosion can be used to measure the intensity and pulse duration of free-electron lasers.

We traced the femtosecond nuclear-wave-packet dynamics in ionic states of oxygen and nitrogen diatomic molecules employing 38-eV XUV-pump--XUV-probe experiments at the free-electron laser in Hamburg (FLASH). The nuclear dynamics is monitored via the detection of coincident ionic fragments using a reaction microscope and a split-mirror setup to generate the pump and probe pulses. By comparing measured kinetic-energy-release spectra with classical and quantum-mechanical simulations, we identified electronic states of the molecular ions that are populated by ionization of the neutral molecule. For specific fragment charge states, this comparison allows us to assess the relevance of specific dissociation paths.

Two-color ($\mathrm{x}\mathrm{\text{\ensuremath{-}}}\mathrm{ray}+\mathrm{\text{infrared}}$) electron spectroscopy is used for investigating laser-assisted $KLL$ Auger decay following $1s$ photoionization of atomic Ne with few-femtosecond x-ray pulses from the Linac Coherent Light Source. In an angle-resolved experiment, the overall width of the laser-modified Auger-electron spectrum and its structure change significantly as a function of the emission angle. The spectra are characterized by a strong intensity variation of the sidebands revealing a gross structure. This variation is caused, as predicted by theory, by the interference of electrons emitted at different times within the duration of one optical cycle of the infrared dressing laser, which almost coincides with the lifetime of the Ne $1s$ vacancy.

We introduce a time-domain approach to explore rotational dynamics caused by intramolecular coupling or the interaction with dissipative media. It pushes the time resolution toward the ultimate limit determined by the rotational period. Femtosecond pulses create a coherent superposition of two rotational states of carbon monoxide. The wave-packet motion is observed by subsequent Coulomb explosion, which results in a time-dependent asymmetry of spatial fragmentation patterns. The asymmetry oscillation prevails for at least 1 ns, covering more than 300 periods with no decoherence. Long time scans will allow weak perturbations of the order of $\ensuremath{\Delta}E/E={10}^{\ensuremath{-}4}$ to be discerned. Our conclusions are confirmed by a fully quantum-mechanical model.

Using a split-mirror stage combined with a reaction microscope, nonlinear autocorrelation traces of XUV pulses from the Free-electron LASer at Hamburg were recorded for ${\mathrm{N}}_{2}$ multiphoton-induced fragmentation into noncoincident ${\mathrm{N}}^{2+}$ and coincident ${\mathrm{N}}^{2+}+{\mathrm{N}}^{2+}$ channels. We find a pulse duration of $40\ifmmode\pm\else\textpm\fi{}10 \mathrm{fs}$ along with a sharp spike pointing to a coherence time of $4\ifmmode\pm\else\textpm\fi{}1 \mathrm{fs}$, almost twice as short as in previous observations. Both are well reproduced by a simulation based on the partial-coherence model that includes the molecular dynamics leading to an $~12\ensuremath{-}\mathrm{fs}$ substructure in the trace.

Two-photon excitation of a single-photon forbidden Auger resonance has been observed and investigated using the intense extreme ultraviolet radiation from the free electron laser in Hamburg. At the wavelength 26.9 nm (46 eV) two photons promoted a 3d core electron to the outer 4d shell. The subsequent Auger decay, as well as several nonlinear above threshold ionization processes, were studied by electron spectroscopy. The experimental data are in excellent agreement with theoretical predictions and analysis of the underlying multiphoton processes.

We investigate ultrafast (fs) electron dynamics in a liquid hydrogen sample, isochorically and volumetrically heated to a moderately coupled plasma state. Thomson scattering measurements using 91.8 eV photons from the free-electron laser in Hamburg (FLASH at DESY) show that the hydrogen plasma has been driven to a nonthermal state with an electron temperature of 13 eV and an ion temperature below 0.1 eV, while the free-electron density is 2.8x10{20} cm{-3}. For dense plasmas, our experimental data strongly support a nonequilibrium kinetics model that uses impact ionization cross sections based on classical free-electron collisions.

