Search

Abstract Time-resolved X-ray photoelectron spectroscopy (TXPS) is a well-established technique to probe coherent nuclear wavepacket dynamics using both table-top and free-electron-based ultrafast X-ray lasers. Energy resolution, however, becomes compromised for a very short pulse duration in the sub-femtosecond range. By resonantly tuning the X-ray pulse to core-excited states undergoing Auger decay, this drawback of TXPS can be mitigated. While resonant Auger-electron spectroscopy (RAS) can recover the vibrational structures not hidden by broadband excitation, the full reconstruction of the wavepacket is a standing challenge. Here, we theoretically demonstrate how the complete information of a nuclear wavepacket, i.e., the populations and relative phases of the vibrational states constituting the wavepacket, can be retrieved from time-resolved RAS (TRAS) measurements. Thus, TRAS offers key insights into coupled nuclear and electronic dynamics in complex systems on ultrashort timescales, providing an alternative to leverage femtosecond and attosecond X-ray probe pulses.

The evolution of charge carriers in photoexcited room temperature ZnO nanoparticles in solution is investigated using ultrafast ultraviolet photoluminescence spectroscopy, ultrafast Zn K-edge absorption spectroscopy, and ab initio molecular dynamics (MD) simulations. The photoluminescence is excited at 4.66 eV, well above the band edge, and shows that electron cooling in the conduction band and exciton formation occur in

Determination and interpretation of Wigner-like photoionisation delays is one of the most active fields of attosecond science. Previous results have suggested that large photoionisation delays are associated with structured continua, but a quantitative relation between photoionisation cross sections and time delays has been missing. Here, we derive a Kramers–Kronig-like relation between these quantities and demonstrate its validity for (anti)resonances. This new concept defines a topological analysis, which rationalises the sign of photoionisation delays and thereby sheds new light on a long-standing controversy regarding the sign of the photoionisation delay near the Ar 3 s Cooper minimum. Our work bridges traditional photoionisation spectroscopy with attosecond chronoscopy and offers new methods for analysing and interpreting photoionisation delays.

Understanding the photoelectron emission time after the interaction of photon with atoms and molecules is of fundamental interest. Here the authors examine the role of partial waves to the photoionization phase shift of atoms using an attosecond clock and electron-ion coincidence spectroscopy.

X-ray absorption and X-ray free electron lasers are important tools to study chemical and structural dynamics, but spectral details like pre-edge features are inherently hard to detect. Here, the authors show that stochastic spectroscopy can yield similar spectral information to monochromatic spectroscopies, while increasing signal yield and reducing acquisition time.

Characterizing and controlling the uniformity of nanoparticles is crucial for their application in science and technology because crystalline defects in the nanoparticles strongly affect their unique properties. Recently, ultra-short and ultra-bright X-ray pulses provided by X-ray free-electron lasers (XFELs) opened up the possibility of structure determination of nanometre-scale matter with Å spatial resolution. However, it is often difficult to reconstruct the 3D structural information from single-shot X-ray diffraction patterns owing to the random orientation of the particles. This report proposes an analysis approach for characterizing defects in nanoparticles using wide-angle X-ray scattering (WAXS) data from free-flying single nanoparticles. The analysis method is based on the concept of correlated X-ray scattering, in which correlations of scattered X-ray are used to recover detailed structural information. WAXS experiments of xenon nanoparticles, or clusters, were conducted at an XFEL facility in Japan by using the SPring-8 Ångstrom compact free-electron laser (SACLA). Bragg spots in the recorded single-shot X-ray diffraction patterns showed clear angular correlations, which offered significant structural information on the nanoparticles. The experimental angular correlations were reproduced by numerical simulation in which kinematical theory of diffraction was combined with geometric calculations. We also explain the diffuse scattering intensity as being due to the stacking faults in the xenon clusters.

With the emergence of X-ray free-electron lasers, it is possible to investigate the structure of nanoscale samples by employing coherent diffractive imaging in the X-ray spectral regime. In this work, we developed a refinement method for structure reconstruction applicable to low-quality coherent diffraction data. The method is based on the gradient search method and considers the missing region of a diffraction pattern and the small number of detected photons. We introduced an initial estimate of the structure in the method to improve the convergence. The present method is applied to an experimental diffraction pattern of an Xe cluster obtained in an X-ray scattering experiment at the SPring-8 Angstrom Compact free-electron LAser (SACLA) facility. It is found that the electron density is successfully reconstructed from the diffraction pattern with a large missing region, with a good initial estimate of the structure. The diffraction pattern calculated from the reconstructed electron density reproduced the observed diffraction pattern well, including the characteristic intensity modulation in each ring. Our refinement method enables structure reconstruction from diffraction patterns under difficulties such as missing areas and low diffraction intensity, and it is potentially applicable to the structure determination of samples that have low scattering power.

