Time-Resolved X-ray Scattering Research Articles

A time-dependent momentum-resolved scattering approach to core-level spectroscopies

While new light sources allow for unprecedented resolution in experiments with X-rays, a theoretical understanding of the scattering cross-section is lacking. In the particular case of strongly correlated electron systems, numerical techniques are quite limited, since conventional approaches rely on calculating a response function (Kramers-Heisenberg formula) that is obtained from a perturbative analysis of scattering processes in the frequency domain. This requires a knowledge of a full set of eigenstates in order to account for all intermediate processes away from equilibrium, limiting the applicability to small tractable systems. In this work, we present an alternative paradigm, recasting the problem in the time domain and explicitly solving the time-dependent Schrödinger equation without the limitations of perturbation theory: a faithful simulation of the scattering processes taking place in actual experiments, including photons and core electrons. We show how this approach can yield the full time and momentum resolved Resonant Inelastic X-Ray Scattering (RIXS) spectrum of strongly interacting many-body systems. We demonstrate the formalism with an application to Mott insulating Hubbard chains using the time-dependent density matrix renormalization group method, which does not require a priory knowledge of the eigenstates and can solve very large systems with dozens of orbitals. This approach can readily be applied to systems out of equilibrium without modification and generalized to other spectroscopies.

Transient vibration and product formation of photoexcited CS2 measured by time-resolved x-ray scattering.

We have observed details of the internal motion and dissociation channels in photoexcited carbon disulfide (CS2) using time-resolved x-ray scattering (TRXS). Photoexcitation of gas-phase CS2 with a 200nm laser pulse launches oscillatory bending and stretching motion, leading to dissociation of atomic sulfur in under a picosecond. During the first 300fs following excitation, we observe significant changes in the vibrational frequency as well as some dissociation of the C-S bond, leading to atomic sulfur in the both 1D and 3P states. Beyond 1400fs, the dissociation is consistent with primarily 3P atomic sulfur dissociation. This channel-resolved measurement of the dissociation time is based on our analysis of the time-windowed dissociation radial velocity distribution, which is measured using the temporal Fourier transform of the TRXS data aided by a Hough transform that extracts the slopes of linear features in an image. The relative strength of the two dissociation channels reflects both their branching ratio and differences in the spread of their dissociation times. Measuring the time-resolved dissociation radial velocity distribution aids the resolution of discrepancies between models for dissociation proposed by prior photoelectron spectroscopy work.

Nucleation Kinetics and Structure Evolution of Quasi-Two-Dimensional ZnO at the Air-Water Interface: An In Situ Time-Resolved Grazing Incidence X-ray Scattering Study.

The design and synthesis of high-quality two-dimensional (2D) materials with desired morphology are essential for property control. One critical challenge that impedes the understanding and control of 2D crystal nucleation and growth is the inability of direct observation of the nanocrystal evolution process with high enough time resolution. Here, we demonstrated an in situ X-ray scattering approach that directly reveals 2D wurtzite ZnO nanosheet growth at the air-water interface. The time-resolved grazing incidence X-ray diffraction (GID) and grazing incidence X-ray off-specular scattering (GIXOS) results uncovered a lateral to vertical growth kinetics switch phenomenon in the ZnO nanosheet growth. This switch represents the 2D to three-dimensional (3D) crystal structure evolution, which governs the size and thickness of nanosheets, respectively. This phenomenon can guide 2D nanocrystal synthesis with rationally controlled size and thickness. Our work opens a new pathway toward the understanding of 2D nanomaterial growth kinetics based on time-resolved liquid surface grazing incidence X-ray techniques.

Functional Expansion of G-Protein-Coupled Receptor Shown by Time-Resolved X-Ray Scattering

Visual rhodopsin forms an excellent archetype for studying spatial and temporal protein dynamics in G-protein-coupled receptors (GPCR) of the human genome. We have proposed a volumetric expansion in rhodopsin following its photoactivation [1]; however, a time-resolved description is lacking. Here we show the first time-resolved X-ray scattering-based detection of such an expansion of rhodopsin leading to the signaling state. We conducted time-resolved small-angle and wide-angle scattering (TR-SAXS and TR-WAXS, respectively) of rhodopsin solubilized in CHAPS detergent at the BioCARS synchrotron (Advanced Photon Source).

