Ultrafast X-ray Research Articles

URSA-PQ: A Mobile and Flexible Pump-Probe Instrument for Gas Phase Samples at the FLASH Free Electron Laser

We present a highly flexible and portable instrument to perform pump-probe spectroscopy with an optical and an X-ray pulse in the gas phase. The so-called URSA-PQ (German for ‘Ultraschnelle Röntgenspektroskopie zur Abfrage der Photoenergiekonversion an Quantensystemen’, Engl. ‘ultrafast X-ray spectroscopy for probing photoenergy conversion in quantum systems’) instrument is equipped with a magnetic bottle electron spectrometer (MBES) and tools to characterize the spatial and temporal overlap of optical and X-ray laser pulses. Its adherence to the CAMP instrument dimensions allows for a wide range of sample sources as well as other spectrometers to be included in the setup. We present the main design and technical features of the instrument. The MBES performance was evaluated using Kr M4,5NN Auger lines using backfilled Kr gas, with an energy resolution ΔE/E ≅ 1/40 in the integrating operative mode. The time resolution of the setup at FLASH 2 FL 24 has been characterized with the help of an experiment on 2-thiouracil that is inserted via the instruments’ capillary oven. We find a time resolution of 190 fs using the molecular 2p photoline shift and attribute this to different origins in the UV-pump—the X-ray probe setup.

Tracking the Local Structure Change during the Photoabsorption Processes of Photocatalysts by the Ultrafast Pump-Probe XAFS Method

The birth of synchrotron radiation (SR) facilities and X-ray free electron lasers (XFELs) has led to the development of new characterization tools that use X-rays and opened frontiers in science and technology. Ultrafast X-ray absorption fine structure (XAFS) spectroscopy for photocatalysts is one such significant research technique. Although carrier behavior in photocatalysts has been discussed in terms of the band theory and their energy levels in reciprocal space (k-space) based on optical spectroscopic results, it has rarely been discussed where photocarriers are located in real-space (r-space) based on direct observation of the excited states. XAFS provides information on the local electronic and geometrical structures around an X-ray-absorbing atom and can address photocarrier dynamics in the r-space observed from the X-ray-absorbing atom. In this article, we discuss the time dependent structure change of tungsten trioxide (WO3) and bismuth vanadate (BiVO4) photocatalysts studied by the ultrafast pump-probe XAFS method in the femtosecond to nanosecond time scale with the Photon Factory Advanced Ring (PF-AR) and the SPring-8 Angstrom Compact free-electron LAser (SACLA). WO3 shows a femtosecond decay process of photoexcited electrons followed by a structural change to a metastable state with a hundred picosecond speed, which is relaxed to the ground-state structure with a nanosecond time constant. The Bi L3 edge of BiVO4 shows little contribution of the Bi 6s electron to the photoabsorption process; however, it is sensitive to the structural change induced by the photoexcited electron. Time-resolved XAFS measurements in a wide range time domain and with varied wavelengths of the excitation pump laser facilitate understanding of the overall details regarding the photocarrier dynamics that have a significant influence on the photocatalytic performance.

Accuracy of XAS theory for unraveling structural changes of adsorbates: CO on Ni(100)

Studying surface reactions using ultrafast optical pump and x-ray probe experiments relies on accurate calculations of x-ray spectra of adsorbates for the correct identification of the spectral signatures and their dynamical evolution. We show that experimental x-ray absorption can be well reproduced for different binding sites in a static prototype system CO/Ni(100) at a standard density functional theory generalized-gradient-approximation level of theory using a plane-wave basis and pseudopotentials. This validates its utility in analyzing ultrafast x-ray probe experiments. The accuracy of computed relative core level binding energies is about 0.2 eV, representing a lower limit for which spectral features can be resolved with this method. We also show that the commonly used Z + 1 approximation gives very good core binding energy shifts overall. However, we find a discrepancy for CO adsorbed in the hollow site, which we assign to the significantly stronger hybridization in hollow bonding than in on-top.

Direct observation on supersonic microprojectile penetration of carbon fiber composites with ultrafast synchrotron X-ray phase contrast imaging

High-speed penetration into carbon fiber composites is of fundamental importance to materials science and impact engineering, but research along this line suffers considerably from the lack of direct experimental observations. Here we investigate such penetration dynamics of a unidirectional carbon fiber reinforced epoxy (UCFRE) composite, with a combination of in situ, ultrafast, synchrotron phase contrast imaging and finite element (FE) analysis. The experiments yield the first direct observation on projectile trajectories and fiber-scale deformation and damage in the UCFRE composites during supersonic microprojectile penetration, for different fiber orientations (0∘−90∘ from the impact direction) and projectile velocities 600−850m.s-1, at unprecedented temporal (∼100 ps) and spatial (5 μm) resolutions. The maximum penetration depth decreases with increasing fiber orientation angles, as a result of anisotropic damage evolution in the composite sample. Strain localizations are prone to develop along a direction perpendicular to the fiber orientation, while the damage or cavity region, along the fiber direction. FE modeling with a three-dimensional Hashin criterion yields consistent projectile trajectory and cavity morphology with the experimental results. With increasing fiber orientation angles, damage analyses show a transition in the damage mode from fiber compression to matrix compression damage, in line with the increasing maximum penetration depth.