Ultrashort pulse X-ray free electron lasers (XFFLs) provided us with an unprecedented regime of X-ray intensities, revolutionizing ultrafast structure determination and paving the way to the novel field of non-linear X-ray optics. While pioneering studies revealed the formation of a nanoplasma following the interaction of an XFEL pulse with nanometer-scale matter, nanoplasma formation and disintegration processes are not completely understood, and the behavior of trapped electrons in the electrostatic potential of highly charged species is yet to be decrypted. Here we report the behavior of the nanoplasma created by a hard X-ray pulse interacting with xenon clusters by using electron and ion spectroscopy. To obtain a deep insight into the formation and disintegration of XFEL-ignited nanoplasma, we studied the XFEL-intensity and cluster-size dependencies of the ionization dynamics. We also present the time-resolved data obtained by a near-infrared (NIR) probe pulse in order to experimentally track the time evolution of plasma electrons distributed in the XFEL-ignited nanoplasma. We observed an unexpected time delay dependence of the ion yield enhancement due to the NIR pulse heating, which demonstrates that the plasma electrons within the XFEL-ignited nanoplasma are inhomogeneously distributed in space.

Understanding strong X-ray induced phenomena is important for applications of X-ray free-electron laser imaging. Here, the authors show time-resolved measurements of X-ray free-electron laser induced electronic decay of CH2I2 molecule probed with NIR pulses and identify mechanisms behind different transient states lifetimes.

X-ray free electron lasers (XFELs) deliver intense, coherent, femtosecond X-ray laser pulses. They are opening new research fields of studying ultrafast electronic and structural dynamics in various forms of matter and interaction of intense and short X-ray pulses with matter. For such studies, atoms, molecules, and atomic clusters in the gas phase may provide us with ideal platforms as various levels of experimental methods and theory are within reach. Developing experimental setups of electron and ion spectroscopies for gas-phase experiments at SACLA, a low-repetition rate X-ray free electron laser facility in Japan, we have studied the interaction of intense, short X-ray pulses with atoms, molecules, and atomic clusters. Via these experimental investigations and modelling efforts, we gain insight into the interaction of these intense pulses with the single-particle targets and extracted the information about how they are modified during the X-ray pulse duration. We summarize these studies in the present review. The information reported here will be important for any XFEL applications as the scattering signal used to image any targets may be strongly influenced by the XFEL-induced dynamics that occur in the target within the pulse duration.

A shape resonance emerges during the light absorption in many molecules with a gigantic burst amplitude and a lifetime of hundreds of attoseconds. Recent advances in attosecond metrology revealed the attosecond lifetime of the shape resonance. For a heteronuclear molecule, the asymmetric initial state and landscape of the molecular potential would lead to an asymmetric shape resonance, whose effect, however, has not been characterized yet. Here, we employ an attosecond interferometer to investigate the molecular-frame photoionization time delay in the vicinity of the shape resonance of the NO molecule. Driven by photons with energy ranging from 23.8 eV to 36.5 eV, a 150 attosecond difference in the time delay is observed between photoemission from the N/O end. Our quantum scattering theoretical simulations reproduce well our experimental findings. It illustrates that the asymmetric time delay originates from the interference between resonant and nonresonant photoionization pathways.

Availability of intense hard X-ray pulses allows exploration of multiple ionization effects in heavier elements. Here, the authors measure the complex charge state distributions of xenon and found a reasonable agreement by comparing with the model including the relativistic and resonance effects.

X-ray Free Electron Lasers (FELs) deliver coherent X-ray pulses, combining unprecedented power densities of up to 1020 W/cm2 and extremely short pulse durations down to hundreds of attoseconds [...]

Free-electron lasers (FELs) can produce radiation in the short wavelength range extending from the extreme ultraviolet (XUV) to the X-rays with a few to a few tens of femtoseconds pulse duration. These facilities have enabled significant breakthroughs in the field of atomic, molecular, and optical physics, implementing different schemes based on two-color photoionization mechanisms. In this article, we present the generation of attosecond pulse trains (APTs) at the seeded FEL FERMI using the beating of multiple phase-locked harmonics. We demonstrate the complex attosecond waveform shaping of the generated APTs, exploiting the ability to manipulate independently the amplitudes and the phases of the harmonics. The described generalized attosecond waveform synthesis technique with an arbitrary number of phase-locked harmonics will allow the generation of sub-100 as pulses with programmable electric fields.