X-ray scattering signatures of early-time accelerations in iodine dissociation

Ultrafast time-resolved x-ray scattering (TRXS) from a photoexcited molecular ensemble measures a distribution S(Q, τ) of the x-ray momentum transfer Q and pump-probe delay τ in which all modes of motion induced by the excitation overlap. Frequency-resolved x-ray scattering (FRXS) based on separates each oscillation and dissociation channel in the TRXS data, enabling measurements of vibrational frequencies and phases, and dissociation velocities and time shifts. Here we extend FRXS analysis to study early-time accelerations as well. We show how these appear as diffuse scattering patterns with characteristic phase evolution in FXRS and we develop a set of transformations that isolate individual channels to measure the early-time accelerated motion. This procedure is used to analyze diatomic iodine x-ray scattering data with multiple dissociations, and the ability of this technique to characterize early-time accelerations of one dissociation channel even in the presence of another dissociation is demonstrated.

Time-resolved resonant elastic soft x-ray scattering at Pohang Accelerator Laboratory X-ray Free Electron Laser.

Resonant elastic x-ray scattering has been widely employed for exploring complex electronic ordering phenomena, such as charge, spin, and orbital order, in particular, in strongly correlated electronic systems. In addition, recent developments in pump-probe x-ray scattering allow us to expand the investigation of the temporal dynamics of such orders. Here, we introduce a new time-resolved Resonant Soft X-ray Scattering (tr-RSXS) endstation developed at the Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL). This endstation has an optical laser (wavelength of 800 nm plus harmonics) as the pump source. Based on the commissioning results, the tr-RSXS at PAL-XFEL can deliver a soft x-ray probe (400 eV-1300 eV) with a time resolution of ∼100 fs without jitter correction. As an example, the temporal dynamics of a charge density wave on a high-temperature cuprate superconductor is demonstrated.

Characterizing Multiphoton Excitation Using Time-Resolved X-ray Scattering

Molecular iodine was photoexcited by a strong 800 nm laser, driving several channels of multiphoton excitation. The motion following photoexcitation was probed using time-resolved X-ray scattering, which produces a scattering map $S(Q,\tau)$. Temporal Fourier transform methods were employed to obtain a frequency-resolved X-ray scattering signal $\tilde{S}(Q,\omega)$. Taken together, $S(Q,\tau)$ and $\tilde{S}(Q,\omega)$ separate different modes of motion, so that mode-specific nuclear oscillatory positions, oscillation amplitudes, directions of motions, and times may be measured accurately. Molecular dissociations likewise have a distinct signature, which may be used to identify both velocities and dissociation time shifts, and also can reveal laser-induced couplings among the molecular potentials.

Probing Electronic Strain Generation by Separated Electron-Hole Pairs Using Time-Resolved X-ray Scattering

Photogeneration of excess charge carriers in semiconductors produces electronic strain. Under transient conditions, electron-hole pairs may be separated across a potential barrier. Using time-resolved X-ray diffraction measurements across an intrinsic AlGaAs/n-doped GaAs interface, we find that the electronic strain is only produced by holes, and that electrons are not directly observable by strain measurements. The presence of photoinduced charge carriers in the n-doped GaAs is indirectly confirmed by delayed heat generation via recombination.

Characterizing dissociative motion in time-resolved x-ray scattering from gas-phase diatomic molecules