Visualizing conical intersection passages via vibronic coherence maps generated by stimulated ultrafast X-ray Raman signals

The rates and outcomes of virtually all photophysical and photochemical processes are determined by conical intersections. These are regions of degeneracy between electronic states on the nuclear landscape of molecules where electrons and nuclei evolve on comparable timescales and thus become strongly coupled, enabling radiationless relaxation channels upon optical excitation. Due to their ultrafast nature and vast complexity, monitoring conical intersections experimentally is an open challenge. We present a simulation study on the ultrafast photorelaxation of uracil, based on a quantum description of the nuclei. We demonstrate an additional window into conical intersections obtained by recording the transient wavepacket coherence during this passage with an X-ray free-electron laser pulse. Two major findings are reported. First, we find that the vibronic coherence at the conical intersection lives for several hundred femtoseconds and can be measured during this entire time. Second, the time-dependent energy-splitting landscape of the participating vibrational and electronic states is directly extracted from Wigner spectrograms of the signal. These offer a physical picture of the quantum conical intersection pathways through visualizing their transient vibronic coherence distributions. The path of a nuclear wavepacket in the vicinity of the conical intersection is directly mapped by the proposed experiment.

A Smart Multi-Plane Detector Design for Ultrafast Electron Beam X-ray Computed Tomography.

In this paper, a smart detector design for novel multi-plane ultrafast electron beam X-ray computed tomography is presented. The concept is based on multi-plane electron beam scanning on a transparent X-ray target and elongated cuboid-shape scintillation detectors for radiation detection over an extended axial scanning range. The optical part of the scintillation detector acts as both an X-ray sensitive scintillator with a fast time response and a light guide. With that, we reduce detector complexity, number of detector elements, overall power consumption, and detector costs. We investigated the performance of this new multi-plane detector design with an evaluation detector setup that is made of cerium doped lutetium yttrium oxyorthosilicate (LYSO:Ce) as scintillation material and an avalanche photodiode (APD) array. Thereby, we assessed two design variants: A monolithic LYSO bar detector and a sandwich detector made of multiple LYSO crystals and glass light-guides. Both types reveal excellent linear detector responses, long-term stabilities, and comparable signal qualities.

Diffractive imaging in a flash

Ultrafast Imaging Ultrashort light pulses on the time scale of attoseconds provide a window into some of the fastest electronic effects occurring in solid-state systems. Obtaining structural information through coherent diffractive imaging is usually done with monochromatic x-ray sources. However, ultrashort pulses are inherently broadband, and getting transient structural information on such short time scales is challenging. Rana et al. describe a method that works with the broadband nature of ultrashort pulses. They split the pulses into 17 different wavelengths and then used an algorithm to computationally stitch together the diffraction patterns from each wavelength to reveal the structural image optimized across all wavelengths. Demonstrating the technique at optical wavelengths illustrates the feasibility of applying the method to ultrafast x-ray pulses. Phys. Rev. Lett. 125 , 086101 (2020).

Relativistic mid-wavelength infrared pulses generated in intense-laser mass-limited target interactions

Infrared spectroscopy, ultra-fast x-ray high harmonic generation, and time-resolved imaging of molecular structures benefit from the availability of intense mid-infrared wavelength pulses. Here we present a new approach to generating these, in which an intense short laser pulse is incident upon a near-critical density, spherical, mass-limited carbon target. After the laser pulse interaction, the carbon ions produced form a central force field. Plasma electrons accelerated by the laser return to the positively charged carbon target under the action of this field. The energy of these electrons is reduced and is lower than their energy in the laser field. These low-energy refluxing electrons start to oscillate with a rotating figure-of-eight motion around the positively charged carbon target and emit relativistically intense mid-infrared pulses with the wavelength in the range 1 to 4 μm.

Experimental analysis of gas phase dynamics in a lab scale bubble column operated with deionized water and NaOH solution under uniform bubbly flow conditions

In this study we present an investigation on the impact of sparger configuration and fluid properties on the local gas phase dynamics for up to 17% total gas holdup. Experiments in a bubble column with 100 mm inner diameter were conducted with deionized water and NaOH solution of up to cNaOH = 32 mmol·l−1. Ultrafast X-ray computed tomography (UFXCT) was applied to obtain local hydrodynamic parameters, such as gas holdup distribution, bubble sizes and average bubble rise velocities. Radial gas holdup profiles show wall peaking, which is attributed to the uniform bubbly flow regime generated by the needle sparger used in this study. The addition of NaOH leads to similar gas holdup values but significant changes on local bubbly dynamics, such as bubble size distribution and axial gas phase velocity. The Sauter mean diameter was found to decrease with increasing concentration of NaOH, whereby the decrease is larger at higher gas flow rate.

