X-ray Correlation Spectroscopy Research Articles

Brownian and advective dynamics in microflow studied by coherent X-ray scattering experiments.

Combining microfluidics with coherent X-ray illumination offers the possibility to not only measure the structure but also the dynamics of flowing samples in a single-scattering experiment. Here, the power of this combination is demonstrated by studying the advective and Brownian dynamics of colloidal suspensions in microflow of different geometries. Using an experimental setup with a fast two-dimensional detector and performing X-ray correlation spectroscopy by calculating two-dimensional maps of the intensity auto-correlation functions, it was possible to evaluate the sample structure and furthermore to characterize the detailed flow behavior, including flow geometry, main flow directions, advective flow velocities and diffusive dynamics. By scanning a microfocused X-ray beam over a microfluidic device, the anisotropic auto-correlation functions of driven colloidal suspensions in straight, curved and constricted microchannels were mapped with the spatial resolution of the X-ray beam. This method has not only a huge potential for studying flow patterns in complex fluids but also to generally characterize anisotropic dynamics in materials.

A liquid jet setup for x-ray scattering experiments on complex liquids at free-electron laser sources.

In this paper we describe a setup for x-ray scattering experiments on complex fluids using a liquid jet. The setup supports Small and Wide Angle X-ray Scattering (SAXS/WAXS) geometries. The jet is formed by a gas-dynamic virtual nozzle (GDVN) allowing for diameters ranging between 1 μm and 20 μm at a jet length of several hundred μm. To control jet properties such as jet length, diameter, or flow rate, the instrument is equipped with several diagnostic tools. Three microscopes are installed to quantify jet dimensions and stability in situ. The setup has been used at several beamlines performing both SAXS and WAXS experiments. As a typical example we show an experiment on a colloidal dispersion in a liquid jet at the X-ray Correlation Spectroscopy instrument at the Linac Coherent Light Source free-electron laser.

The X-ray Correlation Spectroscopy instrument at the Linac Coherent Light Source.

The X-ray Correlation Spectroscopy instrument is dedicated to the study of dynamics in condensed matter systems using the unique coherence properties of free-electron lasers. It covers a photon energy range of 4-25 keV. The intrinsic temporal characteristics of the Linac Coherent Light Source, in particular the 120 Hz repetition rate, allow for the investigation of slow dynamics (milliseconds) by means of X-ray photon correlation spectroscopy. Double-pulse schemes could probe dynamics on the picosecond timescale. A description of the instrument capabilities and recent achievements is presented.

The potential of future light sources to explore the structure and function of matter.

Structural studies in general, and crystallography in particular, have benefited and still do benefit dramatically from the use of synchrotron radiation. Low-emittance storage rings of the third generation provide focused beams down to the micrometre range that are sufficiently intense for the investigation of weakly scattering crystals down to the size of several micrometres. Even though the coherent fraction of these sources is below 1%, a number of new imaging techniques have been developed to exploit the partially coherent radiation. However, many techniques in nanoscience are limited by this rather small coherent fraction. On the one hand, this restriction limits the ability to study the structure and dynamics of non-crystalline materials by methods that depend on the coherence properties of the beam, like coherent diffractive imaging and X-ray correlation spectroscopy. On the other hand, the flux in an ultra-small diffraction-limited focus is limited as well for the same reason. Meanwhile, new storage rings with more advanced lattice designs are under construction or under consideration, which will have significantly smaller emittances. These sources are targeted towards the diffraction limit in the X-ray regime and will provide roughly one to two orders of magnitude higher spectral brightness and coherence. They will be especially suited to experiments exploiting the coherence properties of the beams and to ultra-small focal spot sizes in the regime of several nanometres. Although the length of individual X-ray pulses at a storage-ring source is of the order of 100 ps, which is sufficiently short to track structural changes of larger groups, faster processes as they occur during vision or photosynthesis, for example, are not accessible in all details under these conditions. Linear accelerator (linac) driven free-electron laser (FEL) sources with extremely short and intense pulses of very high coherence circumvent some of the limitations of present-day storage-ring sources. It has been demonstrated that their individual pulses are short enough to outrun radiation damage for single-pulse exposures. These ultra-short pulses also enable time-resolved studies 1000 times faster than at standard storage-ring sources. Developments are ongoing at various places for a totally new type of X-ray source combining a linac with a storage ring. These energy-recovery linacs promise to provide pulses almost as short as a FEL, with brilliances and multi-user capabilities comparable with a diffraction-limited storage ring. Altogether, these new X-ray source developments will provide smaller and more intense X-ray beams with a considerably higher coherent fraction, enabling a broad spectrum of new techniques for studying the structure of crystalline and non-crystalline states of matter at atomic length scales. In addition, the short X-ray pulses of FELs will enable the study of fast atomic dynamics and non-equilibrium states of matter.

