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The relaxation dynamics of electronically excited 3He and 4He clusters and droplets is investigated using time-correlated near-infrared and visible (NIR/VIS) fluorescence excitation spectroscopy.

The relaxation dynamics of electronically excited

Optically responsive materials are present in everyday life, from screens to sensors. However, fabricating large-area, fossil-free materials for functional biocompatible applications is still a challenge today. Nanocelluloses from various sources, such as wood, can provide biocompatibility and are emerging candidates for templating organic optoelectronics. Silver (Ag) in its nanoscale form shows excellent optical properties. Herein, we combine both materials using thin-film large-area spray-coating to study the fabrication of optical response applications. We characterize the Ag nanoparticle formation by X-ray scattering and UV–vis spectroscopy in situ during growth on the nanocellulose template. The morphology and optical properties of the nanocellulose film are compared to the rigid reference surface SiO2. Our results clearly show the potential to tailor the energy band gap of the resulting hybrid material.

The journal of physical chemistry letters 13(36), 8470 - 8476 (2022). doi:10.1021/acs.jpclett.2c01619, Femtosecond extreme ultraviolet wave packet interferometry (XUV-WPI) was applied to study resonant interatomic Coulombic decay (ICD) in the HeNe dimer. The high demands on phase stability and sensitivity for vibronic XUV-WPI of molecular-beam targets are met using an XUV phase-cycling scheme. The detected quantum interferences exhibit vibronic dephasing and rephasing signatures along with an ultrafast decoherence assigned to the ICD process. A Fourier analysis reveals the molecular absorption spectrum with high resolution. The demonstrated experiment shows a promising route for the real-time analysis of ultrafast ICD processes with both high temporal and high spectral resolution., Published by ACS, Washington, DC

High power mid-infrared (MID-IR) laser systems delivering ultrashort pulses at high repetition rates are of considerable interest for vibrational spectroscopy, label-free microscopy, ultrafast dynamics and HHG studies. To meet these requirements, we developed an optical parametric chirped-pulse amplifier (OPCPA) with a difference frequency generation (DFG) stage to provide an efficient platform to down-convert a 200 W Yb-YAG pump laser into the MID-IR range. The MID-IR pulses are tunable from 4.2-11 μm, with a maximum pulse energy around 9 μm with 2.2 μJ at 200 kHz supporting a Fourier limit of 50 fs. In addition, thermal parameters of various nonlinear crystals are reviewed for high power MID-IR OPCPAs.

Physical review / A 104(1), 013114 (2021). doi:10.1103/PhysRevA.104.013114, We study theoretically x-ray absorption and ionization spectra of an atom or molecule by two coherent x-ray pulses that show a relative phase shift resulting in a time delay of the pulse envelopes. We demonstrate that the phase modulation of the spectra is shifted with respect to the phase oscillation comb of the x-ray double pulse. The reason for this shift is the dynamics of the process defined by the interplay of the delay time, the pulse width, the detuning, and the lifetime of the core-excited state., Published by Inst., Woodbury, NY

Journal of physics / B 55(7), 074002 (2022). doi:10.1088/1361-6455/ac5f74, Interferometric pump–probe experiments in the extreme ultraviolet (XUV) domain are experimentally very challenging due to the high phase stability required between the XUV pulses. Recently, an efficient phase stabilization scheme was introduced for seeded XUV free electron lasers (FELs) combining shot-to-shot phase modulation with lock-in detection Wituschek et al (2020 Nat. Commun. 11 883). This method stabilized the seed laser beampath on the fundamental ultraviolet wavelength to a high degree. Here, we extend this scheme including the stabilization of the XUV beampath, incorporating phase fluctuations from the FEL high gain harmonic generation process. Our analysis reveals a clear signal improvement with the new method compared to the previous stabilization scheme., Published by IOP Publ., Bristol