When a nanoparticle is irradiated by an intense laser pulse, it turns into a nanoplasma, a transition that is accompanied by many interesting nonequilibrium dynamics. So far, most experiments on nanoplasmas use ion measurements, reflecting the outside dynamics in the nanoparticle. Recently, the direct observation of the ultrafast structural dynamics on the inside of the nanoparticle also became possible with the advent of x-ray free electron lasers (XFELs). Here, we report on combined measurements of structural dynamics and speeds of ions ejected from nanoplasmas produced by intense near-infrared laser irradiations, with the control of the initial plasma conditions accomplished by widely varying the laser intensity (9×10^{14} W/cm^{2} to 3×10^{16} W/cm^{2}). The structural change of nanoplasmas is examined by time-resolved x-ray diffraction using an XFEL, while the kinetic energies of ejected ions are measured by an ion time-of-fight method under the same experimental conditions. We find that the timescale of crystalline disordering in nanoplasmas strongly depends on the laser intensity and scales with the inverse of the average speed of ions ejected from the nanoplasma. The observations support a recently suggested scenario for nanoplasma dynamics in the wide intensity range, in which crystalline disorder in nanoplasmas is caused by a rarefaction wave propagating at a speed comparable with the average ion speed from the surface toward the inner crystalline core. We demonstrate that the scenario is also applicable to nanoplasma dynamics in the hard x-ray regime. Our results connect the outside nanoplasma dynamics to the loss of structure inside the sample on the femtosecond timescale.

X-ray free-electron lasers produce bright femtosecond X-ray pulses. Here, the authors use a two-colour X-ray free-electron laser beam for simultaneous two-wavelength data collection and show that protein structures can be determined with multiple wavelength anomalous dispersion phasing, which is important for difficult-to-phase projects.

We have performed a full-dimensional theoretical study of vibrationally resolved photoelectron emission from the valence shell of the water molecule by using an extension of the static-exchange density functional theory that accounts for ionization as well as for vibrational motion in the symmetric stretching, antisymmetric stretching, and bending modes. At variance with previous studies performed in centrosymmetric molecules, where vibrationally resolved spectra are mostly dominated by the symmetric stretching mode, in the present case, all three modes contribute to the calculated spectra, including intermode couplings. We have found that diffraction of the ejected electron by the various atomic centers is barely visible in the ratios between vibrationally resolved photoelectron spectra corresponding to different vibrational states of the remaining H2O+ cation (the so-called v-ratios), in contrast to the prominent oscillations observed in K-shell ionization of centrosymmetric molecules, including those that only contain hydrogen atoms around the central atoms, e.g., CH4. To validate the conclusions of our work, we have carried out synchrotron radiation experiments at the SOLEIL synchrotron and determined photoelectron spectra and v-ratios for H2O in a wide range of photon energies, from threshold up to 150 eV. The agreement with the theoretical predictions is good.

In preparation for the upcoming home delivery services that rely on Unmanned Aerial Vehicles (UAVs), we developed a new multi-hop radio network that is laid over a smart meter network transferring electric energy information only. In this network, a UAV follows, for navigation purposes, the topology of a virtual network overlaid on the physical smart meter network. We established a service management control method which does not rely on image analysis or map information processing, i.e. processes that consume precious power resources of the UAV. Instead, navigation is based on the routing technology. The current distance between the UAV and a node of the smart meter network is measured by means of the radio transmission loss value, therefore determining the position of the UAV. A two-layer network model has been proposed. One layer consists of a network of nodes in a residential area with scattered buildings – a location that is safer to navigate – while the other is an access network of nodes in a densely populated area. Then, we proposed methods to determine the direction of movement towards the next hop node on the data-link layer and the end node on the network layer, which is the target destination. We implemented a software-based test system and verified the effectiveness of the proposed methods.