Physical review letters 105, 263002 (2010). doi:10.1103/PhysRevLett.105.263002, Published by APS, College Park, Md.

The advent of free electron laser (FEL) facilities capable of delivering high intensity pulses in the extreme-UV to X-ray spectral range has opened up a wide vista of opportunities to study and control light matter interactions in hitherto unexplored parameter regimes. In particular, current short wavelength FELs can uniquely drive non-linear processes mediated by inner shell electrons and in fields where the photon energy can be as high as 10 keV and so the corresponding optical period reaches below one attosecond. Combined with ultrafast optical lasers, or simply employing wavefront division, pump probe experiments can be performed with femtosecond time resolution. As single photon ionization of atoms and molecules is by now very well understood, they provide the ideal targets for early experiments by which not only FELs can be characterised and benchmarked but can also be the natural departure point in the hunt for non-linear behaviour of atomistic systems bathed in laser fields of ultrahigh photon ene...

In this Letter, we report the pioneering use of free electron laser radiation for the investigation of periodic crystalline structures. The diffraction properties of silver behenate single nanocrystals (5.8 nm periodicity) with the dimensions of $20\text{ }\text{ }\mathrm{nm}\ifmmode\times\else\texttimes\fi{}20\text{ }\text{ }\mathrm{nm}\ifmmode\times\else\texttimes\fi{}20\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ and as powder with grain sizes smaller than 200 nm were investigated with 8 nm free electron laser radiation in single-shot modus with 30 fs long free electron laser pulses. This work emphasizes the possibility of using soft x-ray free electron laser radiation for these crystallographic studies on a nanometer scale.

By use of high intensity XUV radiation from the FLASH free-electron laser at DESY, we have created highly excited exotic states of matter in solid-density aluminum samples. The XUV intensity is sufficiently high to excite an inner-shell electron from a large fraction of the atoms in the focal region. We show that soft-x-ray emission spectroscopy measurements reveal the electronic temperature and density of this highly excited system immediately after the excitation pulse, with detailed calculations of the electronic structure, based on finite-temperature density functional theory, in good agreement with the experimental results.

We present collective Thomson scattering with soft x-ray free electron laser radiation as a method to track the evolution of warm dense matter plasmas with ∼200 fs time resolution. In a pump-probe scheme an 800 nm laser heats a 20 ∼m hydrogen droplet to the plasma state. After a variable time delay in the order of ps the plasma is probed by an x-ray ultra violet (XUV) pulse which scatters from the target and is recorded spectrally. Alternatively, in a self-Thomson scattering experiment, a single XUV pulse heats the target while a portion of its photons are being scattered probing the target. From such inelastic x-ray scattering spectra free electron temperature and density can be inferred giving insight on relaxation time scales in plasmas as well as the equation of state. We prove the feasibility of this method in the XUV range utilizing the free electron laser facility in Hamburg, FLASH. We recorded Thomson scattering spectra for hydrogen plasma, both in the self-scattering and in the pump-probe mode using optical laser heating. © 2009 SPIE-.

Journal of physics / B 42, 141002 (2009). doi:10.1088/0953-4075/42/14/141002, Published by IOP Publ., Bristol

A simple theoretical model is used to interpret recent experimental results for two-photon double ionization (DI) of D2 at 38 eV. We show that the measured kinetic energy distribution associated with emission of two protons can be interpreted as a sum of two processes: a sequential and an instantaneous absorption of the two incident photons. These processes lead to peaks in di erent regions of the spaectrum.