Physical review / A covering atomic, molecular, and optical physics and quantum information 101(6), 063412 (2020). doi:10.1103/PhysRevA.101.063412, Focused short-wavelength free-electron laser (FEL) pulses interacting with gas phase samples can induce by inner-shell ionization a short-lived population inversion, followed by coherent collective emission of directed, short, and strong radiation bursts. We extend our studies into the warm-dense matter (WDM) regime by investigating the nanoplasmas produced in an ensemble of nanometer-sized clusters by FEL irradiation. Here, additional pathways can also lead to strong, laserlike emission: Electron-ion collisions can yield a long-lived population inversion, and subsequent amplified spontaneous emission. We observe amplified spontaneous emission (ASE) in the extreme ultraviolet in xenon clusters excited by soft x-ray FEL pulses, we diagnose the generated nanoplasmas by fluorescence spectroscopy, and we study under various cluster and FEL parameters the directed ASE from the Xe$^{2+}$ 65 nm line. We show its exponential increase as a function of FEL irradiation power, and an accompanying collisional broadening of the emission spectra. These findings are corroborated by extensive numerical simulations based on theory, combining detailed hydrodynamic and kinetic simulations with time-dependent calculations of radiation transport, amplification, and collective emission in the WDM nanoplasma. Our theoretical findings underline that population inversion is due to electron-ion collisions and that the observed decoherence processes can be empirically characterized by a phenomenological decoherence time in the range of 100–200 fs., Published by Inst., Woodbury, NY

Free-electron lasers (FELs) generate femtosecond XUV and X-ray pulses at peak powers in the gigawatt range. The FEL user facility FLASH at DESY (Hamburg, Germany) is driven by a superconducting linear accelerator with up to 8000 pulses per second. Since 2014, two parallel undulator beamlines, FLASH1 and FLASH2, have been in operation. In addition to the main undulator, the FLASH1 beamline is equipped with an undulator section, sFLASH, dedicated to research and development of fully coherent extreme ultraviolet photon pulses using external seed lasers. In this contribution, the first simultaneous lasing of the three FELs at 13.4 nm, 20 nm and 38.8 nm is presented.

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.

The XUV free electron laser FLASH, has been recently operated in the high-gain harmonic generation (HGHG) mode. We characterized the laser-induced energy modulation, as well as the temporal profile of the seeded FEL pulses. FEL saturation was reached for the 7th harmonic of the 266 nm seed laser.

Scientific reports 7(1), 2431(2017). doi:10.1038/s41598-017-02184-3, Free-electron lasers are unique sources of intense and ultra-short x-ray pulses that led to major scientific breakthroughs across disciplines from matter to materials and life sciences. The essential element of these devices are micrometer-sized electron bunches with high peak currents, low energy spread, and low emittance. Advanced FEL concepts such as seeded amplifiers rely on the capability of analyzing and controlling the electron beam properties with few-femtosecond time resolution. One major challenge is to extract tomographic slice parameters instead of projected electron beam properties. Here, we demonstrate that a radio-frequency deflector in combination with a dipole spectrometer not only allows for single-shot extraction of a seeded FEL pulse profile, but also provides information on the electron slice emittance and energy spread. The seeded FEL power profile can be directly related to the derived slice emittance as a function of intra-bunch coordinate with a resolution down to a few femtoseconds., Published by Springer Nature, London

Direct seeding with a high-harmonic generation (HHG) source can improve the spectral, temporal, and coherence properties of a free-electron laser (FEL) and shall reduce intensity and arrival-time fluctuations. In the seeding experiment sFLASH at the extreme ultraviolet FEL in Hamburg FLASH, which operates in the self-amplified spontaneous emission mode (SASE), the 21st harmonic of an 800 nm laser is refocused into a dedicated seeding undulator. For seeding, the external light field has to overcome the noise level of SASE; therefore, an efficient coupling between seed pulse and electron bunch is mandatory. Thus, an HHG beam with a proper divergence, width, beam quality, Rayleigh length, pointing stability, single-shot pulse energy, and stability in the 21st harmonic is needed. Here, we present the setup of the HHG source that seeds sFLASH at 38.1 nm, the optimization procedures, and the necessary diagnostics.

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.

We report on the x-ray absorption of Warm Dense Matter experiment at the FLASH Free Electron Laser (FEL) facility at DESY. The FEL beam is used to produce Warm Dense Matter with soft x-ray absorption as the probe of electronic structure. A multilayer-coated parabolic mirror focuses the FEL radiation, to spot sizes as small as 0.3μm in a ∼15fs pulse of containing >10 12 photons at 13.5 nm wavelength, onto a thin sample. Silicon photodiodes measure the transmitted and reflected beams, while spectroscopy provides detailed measurement of the temperature of the sample. The goal is to measure over a range of intensities approaching 10 18 W/cm 2. Experimental results will be presented along with theoretical calculations. A brief report on future FEL efforts will be given. © 2009 SPIE.