Downward jetting of a dynamic Leidenfrost drop

Using ultrafast x-ray imaging, downward jetting of a Leidenfrost drop is observed and studied for the first time. The downward jetting is caused by capillary waves propagating on the bottom liquid-vapor interface during retraction of the drop. The downward jetting can be inhibited in the case of viscous damping of capillary waves. A jetting criterion depending on the Ohnesorge and Weber numbers is suggested: (Oh)(We${}^{2})\ensuremath{\le}66\ifmmode\pm\else\textpm\fi{}10$.

Direct Spectroscopic Observation of the Hole Polaron in Lead Halide Perovskites

Intrinsic photophysical origin of lead halide perovskites (LHPs) succeeding in optolectronic applications still remains hotly debated. Here, by using the ultrafast X-ray transient absorption spectroscopy, we successfully tracked the fate of photogenerated charge carriers at room temperature within the thin films of two classic LHPs, namely MAPbBr3 (MA: CH3NH3) and FAPbBr3 (FA: CH(NH2)2). We clearly observed in both thin films that the hole polaron is formed by localizing the photogenerated hole at Br 4p orbital and concurrently distorting the local structure surrounding Br atom after the photoexcitation. Furthermore, the bigger cation FA in the cavity of [PbBr6]4- octahedral framework induces larger hole polaron effect due to its p orbital hybridization into valence and conduction bands, correlating with the slower charge carrier recombination dynamics. Our direct experimental observation of the localized hole polaron in perovskites should advance the fundamental comprehension of charge carrier behavior within LHPs and their related devices.

Challenges and tolerances for a compact and hybrid ultrafast X-ray pulse source based on RF and THz technologies

We present an in-depth tolerance study and investigation of the main challenges towards the realization of a hybrid compact ultrafast (fs to sub-fs) X-ray pulse source based on the combination of a conventional S-band gun as electron source and a THz-driven dielectric-loaded waveguide as post-acceleration and compression structure. This study allows us determining which bunch properties are the most affected, and in which proportion, for variations of the parameters of all the beamline elements compared to their nominal values. This leads to a definition of tolerances for the misalignments of the elements and the jitter of their parameters, which are compared to the state-of-the-art in terms of alignment precision and stability of operation parameters. The most challenging aspects towards the realization of the proposed source, including THz generation and manufacturing of the dielectric-loaded waveguide, are finally summarized and discussed.

Structural Heterogeneity in Single Particle Imaging Using X-ray Lasers.

One of the challenges facing single particle imaging with ultrafast X-ray pulses is the structural heterogeneity of the sample to be imaged. For the method to succeed with weakly scattering samples, the diffracted images from a large number of individual proteins need to be averaged. The more the individual proteins differ in structure, the lower the achievable resolution in the final reconstructed image. We use molecular dynamics to simulate two globular proteins in vacuum, fully desolvated as well as with two different solvation layers, at various temperatures. We calculate the diffraction patterns based on the simulations and evaluate the noise in the averaged patterns arising from the structural differences and the surrounding water. Our simulations show that the presence of a minimal water coverage with an average 3 Å thickness will stabilize the protein, reducing the noise associated with structural heterogeneity, whereas additional water will generate more background noise.

Femtosecond synchronization of multiple mode-locked lasers and a microwave oscillator by multicolor electro-optic sampling.

Simple multicolor electro-optic sampling-based femtosecond synchronization of multiple mode-locked lasers is demonstrated. Parallel timing error detection between each laser and a common microwave is achieved by wavelength division multiplexing and demultiplexing. The parallel timing error detection enables simultaneous femtosecond synchronization of more than two mode-locked lasers to the microwave oscillator, even when the lasers have different repetition rates. The residual root-mean-square (rms) timing jitter of laser-laser synchronization measured by an optical cross correlator is 2.6 fs (integration bandwidth, 100 Hz-1 MHz), which is limited by the actuator bandwidth in the laser oscillator. The long-term rms timing drift and frequency instability of laser-microwave synchronization are 7.1 fs (over 10,000 s) and 5.5×10-18 (over 2000 s averaging time), respectively. As a versatile and reconfigurable tool for laser-laser and laser-microwave synchronization, the demonstrated method can be used for various applications ranging from ultrafast x-ray and electron science facilities to dual- and triple-comb spectroscopy.

Angular Momentum in Rotating Superfluid Droplets.