Time-resolved x-ray scattering (TRXS) measures internuclear separations in a molecule following laser-induced photoexcitation. The molecular dynamics induced by the excitation laser may lie on one or several bound or dissociative electronic states. Time-resolved x-ray scattering from these states can be difficult to isolate because they generally overlap in the angle-resolved x-ray scattering pattern $I(x,y,\ensuremath{\tau})$, where $\ensuremath{\tau}$ is the pump-probe delay and $(x,y)$ are the physical pixel positions. Here we show how standard transform methods can isolate the dynamics from individual states. We form the temporal Fourier transform $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{I}(x,y,\ensuremath{\omega})={\ensuremath{\int}}_{\ensuremath{-}\ensuremath{\infty}}^{+\ensuremath{\infty}}d\ensuremath{\tau}\phantom{\rule{0.16em}{0ex}}{e}^{\ensuremath{-}i\ensuremath{\omega}\ensuremath{\tau}}I(x,y,\ensuremath{\tau})$. This frequency-resolved x-ray scattering (FRXS) signal segregates the bound states according to their vibrational frequencies ${\ensuremath{\omega}}_{i}$ and also displays dissociative states along straight lines $\ensuremath{\omega}=vQ$, where the slope $v$ is the rate of increase of the internuclear distance and $Q$ is the momentum transfer between the incident and scattered x-ray photon. We derive this relation and use FRXS to extract state-specific dynamics from experimental TRXS from molecular iodine following a 520-nm pump. Dynamics observed include one- and two-photon dissociation of the ${}^{1}{\mathrm{\ensuremath{\Pi}}}_{u}$ and ${}^{1}{\mathrm{\ensuremath{\Sigma}}}_{g}{}^{+}$ excited states and vibrational wave packets on the $B{\phantom{\rule{0.16em}{0ex}}}^{3}{\mathrm{\ensuremath{\Pi}}}_{0u}{}^{+}$ state.

In Situ Time-Resolved X-ray Scattering Study of Isotactic Polypropylene in Additive Manufacturing.

Using a specially designed apparatus, which collects simultaneous temperature and X-ray scattering data, we performed in situ measurements of the filament during MatEx 3D printing. The data show that the MatEx 3D printing extrusion process provides sufficient shear to form shish-kebab structures, which initially nucleate at the filament surface and spread into the filament core. Time-resolved measurements show that the kebab component near the surface relaxes after deposition of the second filament and enhances chain diffusion across the interface. SEM images indicate near complete interfacial merging of the filaments, which results in excellent mechanical properties.

On the limits of observing motion in time-resolved X-ray scattering.

Limits on the ability of time-resolved X-ray scattering (TRXS) to observe harmonic motion of amplitude, A and frequency, ω0, about an equilibrium position, R0, are considered. Experimental results from a TRXS experiment at the LINAC Coherent Light Source are compared to classical and quantum theories that demonstrate a fundamental limitation on the ability to observe the amplitude of motion. These comparisons demonstrate dual limits on the spatial resolution through Qmax and the temporal resolution through ωmax for observing the amplitude of motion. In the limit where ωmax ≈ ω0, the smallest observable amplitude of motion is A = 2π/Qmax. In the limit where ωmax≥2ω0, A≤2π/Qmax is observable provided there are sufficient statistics.This article is part of the theme issue ‘Measurement of ultrafast electronic and structural dynamics with X-rays’.

Structural and Thermal Characterisation of Nanofilms by Time-Resolved X-ray Scattering.

High time resolution in scattering analysis of thin films allows for determination of thermal conductivity by transient pump-probe detection of dissipation of laser-induced heating, TDXTS. We describe an approach that analyses the picosecond-resolved lattice parameter reaction of a gold transducer layer on pulsed laser heating to determine the thermal conductivity of layered structures below the transducer. A detailed modeling of the cooling kinetics by a Laplace-domain approach allows for discerning effects of conductivity and thermal interface resistance as well as basic depth information. The thermal expansion of the clamped gold film can be calibrated to absolute temperature change and effects of plastic deformation are discriminated. The method is demonstrated on two extreme examples of phononic barriers, isotopically modulated silicon multilayers with very small acoustic impedance mismatch and silicon-molybdenum multilayers, which show a high resistivity.

Revealing Fast Structural Dynamics in pH-Responsive Peptides with Time-Resolved X-ray Scattering.

Many biomaterials can adapt to changes in the local biological environment (such as pH, temperature, or ionic composition) in order to regulate function or deliver a payload. Such adaptation to environmental perturbation is typically a hierarchical process that begins with a response at a local structural level and then propagates to supramolecular and macromolecular scales. Understanding fast structural dynamics that occur upon perturbation is important for rational design of functional biomaterials. However, few nanosecond time-resolved methods can probe both intra- and intermolecular scales simultaneously with a high structural resolution. Here, we utilize time-resolved X-ray scattering to probe nanosecond to microsecond structural dynamics of poly-l-glutamic acid undergoing protonation via a pH jump initiated by photoexcitation of a photoacid. Our results provide insights into the protonation-induced hierarchical changes in packing of peptide chains, formation of a helical structure, and the associated collapse of the peptide chain.