New journal of physics 18(11), 113055 (2016). doi:10.1088/1367-2630/18/11/113055, Fullerene complexes may play a key role in the design of future molecular electronics and nanostructured devices with potential applications in light harvesting using organic solar cells. Charge and energy flow in these systems is mediated by many-body effects. We studied the structure and dynamics of laser-induced multi-electron excitations in isolated C60 by two-photon photoionization as a function of excitation wavelength using a tunable fs UV laser and developed a corresponding theoretical framework on the basis of ab initio calculations. The measured resonance line width gives direct information on the excited state lifetime. From the spectral deconvolution we derive a lower limit for purely electronic relaxation on the order of ${\tau }_{\mathrm{el}}={10}_{-3}^{+5}$ fs. Energy dissipation towards nuclear degrees of freedom is studied with time-resolved techniques. The evaluation of the nonlinear autocorrelation trace gives a characteristic time constant of ${\tau }_{\mathrm{vib}}=400\pm 100$ fs for the exponential decay. In line with the experiment, the observed transient dynamics is explained theoretically by nonadiabatic (vibronic) couplings involving the correlated electronic, the nuclear degrees of freedom (accounting for the Herzberg–Teller coupling), and their interplay., Published by Dt. Physikalische Ges., [Bad Honnef]

A 100 kHz high harmonic source with record high >10$^{11}$ photons/s in single harmonics between 55-73 eV is presented. The unique capabilities are underlined by using it for coincidence experiments and measurements on magnetic samples.

Unraveling and controlling chemical dynamics requires techniques to image structural changes of molecules with femtosecond temporal and picometer spatial resolution. Ultrashort-pulse x-ray free-electron lasers have significantly advanced the field by enabling advanced pump-probe schemes. There is an increasing interest in using table-top photon sources enabled by high-harmonic generation of ultrashort-pulse lasers for such studies. We present a novel high-harmonic source driven by a 100 kHz fiber laser system, which delivers 10sup11/supphotons/s in a single 1.3 eV bandwidth harmonic at 68.6 eV. The combination of record-high photon flux and high repetition rate paves the way for time-resolved studies of the dissociation dynamics of inner-shell ionized molecules in a coincidence detection scheme. First coincidence measurements on CHsub3/subI are shown and it is outlined how the anticipated advancement of fiber laser technology and improved sample delivery will, in the next step, allow pump-probe studies of ultrafast molecular dynamics with table-top XUV-photon sources. These table-top sources can provide significantly higher repetition rates than the currently operating free-electron lasers and they offer very high temporal resolution due to the intrinsically small timing jitter between pump and probe pulses.

We report a comprehensive investigation of the electronically excited states of helium clusters and droplets of sizes ranging from a few to several 10(7) atoms using time-resolved fluorescence excitation spectroscopy and quantum chemical ab initio calculations. We employ various approaches for our analysis considering the lifetime-dependence of the fluorescence intensity, spectral shifts, intensity scaling with cluster size, isotopic dependence, and density-dependence of the calculated electron wave function radii. A unique feature of helium clusters and droplets is their radially varying particle density. Our results show that short-lived fluorescence is sensitive to regions of increased density and probes excitations located in the bulk volume, whereas long-lived fluorescence is sensitive to regions of reduced density such as for small clusters or for the surface of large droplets. Spectra of (3)He droplets serve as a reference for low density, but are free from contributions of small clusters. This allows us to distinguish regions of reduced density as these can be due to both surface states or small clusters. Our analysis reveals a picture where spectral features are related to regions of different density due to isotopic composition, cluster size, and surface or bulk volume location of the excitations. The 2s and 2p related excitations appear as blue-shifted wings for small clusters or for excited atoms within the surface layer, whereas in the bulk-volume of large droplets, they appear as distinct bands with large intensities, dominating the entire spectrum. Excitations at energies higher than 23 eV are unambiguously assigned to regions of low and medium density location within the deeper parts of the surface layer but show no relation to the bulk volume. Our findings support the idea that in liquid helium high-lying states and, in particular, Rydberg states are quenched in favor of the 2s and 2p excitations.