Quantum mechanically, photoionization can be fully described by the complex photoionization amplitudes that describe the transition between the ground state and the continuum state. Knowledge of the value of the phase of these amplitudes has been a central interest in photoionization studies and newly developing attosecond science, since the phase can reveal important information about phenomena such as electron correlation. We present a new attosecond-precision interferometric method of angle-resolved measurement for the phase of the photoionization amplitudes, using two phase-locked extreme ultraviolet pulses of frequency ω and 2ω, from a free-electron laser. Phase differences Δη[over ˜] between one- and two-photon ionization channels, averaged over multiple wave packets, are extracted for neon 2p electrons as a function of the emission angle at photoelectron energies 7.9, 10.2, and 16.6 eV. Δη[over ˜] is nearly constant for emission parallel to the electric vector but increases at 10.2 eV for emission perpendicular to the electric vector. We model our observations with both perturbation and ab initio theory and find excellent agreement. In the existing method for attosecond measurement, reconstruction of attosecond beating by interference of two-photon transitions (RABBITT), a phase difference between two-photon pathways involving absorption and emission of an infrared photon is extracted. Our method can be used for extraction of a phase difference between single-photon and two-photon pathways and provides a new tool for attosecond science, which is complementary to RABBITT.

A central motivation for the development of x-ray free-electron lasers has been the prospect of time-resolved single-molecule imaging with atomic resolution. Here, we show that x-ray photoelectron diffraction—where a photoelectron emitted after x-ray absorption illuminates the molecular structure from within—can be used to image the increase of the internuclear distance during the x-ray-induced fragmentation of an O_{2} molecule. By measuring the molecular-frame photoelectron emission patterns for a two-photon sequential K-shell ionization in coincidence with the fragment ions, and by sorting the data as a function of the measured kinetic energy release, we can resolve the elongation of the molecular bond by approximately 1.2 a.u. within the duration of the x-ray pulse. The experiment paves the road toward time-resolved pump-probe photoelectron diffraction imaging at high-repetition-rate x-ray free-electron lasers.

The high energy densities deposited in materials by focused X-ray laser pulses generate shock waves which travel away from the irradiated region, and can generate complex wave patterns or induce phase changes. We determined the time-pressure histories of shocks induced by X-ray laser pulses in liquid water microdrops, by measuring the surface velocity of the microdrops from images recorded during the reflection of the shock at the surface. Measurements were made with ~30 µm diameter droplets using 10 keV X-rays, for X-ray pulse energies that deposited linear energy densities from 3.5 to 120 mJ/m; measurements were also made with ~60 µm diameter drops for a narrower energy range. At a distance of 15 µm from the X-ray beam, the peak shock pressures ranged from 44 to 472 MPa, and the corresponding time-pressure histories of the shocks had a fast quasi-exponential decay with positive pressure durations estimated to range from 2 to 5 ns. Knowledge of the amplitude and waveform of the shock waves enables accurate modeling of shock propagation and experiment designs that either maximize or minimize the effect of shocks.

X-ray free-electron lasers (XFELs) deliver ultrashort coherent laser pulses in the X-ray spectral regime, enabling novel investigations into the structure of individual nanoscale samples. In this work, we demonstrate how single-shot small-angle X-ray scattering (SAXS) measurements combined with fluorescence and ion time-of-flight (TOF) spectroscopy can be used to obtain size- and structure-selective evaluation of the light-matter interaction processes on the nanoscale. We recorded the SAXS images of single xenon clusters using XFEL pulses provided by the SPring-8 Angstrom compact free-electron laser (SACLA). The XFEL fluences and the radii of the clusters at the reaction point were evaluated and the ion TOF spectra and fluorescence spectra were sorted accordingly. We found that the XFEL fluence and cluster size extracted from the diffraction patterns showed a clear correlation with the fluorescence and ion TOF spectra. Our results demonstrate the effectiveness of the multispectroscopic approach for exploring laser−matter interaction in the X-ray regime without the influence of the size distribution of samples and the fluence distribution of the incident XFEL pulses.

Single-photon laser enabled Auger decay (spLEAD) is an electronic de-excitation process which was recently predicted and observed in Ne. We have investigated it using bichromatic phase-locked free electron laser radiation and extensive angle-resolved photoelectron measurements, supported by a detailed theoretical model. We first used separately the fundamental wavelength resonant with the Ne ^+ 2 s –2 p transition, 46.17 nm, and its second harmonic, 23.08 nm, then their phase-locked bichromatic combination. In the latter case the phase difference between the two wavelengths was scanned, and interference effects were observed, confirming that the spLEAD process was occurring. The detailed theoretical model we developed qualitatively predicts all observations: branching ratios between the final Auger states, their amplitudes of oscillation as a function of phase, the phase lag between the oscillations of different final states, and partial cancellation of the oscillations under certain conditions.