The potential of future light sources to explore the structure and function of matter

Structural studies in general and in particular in crystallography have benefited and still do benefit dramatically from the use of synchrotron radiation. Its tuneability is mandatory for multi or single anomalous diffraction experiments, still one of the main methods for solving new crystal structures. Its tuneability is also a key for spectroscopy techniques for the determination of the local atomic environment around and the oxidation state of an absorbing atom. These techniques are powerful tools e.g. in chemistry to study reactions, and can be applied not only to crystalline matter. Low emittance storage rings of the third generation with their highly brilliant X-ray beams enable us to focus beams down to the micrometer range intense enough for the investigation of weakly scattering crystals down to the size of several micrometers. Considering these highly intense beams, if it comes to structural studies using X-ray, what are still the limitations of the most modern storage ring sources? The length of individual X-ray pulses is in the order of 100 ps, which is sufficient to trace structural changes of larger groups or the diffusion of atoms over larger atomic distances. However, fast processes as they occur e.g. during vision or photosynthesis are not accessible by these means. Also the coherent fraction of the radiation of present day storage rings in the X-ray regime is rather low (i.g. < 1 %). This limits on one hand our ability to study the structure and dynamic of non-crystalline materials by methods exploiting the coherence properties of the beam like coherent diffractive imaging and X-ray correlation spectroscopy, respectively. On the other hand the flux in an ultra small diffraction-limited focus is limited as well. Upcoming linac driven free electron laser (FEL) sources with extremely short (sub 100 fs) and intense pulse (~10^12 ph) of very high coherence circumvent some of the limitations of present day storage rings. It has been demonstrated that their individual pulses are short enough to outrun radiation damage for single pulse exposures. First structures from sub micrometer crystals using an X-ray FEL (LCLS, Stanford) have already been published. These ultra short pulses also enable time resolved studies 1000 times faster than at standard storage ring sources. Meanwhile new storage rings with more aggressive lattice designs are under construction or under consideration with significantly smaller emittances. These sources target towards the diffraction limit in the X-ray regime and will provide roughly one to two orders of magnitude higher brilliance and coherence. They will be especially suited to those experiments exploiting the coherence properties of the beams and to ultra small focal spot sizes in the several nm regime. Developments at various places are ongoing for a totally new type of X-ray source combining a linac with a storage ring. This so called energy recovery linacs (ERL) promise to provide pulses almost as short as at an FEL with brilliances and multi-user capabilities comparable to a diffraction limited storage ring. The contribution will try to give an overview of the stage of development of the various source projects and their possible impact on structural studies in future.

ALS-II, a Potential Soft X-ray, Diffraction Limited Upgrade of the Advanced Light Source

The Advanced Light Source (ALS) at Berkeley Lab has seen many upgrades over the years, keeping it one of the brightest sources for soft x-rays worldwide. Recent developments in magnet technology and lattice design appear to open the door for very large further increases in brightness [1], particularly by reducing the horizontal emittance, even within the space constraints of the existing tunnel. Initial studies for possible lattices will be presented that could approach the soft x-ray diffraction limit around 2 keV in both planes within the ALS footprint.Emerging scientific applications and experimental methods that would greatly benefit from ring based sources having much higher brightness and transverse coherence than present or near future storage ring facilities include nanometer imaging applications, X-ray correlation spectroscopy, diffraction microscopy, holography, ptychography, and resonant inelastic soft X-ray scattering at high resolution.