The angular momentum of rotating superfluid droplets originates from quantized vortices and capillary waves, the interplay between which remains to be uncovered. Here, the rotation of isolated submicrometer superfluid ^{4}He droplets is studied by ultrafast x-ray diffraction using a free electron laser. The diffraction patterns provide simultaneous access to the morphology of the droplets and the vortex arrays they host. In capsule-shaped droplets, vortices form a distorted triangular lattice, whereas they arrange along elliptical contours in ellipsoidal droplets. The combined action of vortices and capillary waves results in droplet shapes close to those of classical droplets rotating with the same angular velocity. The findings are corroborated by density functional theory calculations describing the velocity fields and shape deformations of a rotating superfluid cylinder.

Investigations on bubbly two-phase flow in a constricted vertical pipe

We report on an experimental investigation of adiabatic bubbly two-phase flow development in a DN50 pipe with a ring-shaped and a baffle-shaped constriction at different superficial velocities of gas (up to jg = 0.1400 m·s−1) and liquid (up to jl = 1.6110 m·s-1) using ultrafast electron beam X-ray computed tomography (UFXCT). From UFXCT images, cross-sectional gas holdup distributions were obtained with a temporal resolution of up to 2,500 frames per second in 18 scanning planes along the pipe. A sophisticated data processing approach was applied to extract gas holdup data immediately from the two-phase flow image stack. Based on that, time-averaged gas holdup of the cross-section and the axial center of the pipe were calculated. In addition, bubble sizes and velocities were determined.

In-situ Synchrotron imaging of keyhole mode multi-layer laser powder bed fusion additive manufacturing

The keyhole mode in laser powder bed fusion (LPBF) additive manufacturing can be associated with excessive porosity and spatter, however, the underlying physics in multilayer build conditions remain unclear. Here, we used ultra-fast synchrotron X-ray imaging to reveal this phenomena. We in investigated melt pool dynamics, keyhole porosity and spatter formation mechanisms and their impact in all layers of the build. We observed that the transient melt pool dynamics associated with the keyhole include: (I) keyhole initiation, (II) keyhole development, and (III) melt pool recovery. Porosity and spatter were associated with stages (II) and (III). We also discovered that droplet spatter can form due to the collapse of the keyhole recoil zone, causing molten particle agglomeration and ejection during stage (III). Our results clarify the transient dynamics behind the keyhole mode in a multi-layer LBPF process and can be used to guide the reduction in porosity and spatter in additive manufacturing.

Ultrafast x-ray and electron scattering of free molecules: A comparative evaluation.

Resolving gas phase molecular motions with simultaneous spatial and temporal resolution is rapidly coming within the reach of x-ray Free Electron Lasers (XFELs) and Mega-electron-Volt (MeV) electron beams. These two methods enable scattering experiments that have yielded fascinating new results, and while both are important methods for determining transient molecular structures in photochemical reactions, it is important to understand their relative merits. In the present study, we evaluate the respective scattering cross sections of the two methods and simulate their ability to determine excited state molecular structures in light of currently existing XFEL and MeV source parameters. Using the example of optically excited N-methyl morpholine and simulating the scattering patterns with shot noise, we find that the currently achievable signals are superior with x-ray scattering for equal samples and on a per-shot basis and that x-ray scattering requires fewer detected signal counts for an equal fidelity structure determination. Importantly, within the independent atom model, excellent structure determinations can be achieved for scattering vectors only to about 5 Å−1, leaving larger scattering vector ranges for investigating vibrational motions and wavepackets. Electron scattering has a comparatively higher sensitivity toward hydrogen atoms, which may point to applications where electron scattering is inherently the preferred choice, provided that excellent signals can be achieved at large scattering angles that are currently difficult to access.

Observation of the molecular response to light upon photoexcitation

When a molecule interacts with light, its electrons can absorb energy from the electromagnetic field by rapidly rearranging their positions. This constitutes the first step of photochemical and photophysical processes that include primary events in human vision and photosynthesis. Here, we report the direct measurement of the initial redistribution of electron density when the molecule 1,3-cyclohexadiene (CHD) is optically excited. Our experiments exploit the intense, ultrashort hard x-ray pulses of the Linac Coherent Light Source (LCLS) to map the change in electron density using ultrafast x-ray scattering. The nature of the excited electronic state is identified with excellent spatial resolution and in good agreement with theoretical predictions. The excited state electron density distributions are thus amenable to direct experimental observation.

Measurement of transient strain induced by two-photon excitation

This paper quantifies the lattice expansion of an insulator upon two photon excitation. The ultrafast x-ray diffraction experiments measure a clean quadratic dependence of the strain on the laser fluency and prove that the pulse intensity is the relevant quantity, as the signal is anti-proportional to the pulse duration. The investigated material Bi:YIG is the standard material for magnonics, where ultrashort laser-pulses can create spin-waves via various mechanisms, including strain modulation of the anisotropy