We theoretically study the photoelectron angular distribution (PAD) from the two-photon single ionization of H and He by femtosecond and attosecond extreme-ultraviolet pulses, based on the time-dependent perturbation theory and simulations with the full time-dependent Schrodinger equation. The PAD is formed by the interference of the s and d continuum wave packets, and, thus, contains the information on the relative phase and amplitude ratio between them. We find that, when a spectrally broadened femtosecond pulse is resonant with an excited level, the PAD substantially changes with pulse width, since the competition between resonant and nonresonant ionization paths, leading to distinct from the scattering phase shift difference, changes with it. In contrast, when the Rydberg manifold is excited, and for the case of above-threshold two-photon ionization, and the PAD do not depend much on pulse width, except for the attosecond region. Thus, the Rydberg manifold and the continuum behave similarly in this respect. For a high-harmonic pulse composed of multiple harmonic orders, while the value is different from that for a single-component pulse, the PAD still rapidly varies with pulse width. The present results illustrate a new way to tailor the continuum wave packet.

Ultrafast electron transfer in dissociating iodomethane and fluoromethane molecules was studied at the Linac Coherent Light Source free-electron laser using an ultraviolet-pump, X-ray-probe scheme. The results for both molecules are discussed with respect to the nature of their UV excitation and different chemical properties. Signatures of long-distance intramolecular charge transfer are observed for both species, and a quantitative analysis of its distance dependence in iodomethane is carried out for charge states up to I21+. The reconstructed critical distances for electron transfer are in good agreement with a classical over-the-barrier model and with an earlier experiment employing a near-infrared pump pulse.

We have measured the angular distributions of high energy photoelectrons of benzene molecules generated by intense infrared femtosecond laser pulses. These electrons arise from the elastic collisions between the benzene ions with the previously tunnel-ionized electrons that have been driven back by the laser field. Theory shows that laser-free elastic differential cross sections (DCSs) can be extracted from these photoelectrons, and the DCS can be used to retrieve the bond lengths of gas-phase molecules similar to the conventional electron diffraction method. From our experimental results, we have obtained the C-C and C-H bond lengths of benzene with a spatial resolution of about 10 pm. Our results demonstrate that laser induced electron diffraction (LIED) experiments can be carried out with the present-day ultrafast intense lasers already. Looking ahead, with aligned or oriented molecules, more complete spatial information of the molecule can be obtained from LIED, and applying LIED to probe photo-excited molecules, a “molecular movie” of the dynamic system may be created with sub-Ångström spatial and few-ten femtosecond temporal resolutions.

X-ray free-electron lasers (XFELs) made available a new regime of x-ray intensities, revolutionizing the ultrafast structure determination and laying the foundations of the novel field of nonlinear x-ray optics. Although earlier studies revealed nanoplasma formation when an XFEL pulse interacts with any nanometer-scale matter, the formation process itself has never been decrypted and its timescale was unknown. Here we show that time-resolved ion yield measurements combined with a near-infrared laser probe reveal a surprisingly ultrafast population (∼12 fs), followed by a slower depopulation (∼250 fs) of highly excited states of atomic fragments generated in the process of XFEL-induced nanoplasma formation. Inelastic scattering of Auger electrons and interatomic Coulombic decay are suggested as the mechanisms populating and depopulating, respectively, these excited states. The observed response occurs within the typical x-ray pulse durations and affects x-ray scattering, thus providing key information on the foundations of x-ray imaging with XFELs.

During the last decades, the advent of the short-wavelength Free Electron Lasers (FELs) in the range from extreme ultraviolet (XUV) to hard X-rays has opened a new research avenue for the investigations of ultrafast electronic and structural dynamics in any form of matter[...]

X-ray free electron lasers (XFELs) providing ultrashort intense pulses of X-rays have proven to be excellent tools to investigate the dynamics of radiation-induced dissociation and charge redistribution in molecules and nanoparticles. Coincidence techniques, in particular multi-ion time-of-flight (TOF) coincident experiments, can provide detailed information on the photoabsorption, charge generation, and Coulomb explosion events. Here we review several such recent experiments performed at the SPring-8 Angstrom Compact free electron LAser (SACLA) facility in Japan, with iodomethane, diiodomethane, and 5-iodouracil as targets. We demonstrate how to utilize the momentum-resolving capabilities of the ion TOF spectrometers to resolve and filter the coincidence data and extract various information essential in understanding the time evolution of the processes induced by the XFEL pulses.