New journal of physics 20, 013010 (2017). doi:10.1088/1367-2630/aa9b4c, We report on a direct time-domain measurement of the temporal properties of a seeded free-electron laser pulse in the extreme ultraviolet spectral range. Utilizing the oscillating electromagnetic field of terahertz radiation, a single-shot THz streak-camera was applied for measuring the duration as well as spectral phase of the generated intense XUV pulses. The experiment was conducted at FLASH, the free electron laser user facility at DESY in Hamburg, Germany. In contrast to indirect methods, this approach directly resolves and visualizes the frequency chirp of a seeded FEL pulse. The reported diagnostic capability is a prerequisite to tailor amplitude, phase and frequency distributions of FEL beams on demand. In particular, it opens up a new window of opportunities for advanced coherent spectroscopic studies making use of the high degree of temporal coherence expected from a seeded FEL pulse., Published by IOP Publ., [Bad Honnef]

We report on the ionization and nanoplasma dynamics of small xenon clusters irradiated by intense, short pulses of a short-wavelength free-electron laser. Fluorescence spectroscopy indicates that inelastic electron collisions play a prominent role in the formation of the highest charge states. From the spectral distribution an electron temperature of 27 eV is deduced which corresponds to the average excess energy of the Auger- and photoelectrons ionized from individual atoms but trapped in the cluster core. This suggests that fluorescence spectra reflect a very early stage within the nanoplasma dynamics and shows how a part of the kinetic energy of the plasma electrons trapped in the cluster potential is transferred to the ions., Journal of Physics B: Atomic, Molecular and Optical Physics, 48 (17), ISSN:1361-6455, ISSN:0368-3508, ISSN:0953-4075, ISSN:0022-3700

Measurements of the longitudinal phase-space distribution of electron bunches seeded with an external laser were done in order to study the impact of collective effects on seeded microbunches in free-electron lasers. Velocity bunching of a seeded microbunch appears to be a viable alternative to compression with a magnetic chicane under high-gain harmonic generation seeding conditions when the collective effects of Coulomb forces in a drift space and coherent synchrotron radiation in a chicane are considered. Measurements of these effects on seeded electron microbunches were performed with an RF deflecting structure and a dipole magnet which streak out the electron bunch for single-shot images of the longitudinal phase-space distribution. Particle tracking simulations in 3D predicted the compression dynamics of the seeded microbunches with collective effects., Proceedings of the 37th International Free Electron Laser Conference, FEL2015, Daejeon, Republic of Korea

We have studied fluorescence spectra of small Xem and Xem(NF3)k clusters (m,k⩽102) embedded in large host Ne7500 clusters, excited with SR excitation in the VUV spectral region above the Xe+(P1∕22) atomic limit. The IR emission bands due to ionic chromophores Xep+ (p=2,3,4) as well as UV (B–X) and visible (C–A) bands due to desorbed Xe+F− excimer molecules have been observed. The triatomic Xe2F excimer emission has only been observed under low-energy excitation close to the lowest xenon exciton absorption band. A competition between ionic and charge-transfer reaction channels is followed as a function of the cluster sizes m and k.

The electron and nuclear dynamics of C60 fullerenes irradiated with femtosecond laser pulses are investigated with photoelectron and photoion spectroscopy. The focus of this work is the detailed exploration of the population mechanism of Rydberg levels within the excitation process of neutral C60. The effect of excitation wavelength, intensity, chirp, and polarization on the kinetic energy distribution of photoelectrons in single-pulse experiments gives first insight into the underlying processes. In combination with time-resolved two-color pump-probe spectroscopy depending on either pump, or probe pulse intensity, a more complete picture of the interaction can be drawn. The results point towards a very interesting but nevertheless complex behavior including four steps: (i) non-adiabatic multielectron excitation of the HOMO (hu) → LUMO+1 (t1g) transition; (ii) thermalization within the hot electron cloud on a time scale below 100 fs, followed by a coupling of energy to vibrational modes of the molecule via doorway state(s); (iii) population of electronically excited Rydberg states by multiphoton absorption, and (iv) single photon ionization from the excited Rydberg states. This excitation process results in a characteristic sequence of photoelectron lines in the photoemission spectra. The comparison of the experimental results with recent theoretical work gives convincing evidence that non-adiabatic multielectron dynamics (NMED) plays a key role for the understanding of the response of C60 to short-pulse laser radiation.