Effect of Film Thickness on Induction Time of Isothermal Crystallization from the Melt of Poly(3-hydroxybutyrate) Investigated by Time-resolved Grazing-incidence X-ray Scattering Measurements Using Synchrotron Radiation

Effect of Film Thickness on Induction Time of Isothermal Crystallization from the Melt of Poly(3-hydroxybutyrate) Investigated by Time-resolved Grazing-incidence X-ray Scattering Measurements Using Synchrotron Radiation

Nanoparticle-Templated Self-Assembly of Viral Capsids Probed by Time-Resolved Absorbance Spectroscopy and X-Ray Scattering

Viral capsid proteins have the remarkable ability to self-assemble around a cargo core, whether their genome, a polyelectrolyte or an inorganic nanoparticle. Although the equilibrium properties of such composite objects have been broadly investigated, little is known about the kinetic pathways leading to their assembly. By exploiting the plasmon resonance effect of gold nanoparticles, we show that the adsorption of viral proteins can be monitored through the redshift of the resonance wavelength. Kinetic measurements reveal that the process occurs in less than a second. The spatial arrangement of nanoparticles is probed by time-resolved small-angle X-ray scattering, and we observe the formation of transient aggregates with a lifetime in the hour range. The biocompatibility of gold nanoparticles induced by adsorbed proteins is of great importance for their use in biomedicine and the reported results should help better understand the processes of formation.

Dynamics of Quaternary Structure Transitions in R-State Carbonmonoxyhemoglobin Unveiled in Time-Resolved X-ray Scattering Patterns Following a Temperature Jump.

It is well-known that tetrameric hemoglobin binds ligands cooperatively by undergoing a ligand-induced T → R quaternary structure transition, a structure-function relationship that has long served as a model system for understanding allostery in proteins. However, kinetic studies of the reverse, R → T quaternary structure transition following photolysis of carbonmonoxyhemoglobin (HbCO) reveal complex behavior that may be better explained by the presence of two different R quaternary structures coexisting in thermal equilibrium. Indeed, we report here time-resolved small- and wide-angle X-ray scattering (SAXS/WAXS) patterns of HbCO following a temperature jump that not only provide unambiguous evidence for more than one R state, but also unveil the time scale for interconversion between them. Since the time scale for the photolysis-induced R → T transition is likely different for different R-states, this structural heterogeneity must be accounted for to properly explain the kinetic heterogeneity observed in time-resolved spectroscopic studies following photolysis of HbCO.