We report the creation of solid-density aluminum plasma using free-electron laser (FEL) radiation at $13.5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ wavelength. Ultrashort pulses were focused on a bulk Al target, yielding an intensity of $2\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}$. The radiation emitted from the plasma was measured using an xuv spectrometer. Bremsstrahlung and line intensity ratios yield consistent electron temperatures of about $38\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, supported by radiation hydrodynamics simulations. This shows that xuv FELs heat up plasmas volumetrically and homogeneously at warm-dense-matter conditions, which are accurately characterized by xuv spectroscopy.

Few-photon multiple ionization of N2 was studied differentially in a reaction microscope using 44 eV, approximately 25 fs, intense ( approximately 10(13) W/cm(2)) photon pulses from FLASH. Sequential ionization is observed to dominate. For various intermediate charge states N(2)(n+0 we find a considerable excess of photons absorbed compared to the minimum number that would energetically be required. Photoionization of aligned N(2)(n+) ions, produced by photon absorption in sequential steps, is explored and few-photon absorption pathways are traced by inspecting kinetic energy releases and fragment-ion angular distributions.

Recoil-ion momentum distributions for two-photon double ionization of He and Ne (variant Planck's over omega=44 eV) have been recorded with a reaction microscope at FLASH (the free-electron laser at Hamburg) at an intensity of approximately 1 x 10(14) W/cm2 exploring the dynamics of the two fundamental two-photon-two-electron reaction pathways, namely, sequential and direct (or nonsequential) absorption of the photons. We find strong differences in the recoil-ion momentum patterns for the two mechanisms pointing to the significantly different two-electron emission dynamics and thus provide serious constraints for theoretical models.

Few-photon multiple ionization of Ne and Ar atoms by strong vacuum ultraviolet laser pulses from the free-electron laser at Hamburg was investigated differentially with the Heidelberg reaction microscope. The light-intensity dependence of ${\mathrm{Ne}}^{2+}$ production reveals the dominance of nonsequential two-photon double ionization at intensities of $Il6\ifmmode\times\else\texttimes\fi{}{10}^{12}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$ and significant contributions of three-photon ionization as $I$ increases. ${\mathrm{Ne}}^{2+}$ recoil-ion-momentum distributions suggest that two electrons absorbing ``instantaneously'' two photons are ejected most likely into opposite hemispheres with similar energies.

We propose a collective Thomson scattering experiment at the VUV free electron laser facility at DESY (FLASH) which aims to diagnose warm dense matter at near-solid density. The plasma region of interest marks the transition from an ideal plasma to a correlated and degenerate many-particle system and is of current interest, e.g. in ICF experiments or laboratory astrophysics. Plasma diagnostic of such plasmas is a longstanding issue. The collective electron plasma mode (plasmon) is revealed in a pump-probe scattering experiment using the high-brilliant radiation to probe the plasma. The distinctive scattering features allow to infer basic plasma properties. For plasmas in thermal equilibrium the electron density and temperature is determined from scattering off the plasmon mode., Comment: 25 pages, 10 figures. Proceedings of the international conference "Radiative Properties of Hot Dense Matter", Sept. 11-15, 2006, Albufeira, Portugal. To appear in"High Energy Density Physics"

We demonstrate a novel method to monitor the total angular distribution of the spectrum of hard x-ray emission from a plasma generated with femtosecond laser pulses with an intensity of $5\ifmmode\times\else\texttimes\fi{}{10}^{18}\phantom{\rule{0ex}{0ex}}\mathrm{W}/{\mathrm{cm}}^{2}$ on a solid target. Measured and calculated angular distributions of x rays show a pronounced anisotropy for MeV photon energies. We complemented the spectral information by demonstrating a $(\ensuremath{\gamma},n)$ nuclear reaction with a tabletop laser system.

In this work we report the ionization of a gaseous neon target by combining extreme ultraviolet (XUV) radiation from the Free Electron Laser in Hamburg with an intense synchronized optical laser. The photoelectron spectrum dependence of the electrons ejected from singly-charged neon (Ne+) on the relative polarisation of the XUV and optical laser fields was investigated both experimentally and theoretically.