We report on first experiments of rare gas cluster dynamics and spectroscopy using light from the DESY free electron laser at 13 eV. The energy absorption is found to be very efficient and the clusters fragment completely. Non-linear effects appear at a peak power density threshold much lower than for visible light. The data are consistent with a three-step ionization process, but the nature of the first step is still controversial.

As one of the first experiments at the free electron laser of the TESLA Test Facility (TTF) the Coulomb explosion of Xenon clusters irradiated with high intensity pulses at a wavelength of 98 nm has been observed. Classical trajectory calculations have been performed in order to illuminate the energy absorption process. Comparison with typical parameters in the infrared regime shows that above barrier ionization is suppressed due to the fast oscillating field and thermionic ionization prevails.

Energy relaxation processes of photo-excited Kr50¯ clusters covered with a shell of Ar atoms (up to 40), which are embedded inside large Ne7500¯ clusters are investigated with energy resolved fluorescence spectroscopy. In the energy range of the characteristic Ne cluster absorption (16.5–18 eV) a strong energy transfer to the embedded Kr cluster is observed, which results in the desorption of electronically excited Kr* atoms. Kr* atoms move through the Ne cluster, desorb and emit visible and near-infrared light in the vacuum (5p→5s). By coating the Kr clusters with Ar atoms, the Kr lines disappear and 4p→4s transitions of Ar* become dominant. Additionally, new emission bands occur, which are assigned to transitions of perturbed atomic Kr 5p-states inside Ne clusters. Due to the interaction of electronically excited Kr* atoms with neutral Ar atoms in the surrounding shell, several excited Kr states namely 5p [1/2]0 and 5p [3/2]2 decay nonradiatively. This is in agreement with the well-known “energy-gap law...

We have studied the fluorescence of electronically excited OH*, H* and H2O+* dissociation fragments after VUV excitation ( hν≥11.6eV) of rare-gas clusters (Rg = Ne, Ar) doped with H2O molecules. In contrast to a free molecule, where Balmer H-series dominate the UV-visible spectra, only the OH *(A2Σ+↦X2Π) emission band is observed in neon clusters. No emission of excited water ions has been observed. We find that while higher excitation energies (Ne vs. Ar) induce higher vibrational excitation of the OH*(A) fragment, the rotational temperature is lower. This effect is attributed to the difference in the geometric position of the H2O molecule on the surface or inside the Rg-cluster. The rotational relaxation in neon clusters is rapid while the vibrational relaxation is slow because of the coupling with the low energy matrix phonons.

The charge localization process in XeN+ cluster ions (N=40–20 000) is investigated with fluorescence spectroscopy methods. New discrete and continuous luminescence bands in the visible and near infrared spectral range are observed and are assigned to radiative transitions of ionic dimers, trimers and tetramers inside Xe clusters. The bands are related to the 5p5 2P1/2→5p5 2P3/2 transition of electronically excited atomic Xe ions and the 6p→6s transitions of electronically excited Xe neutrals. The dependence of the size of the ionic centers on the Xe cluster size is discussed. In large clusters discrete lines are due to embedded dimer emission and they are identified as 2(1/2)u→1(3/2)g transitions between different vibrational levels. Line positions are blue-shifted by 30 (±1) meV with respect to free molecular dimer ions. The energy shift is due to the interaction of the surrounding neutral Xe cluster atoms with the embedded ionic Xe molecules.

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.

We report on the dynamics of ultrafast heating in cryogenic hydrogen initiated by a ≲300 fs, 92 eV free electron laser x-ray burst. The rise of the x-ray scattering amplitude from a second x-ray pulse probes the transition from dense cryogenic molecular hydrogen to a nearly uncorrelated plasmalike structure, indicating an electron-ion equilibration time of ∼0.9 ps. The rise time agrees with radiation hydrodynamics simulations based on a conductivity model for partially ionized plasma that is validated by two-temperature density-functional theory. © 2014 American Physical Society.