In-Situ X-Ray Observations of Sn Electrodeposition into Anodized Alumina

Aluminum is used widely for various products and application because of its characteristics of high strength, low density and good corrosion resistance. When aluminum is exposed to air a native oxide forms on its surface and protects it from corrosion[1,2]. The native oxide can by an electrochemical method called anodization be grown thicker to give a better corrosion protection of the aluminum[3]. By using acidic electrolytes for the anodization the grown oxide will become porous, these pores can become ordered under certain anodization conditions[4]. The grown Nano-Porous Anodic Aluminum Oxide (NP-AAO) can be used as a template for growing nano wires and can be colored by deposition of metals into the pores. We have followed in-situ the electrochemical deposition of Sn into the NP-AAO by Grazing Transmission Small Angle X-Ray Scattering (GTSAXS), X-Ray Fluorescence (XRF) and X-Ray Absorption Near Edge Structure (XANES). A two-step anodization method was used to create the ordered NP-AAO, this is achieved by anodizing the sample in acidic electrolytes in two anodizing steps with a chemical etch between the steps. The second anodization step was followed in-situ with GTSAXS[5]. To achieve a better deposition into the pores of the oxide, either a barrier layer thinning or a pore widening step was used after the second anodization. These methods will decrease the barrier layer at the bottom of the pores by a controlled down stepping of the anodization potential and make the pores wider trough a chemical etch. Both methods were followed continuously by GTSAXS. The electrochemical deposition of Sn was achieved using an alternating voltage and an electrolyte containing tin ions. We followed the deposition continuously with GTSAXS, while XRF and XANES was measured at intervals during the deposition. Our GTSAXS measurements shows an increase in intensity during deposition, indicating deposition into the pores of the aluminum oxide. From our XRF measurements an increase of deposited Sn could be detected as the XRF signal at the sample increased over time. We used XANES for determining the chemical state of the deposited Sn. Ex-situ FIB-SEM measurements shows the pores filled with Sn, confirming the in-situ measurements. The sample also had a change in color confirming the Sn presence inside the NP-AAO. References L.P.H. Jeurgens, W.G. Sloof, F.D. Tichelaar, C.G. Borsboom, E.J. Mittemeijer, Determination of thickness and composition of aluminium-oxide overlayers on aluminium substrates, Appl. Surf. Sci. 144, 11 (1999)J. Evertsson et al, The thickness of native oxides on aluminum alloys and single crystals, Appl.Surf. Sci. 349, 826 (2015)F. Bertram et al, In situ anodization of aluminum surfaces studied by x-ray reflectivity and electrochemical impedance spectroscopy, J. Appl. Phys. 116, 034902, (2014)W. Lee. and S.-J. Park, Porous Anodic Aluminum Oxide: Anodization and Templated Synthesis of Functional Nanostructures, Chem. Rev. 114, 7487 (2014)Nikolay A. Vinogradov, Gary S. Harlow, Francesco Carlà, Jonas Evertsson, Lisa Rullik, Weronica Linpé, Roberto Felici, and Edvin Lundgren, Observation of Pore Growth and Self-Organization in Anodic Alumina by Time-Resolved X-ray Scattering, ACS Applied Nano Materials, Article ASAP

Delayed auger recombination in silicon measured by time-resolved X-ray scattering

We report a new method of measuring the non-radiative recombination rate in bulk Silicon. Synchrotron time-resolved x-ray scattering (TRXS) combines femtometer spatial sensitivity with nanosecond time resolution to record the temporal evolution of a crystal lattice following intense ultrafast laser excitation. Modeling this data requires an Auger recombination time that is considerably slower than previous measurements, which were made at lower laser intensities while probing only a relatively shallow surface depth. We attribute this difference to an enhanced Coulomb interaction that has been predicted to occur in bulk materials with high densities of photoexcited charge carriers.

Probing Cytochrome c Folding Transitions upon Phototriggered Environmental Perturbations Using Time-Resolved X-ray Scattering.

Direct tracking of protein structural dynamics during folding-unfolding processes is important for understanding the roles of hierarchic structural factors in the formation of functional proteins. Using cytochrome c (cyt c) as a platform, we investigated its structural dynamics during folding processes triggered by local environmental changes (i.e., pH or heme iron center oxidation/spin/ligation states) with time-resolved X-ray solution scattering measurements. Starting from partially unfolded cyt c, a sudden pH drop initiated by light excitation of a photoacid caused a structural contraction in microseconds, followed by active site restructuring and unfolding in milliseconds. In contrast, the reduction of iron in the heme via photoinduced electron transfer did not affect conformational stability at short timescales (<1 ms), despite active site coordination geometry changes. These results demonstrate how different environmental perturbations can change the nature of interaction between the active site and protein conformation, even within the same metalloprotein, which will subsequently affect the folding structural dynamics.

Observation of Pore Growth and Self-Organization in Anodic Alumina by Time-Resolved X-ray Scattering

The anodic oxidation of metals such as aluminum and titanium can lead to the development of self-ordering pores. These pores make excellent templates for a range of nanoscale objects with many applications in nanoscience. Theoretical studies on pore formation have proposed several models for the establishment, growth, and ordering of these pores; however, experimental verification has mostly been limited to ex situ measurements. Here we show that the lateral and vertical pore structure can be probed in situ with high precision, using grazing transmission X-ray scattering. By making use of the high flux available at modern synchrotrons and fitting only the difference between scattering patterns we show the nearly real-time evolution of the pore’s arrangement. We observe no dependence on the substrate crystallographic orientation for domain size or pore separation. We do however observe an anisotropy in the oxide growth rate for the different substrate surfaces. This experimental approach can be applied to ...