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The free expansion of a planar plasma surface is a fundamental non-equilibrium process relevant for various fields but as-yet experimentally still difficult to capture. The significance of the associated spatiotemporal plasma motion ranges from astrophysics and controlled fusion to laser machining, surface high-harmonic generation, plasma mirrors, and laser-driven particle acceleration. Here, we show that x-ray coherent diffractive imaging can surpass existing approaches and enables the quantitative real-time analysis of the sudden free expansion of laser-heated nanoplasmas. For laser-ionized SiO2 nanospheres, we resolve the formation of the emerging nearly self-similar plasma profile evolution and expose the so far inaccessible shell-wise expansion dynamics including the associated startup delay and rarefaction front velocity. Our results establish time-resolved diffractive imaging as an accurate quantitative diagnostic platform for tracing and characterizing plasma expansion and indicate the possibility to resolve various laser-driven processes including shock formation and wave-breaking phenomena with unprecedented resolution.

Free-electron lasers providing ultra-short high-brightness pulses of X-ray radiation have great potential for a wide impact on science, and are a critical element for unravelling the structural dynamics of matter. To fully harness this potential, we must accurately know the X-ray properties: intensity, spectrum and temporal profile. Owing to the inherent fluctuations in free-electron lasers, this mandates a full characterization of the properties for each and every pulse. While diagnostics of these properties exist, they are often invasive and many cannot operate at a high-repetition rate. Here, we present a technique for circumventing this limitation. Employing a machine learning strategy, we can accurately predict X-ray properties for every shot using only parameters that are easily recorded at high-repetition rate, by training a model on a small set of fully diagnosed pulses. This opens the door to fully realizing the promise of next-generation high-repetition rate X-ray lasers., X-ray free-electron lasers, important light sources for materials research, suffer from shot-to-shot fluctuations that necessitate complex diagnostics. Here, the authors apply machine learning to accurately predict pulse properties, using parameters that can be acquired at high-repetition rates.

Many photoinduced processes including photosynthesis and human vision happen in organic molecules and involve coupled femtosecond dynamics of nuclei and electrons. Organic molecules with heteroatoms often possess an important excited-state relaxation channel from an optically allowed ππ* to a dark nπ* state. The ππ*/nπ* internal conversion is difficult to investigate, as most spectroscopic methods are not exclusively sensitive to changes in the excited-state electronic structure. Here, we report achieving the required sensitivity by exploiting the element and site specificity of near-edge soft X-ray absorption spectroscopy. As a hole forms in the n orbital during ππ*/nπ* internal conversion, the absorption spectrum at the heteroatom K-edge exhibits an additional resonance. We demonstrate the concept using the nucleobase thymine at the oxygen K-edge, and unambiguously show that ππ*/nπ* internal conversion takes place within (60 ± 30) fs. High-level-coupled cluster calculations confirm the method’s impressive electronic structure sensitivity for excited-state investigations., Many photo-induced processes such as photosynthesis occur in organic molecules, but their femtosecond excited-state dynamics are difficult to track. Here, the authors exploit the element and site selectivity of soft X-ray absorption to sensitively follow the ultrafast ππ*/nπ* electronic relaxation of hetero-organic molecules.

Intense, coherent X-ray pulses from a free-electron laser can be used to obtain high-resolution morphology of individual sub-micrometre particles in their native state, while at the same time their composition is analysed by mass spectrometry. Aerosol particles are of importance in fields as diverse as materials engineering, toxicology and climate change, yet it is difficult to analyse the structure and properties of these materials in their native environment. This paper reports an in situ method for imaging individual sub-micrometre-sized particles to nanometre resolution in flight using intense X-ray pulses from the Linac Coherent Light Source free-electron laser. The technique can also simultaneously carry out compositional analysis using time-of-flight mass spectrometry. The morphology of micrometre-size particulate matter is of critical importance in fields ranging from toxicology1 to climate science2, yet these properties are surprisingly difficult to measure in the particles’ native environment. Electron microscopy requires collection of particles on a substrate3; visible light scattering provides insufficient resolution4; and X-ray synchrotron studies have been limited to ensembles of particles5. Here we demonstrate an in situ method for imaging individual sub-micrometre particles to nanometre resolution in their native environment, using intense, coherent X-ray pulses from the Linac Coherent Light Source6 free-electron laser. We introduced individual aerosol particles into the pulsed X-ray beam, which is sufficiently intense that diffraction from individual particles can be measured for morphological analysis. At the same time, ion fragments ejected from the beam were analysed using mass spectrometry, to determine the composition of single aerosol particles. Our results show the extent of internal dilation symmetry of individual soot particles subject to non-equilibrium aggregation, and the surprisingly large variability in their fractal dimensions. More broadly, our methods can be extended to resolve both static and dynamic morphology of general ensembles of disordered particles. Such general morphology has implications in topics such as solvent accessibilities in proteins7, vibrational energy transfer by the hydrodynamic interaction of amino acids8, and large-scale production of nanoscale structures by flame synthesis9.