The influence of an intense optical laser on the electronic relaxation of the 3d–5p resonance in atomic Kr has been studied experimentally and theoretically. The resonance profile undergoes a strong modification, observed as a shift and a broadening of the excitation spectrum, as a function of the optical intensity. The theoretical treatment of the process demonstrates the importance of strongly time-dependent dynamics as the origin of the observed phenomena resulting in the ponderomotive shift of the resonance position as well as a competition between resonant and normal Auger decay. The ionization of the excited 5p electron by the optical laser provides the possibility to change the ratio between singly and doubly charged final states by controlling the relaxation of the resonant core hole state via resonant or normal Auger decay.

In this work we report on the ionization of a gaseous neon target by combining extreme ultraviolet (XUV) radiation from the Free Electron Laser in Hamburg with an intense synchronized optical laser. Applying energy-resolved photoelectron spectroscopy, the dependence of the electrons ejected from singly charged neon (Ne+) on the relative polarization of the XUV and optical laser fields was investigated. The electron spectra exhibit a strong dependence on the degree of the relative orientation of the linear polarization vectors. A quantitative analysis was undertaken using multichannel time-dependent perturbation theory in order to reproduce the magnitude of the ejected low kinetic energy electrons as a function of the relative polarization directions and explicitly taking into account the influence of the residual Ne2 + cores (1D,3P). It is concluded that the variation of the photoelectron spectrum with the deviation of XUV and optical field directions is mainly related to the different responses of the magnetic substates of the ejected electrons in their interaction with the laser field.

First experiments on atomic photoionization and molecular dissociation have been performed by taking advantage of the unprecedented characteristics of the free electron laser in Hamburg (FLASH) combined with a separate near-infrared (NIR) femtosecond laser. In a series of two-colour experiments, the photoionization of rare gases in the presence of a strong NIR dressing field as well as the polarization dependence of this process were investigated systematically. A detailed analysis of the partial cross sections for the two-colour two-photon ionization process was carried out for low dressing fields. Higher dressing fields gave rise to multi-photon processes, which were observed and analysed without undesirable interferences, a beneficial consequence of the monochromaticity of the FLASH radiation. The experimental results were compared with theoretical descriptions for two-colour above-threshold ionization obtained by employing second-order perturbation theory and the 'soft-photon' approximation. In addition, complementary information was obtained on the sequential two-photon double ionization of Ne, which was made possible by the short and intense FLASH pulses. As a starting point for future time-resolved studies of molecular dissociation, a proof-of-principle experiment on the hydrogen diatomic system was carried out. In a typical pump–probe arrangement, excited neutral fragments, which were formed during photo-induced dissociation by the FLASH radiation, were identified via single- and multi-photon ionization induced by the time-delayed optical laser.

Multiple ionization and fragmentation of N2 induced by absorption of few photons (44 and 46 eV) has been explored in pump-probe measurements at the free electron laser FLASH [1] at Hamburg. Photo ionization of aligned N2+ ions, produced by photon absorption in sequential steps, is explored and few-photon absorption pathways are traced by inspecting the kinetic energy releases (KER) and the fragment-ion angular distributions as function of the pump-probe delay time.

A method has been developed and applied to measure the beam waist and spot size of a focused soft x-ray beam at the free-electron laser FLASH of the Deutsches Elektronen-Synchrotron in Hamburg. The method is based on a saturation effect upon atomic photoionization and represents an indestructible tool for the characterization of powerful beams of ionizing electromagnetic radiation. At the microfocus beamline BL2 at FLASH, a full width at half maximum focus diameter of (15±2)μm was determined.

Static and time-resolved photoelectron diffraction of laser-aligned polyatomic molecules photoionized by Free-Electron Laser pulses is presented. The results are a proof-of-principle for imaging dynamic structural changes in molecules with femtosecond temporal and Angstrom spatial resolution.