Two-dimensional x-ray correlation spectroscopy of remote core states.

Nonlinear all-X-ray signals that involve large core-atom separation compared to the X-ray wavelengths may not be described by the dipole approximation since they contain additional phase factors. Expressions for the rotationally averaged 2D X-ray photon echo signals from randomly oriented systems that take this position-dependent phase into account for arbitrary ratio between the core separation and the resonant wavelength are presented. Application is made to the Se K-edge of a selenium dipeptide system.

Curcumin containing monoolein aqueous dispersions: A preformulative study

The present study describes the production and characterization of monoolein aqueous dispersions (MAD) as drug delivery systems for curcumin (CR).MAD based on monoolein and different emulsifiers have been produced and characterized. Morphology and dimensional distribution have been investigated by Cryogenic Transmission Electron Microscopy (cryo-TEM), X-ray and Photon Correlation Spectroscopy (PCS).Monoolein in different mixtures with sodium cholate, sodium caseinate, bentonite and poloxamer resulted in heterogeneous dispersions constituted of unilamellar vesicles, cubosomes and sponge type phases, depending on the employed components, as found by cryo-TEM and X-ray studies. CR was encapsulated with entrapment efficiencies depending on the MAD composition, particularly the highest was reached in the case of monoolein/poloxamer/sodium cholate mixture. The same mixture was able to maintain CR stability also after 6months.CR release modalities were in vitro investigated in order to mimic a possible subcutaneous administration of MAD. It was found that MAD constituted of monoolein/poloxamer and monoolein/poloxamer/sodium cholate mixtures were able to sustain CR release.MAD viscous vehicles were produced by xanthan gum.CR percutaneous absorption has been studied in vitro using excised human skin membranes [stratum corneum epidermis (SCE)] mounted into Franz cells. It was found that fluxes (Fn) of CR incorporated in MAD are influenced by the presence of monoolein based nanosystems. In particular xanthan gum based MAD better control CR diffusion from MAD.

Timepix detector at the X-ray Correlation Spectroscopy instrument at LCLS

The Timepix detector can be operated in Time-over-Threshold mode to allow for charge integration measurements as required by short (< 50 fs) x-ray pulses of the Linac Coherent Light Source (LCLS). Initial commissioning activities have started at the X-ray Correlation Spectroscopy (XCS) instrument at LCLS, where speckle patterns have been measured.

The X-ray Correlation Spectroscopy instrument at the Linac Coherent Light Source

The X-ray Correlation Spectroscopy instrument (XCS) at the Linac Coherent Light Source (LCLS) is a dedicated instrument using coherent x-ray scattering techniques to investigate dynamics in condensed matter systems. XCS can probe both slow and ultrafast dynamics on lengthscales of interest. It employs an extensive suite of X-ray instrumentation to tailor the LCLS X-ray beam properties to experimental requirements. Results demonstrating the full transverse coherence of the LCLS beam are presented.

Charge-pump detector for X-ray correlation spectroscopy

Abstract A detector for the X-ray Correlation Spectroscopy (XCS) instrument at the Linac Coherent Light Source (LCLS) in Stanford (CA) is being developed at Brookhaven National Laboratory (BNL). The LCLS is the first operational X-ray free electron laser. It provides extremely bright coherent laser-like X-ray pulses with energy up to 8 keV, shorter than 100 fs and with a repetition rate that will go up to 120 Hz. An ideal detector for XCS experiments should cover a large angular range with high efficiency and provide a proper resolution to resolve the speckle. The requirement for dynamic range is not particularly stringent while a fast readout is needed. In particular, the Charge Pump Detector has to be highly efficient at the energy of 8 keV, provide a dynamic range of 100 photons and a readout noise much better than one photon. The 1024×1024 pixels have to be read within the repetition rate of the laser pulses, that is faster than 8 ms. The pixel size of 56 μm×56 μm is a compromise between charge sharing and small pixel. Working principle and details of the detector will be discussed.