Stimulated electronic x-ray Raman scattering is the building block for several proposed x-ray pump probe techniques, that would allow the study of electron dynamics at unprecedented timescales. We present high spectral resolution data on stimulated electronic x-ray Raman scattering in a gas sample of neon using a self-amplified spontaneous emission x-ray free-electron laser. Despite the limited spectral coherence and broad bandwidth of these sources, high-resolution spectra can be obtained by statistical methods, opening the path to coherent stimulated x-ray Raman spectroscopy. An extension of these ideas to molecules and the results of a recent experiment in CO are discussed.

Intense femtosecond x-ray pulses from free-electron laser sources allow the imaging of individual particles in a single shot. Early experiments at the Linac Coherent Light Source (LCLS) have led to rapid progress in the field and, so far, coherent diffractive images have been recorded from biological specimens, aerosols, and quantum systems with a few-tens-of-nanometers resolution. In March 2014, LCLS held a workshop to discuss the scientific and technical challenges for reaching the ultimate goal of atomic resolution with single-shot coherent diffractive imaging. This paper summarizes the workshop findings and presents the roadmap toward reaching atomic resolution, 3D imaging at free-electron laser sources.

When exposed to ultraintense x-radiation sources such as free electron lasers (FELs) the innermost electronic shell can efficiently be emptied, creating a transient hollow atom or molecule. Understanding the femtosecond dynamics of such systems is fundamental to achieving atomic resolution in flash diffraction imaging of noncrystallized complex biological samples. We demonstrate the capacity of a correlation method called "partial covariance mapping'' to probe the electron dynamics of neon atoms exposed to intense 8 fs pulses of 1062 eV photons. A complete picture of ionization processes competing in hollow atom formation and decay is visualized with unprecedented ease and the map reveals hitherto unobserved nonlinear sequences of photoionization and Auger events. The technique is particularly well suited to the high counting rate inherent in FEL experiments.

We are designing and building the next generation multi-purpose instrumentation especially adapted to accommodate unique large-area, single-photon counting pnCCD detectors together with advanced many-particle ion and electron imaging spectrometers (reaction microscope, REMI; velocity map imaging, VMI; magnetic bottle) for simultaneous detection of scattered and fluorescent photons and charged particles in experiments at the LCLS FEL.

We present our first results of a UV-pump X-ray-probe study of the photoprotection mechanism of thymine. The experiment used element specific Auger spectroscopy and was carried out at the LCLS.

With the first X-ray free-electron laser (FEL), the Linac Coherent Light Source (LCLS), multiphoton ionization has been pushed to a new regime, where atoms and molecules are not just ionized by a series of valence ionizations but "from the inside out". At unprecedented high intensities and short pulse durations in the soft X-ray regime, a series of inner-shell photoionizations followed by cascades of Auger decays was observed to lead to highly charged final states in rare gases such as Ne, Ar, Kr, and Xe. Ion time-of-flight and fluorescence spectra were recorded for different FEL pulse energies and pulse lengths and compared to theoretical models to explain the underlying processes that lead to unexpectedly high charge states in Xe.

Nonlinear absorption of femtosecond kilovolt x-rays in Neon is investigated. A possible two-photon Ionization is observed with the presence of Ne9+ at photon energies below the Ne8+ 1s2 ionization threshold.

We use the unprecedented LCLS peak power to study nonlinear x-ray multiphoton physics in molecules. We report on fundamental questions concerning the creation and decay of double-core-hole vacancies in N2 by short x-ray pulses.

Molecules selectively transform light energy from the sun into other forms of energy like heat, electricity, or chemical energy with high quantum efficiency. The energy conversion process is the result of a correlated motion of electrons and nuclei after photoexcitation, often under breakdown of the Born-Oppenheimer approximation. The element and site selectivity of x-rays allows observing molecular processes from a different point of view compared to ultrafast optical probes [1,2].

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.

We present time-resolved femtosecond photoelectron momentum images and angular distributions of dissociating, laser-aligned 1,4-dibromobenzene (C6H4Br2) molecules measured in a near-infrared pump, soft-x-ray probe experiment performed at an x-ray free-electron laser. The observed alignment dependence of the bromine 2p photoelectron angular distributions is compared to density functional theory calculations and interpreted in terms of photoelectron diffraction. While no clear time-dependent effects are observed in the angular distribution of the Br(2p) photoelectrons, other, low-energy electrons show a pronounced dependence on the time delay between the near-infrared laser and the x-ray pulse. Keywords: free-electron laser, photoelectron diffraction, molecular-frame photoelectron angular distributions, laser alignment, femtosecond pump–probe