For seeding of a free-electron laser, the spatial and temporal overlap of the seed laser pulse and the electron bunch in the modulator is critical. To establish the temporal overlap, the time difference between pulses from the seed laser and spontaneous undulator radiation is reduced to a few pico-seconds with a combination of a photomultiplier tube and a streak camera. Finally, for the precise overlap the impact of the seed laser pulses on the electron bunches is observed. In this contribution, we describe the current experimental setup, discuss the techniques applied to establish the temporal overlap and analyze its stability.

Collective effects and instabilities due to longitudinal space charge and coherent synchrotron radiation can degrade the quality of the ultra-relativistic, high-brightness electron bunches needed for the operation of free-electron lasers. In this contribution, we demonstrate the application of a laser-induced microbunching instability to selectively suppress the SASE process. A significant decrease of photon pulse energies was observed at the free-electron laser FLASH in coincidence with overlap of 800 nm laser pulses and electron bunches within a modulator located approximately 40 meters upstream of the undulators. We discuss the underlying mechanisms based on longitudinal space charge amplification [E.A. Schneidmiller and M.V. Yurkov, Phys. Rev. ST Accel. Beams 13, 110701 (2010)] and present measurements.

The mobility of Xe atoms captured from a cross jet by hydrogen clusters is investigated with time resolved VUV fluorescence excitation spectroscopy. A band blue shifted by 0.5 eV with respect to the 5p→6s transition of free Xe atoms is observed. It is attributed to Xe atoms profoundly embedded in hydrogen clusters and surrounded by several layers of hydrogen. The complete absence of features related to surface sites gives evidence that after the pick up process the xenon atoms penetrate deep into the interior of the hydrogen clusters. This indicates that hydrogen clusters show a liquid-like behavior similar to helium clusters, though their estimated temperature (∼6 K) is much below the triple point of hydrogen (14 K).

Highly charged ions are formed in the center of composite clusters by strong free-electron laser pulses and they emit fluorescence on a femtosecond time scale before competing recombination leads to neutralization of the nanoplasma core. In contrast to mass spectrometry that detects remnants of the interaction, fluorescence in the extreme ultraviolet spectral range provides fingerprints of transient states of high energy density matter. Spectra from clusters consisting of a xenon core and a surrounding argon shell show that a small fraction of the fluorescence signal comes from multiply charged xenon ions in the cluster core. Initially, these ions are as highly charged as the ions in the outer shells of pure xenon clusters with charge states up to at least $11+$.

Initiating the gain process in a free-electron laser (FEL) from an external highly coherent source of radiation is a promising way to improve the pulse properties such as temporal coherence and synchronization performance in time-resolved pump-probe experiments at FEL facilities, but this so-called "seeding" suffers from the lack of adequate sources at short wavelengths. We report on the first successful seeding at a wavelength as short as 38.2 nm, resulting in GW-level, coherent FEL radiation pulses at this wavelength as well as significant second harmonic emission at 19.1 nm. The external seed pulses are about 1 order of magnitude shorter compared to previous experiments allowing an ultimate time resolution for the investigation of dynamic processes enabling breakthroughs in ultrafast science with FELs. The seeding pulse is the 21st harmonic of an 800-nm, 15-fs (rms) laser pulse generated in an argon medium. Methods for finding the overlap of seed pulses with electron bunches in spatial, longitudinal, and spectral dimensions are discussed and results are presented. The experiment was conducted at FLASH, the FEL user facility at DESY in Hamburg, Germany. (Less)

Femtosecond x-ray laser flashes with power densities of up to ${10}^{14}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$ at 13.7 nm wavelength were scattered by single xenon clusters in the gas phase. Similar to light scattering from atmospheric microparticles, the x-ray diffraction patterns carry information about the optical constants of the objects. However, the high flux of the x-ray laser induces severe transient changes of the electronic configuration, resulting in a tenfold increase of absorption in the developing nanoplasma. The modification in opaqueness can be correlated to strong atomic charging of the particle leading to excitation of ${\mathrm{Xe}}^{4+}$. It is shown that single-shot single-particle scattering on femtosecond time scales yields insight into ultrafast processes in highly excited systems where conventional spectroscopy techniques are inherently blind.

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.