Probing Multiple Core−Hole Interactions in the Nitrogen K-Edge of DNA Base Pairs by Multidimensional Attosecond X-ray Spectroscopy. A Simulation Study

Two-dimensional X-ray correlation spectroscopy (2DXCS) signals of the isolated DNA bases and Watson-Crick base pairs which contain multiple absorbing nitrogen atoms are calculated. Core-hole excited states are calculated using density functional theory with the B3LYP functional and 6-311G** basis set. Sum over states calculations of the signals reveal changes in cross-peak intensities between hydrogen-bonded and stacked base pairs. Nucleobase analogues are proposed for investigating base-stacking and hydrogen-bonding interactions.

Probing heterogeneous dynamics of a repulsive colloidal glass by time resolved x-raycorrelation spectroscopy

We present here experimental results obtained by time resolved x-ray correlationspectroscopy, revealing the heterogeneous nature of the dynamics down to thenanoscale in a dense dispersion of magnetic nanoparticles. The dynamicalsusceptibility, which quantifies the amplitude of the dynamical fluctuations, isinvestigated in a repulsive colloidal glass. Its value is an order of magnitudesmaller than the ones found in gels and is length scale independent in theQ-range of the x-ray scattering experiment.

Polarization effects in coherent scattering from magnetic specimen: Implications for x-ray holography, lensless imaging, and correlation spectroscopy

We present resonant coherent small-angle scattering from sub-mm magnetic domains in an in-line holography geometry. By varying the polarization of the incident soft x rays the interference between the reference wave propagating from a pinhole and scattered object wave can be switched ‘‘on’’ or ‘‘off.’’ The referenceobject interference contribution in the scattering can be isolated by combining measurements with right and left circular polarizations. The implications for spatial and temporal interference experiments of magnetic specimen such as holography, lensless imaging, or photon correlation spectroscopy are discussed.

Dynamics of concentrated colloidal suspensions probed by x-ray correlation spectroscopy

We report x-ray intensity fluctuation spectroscopy (XIFS) measurements of the equilibrium dynamics of colloidal suspensions of charge-stabilized ${\mathrm{Sb}}_{2}{\mathrm{O}}_{5}$ spheres of radius $250\ifmmode\pm\else\textpm\fi{}40 \mathrm{\AA{}}$ in glycerol. The dynamics were investigated for times between 4 and several thousand seconds, and for wave vectors, $Q$, from 0.003 to 0.012 ${\mathrm{\AA{}}}^{\mathrm{\ensuremath{-}}1}$, well beyond wave vectors that can be studied with visible light. The unique advantages of XIFS for studies of concentrated suspensions composed of refractive-index-mismatched components have not previously been explored. Here, we study a dilute sample (sample I) and a concentrated sample (sample II). For sample I, the diffusion coefficient is independent of $Q$, with a value consistent with the Stokes-Einstein relation. For sample II (effective hard-sphere volume fraction, $\ensuremath{\varphi}=0.18)$, the diffusion coefficient exhibits a minimum at the peak of the structure factor, indicating modifications to Brownian behavior due to interparticle interactions.

Dynamic light scattering with a CCD camera

We have successfully implemented a method to measure intensity autocorrelation functions with a CCD camera and a fast computer. We report light scattering experiments on solutions of diffusing latex spheres in glycerol and compare our results to those obtained with a conventional hardwired electronic correlator. Averaging allows significant reduction of measurement times and makes this technique suitable for the study of systems with very long time scales. Moreover, the CCD setup allows measurement of two-time correlation functions under nonequilibrium conditions where conventional correlators which rely on time-averaging fail. The new method may offer significant advantage for low-intensity applications like x-ray correlation spectroscopy.