xcs overview

XCSX-ray Correlation Spectroscopy

XCS Overview

The unprecedented brilliance and narrow pulse duration of the Linac Coherence Light Source provides a unique opportunity to observe dynamical changes of large groups of atoms in condensed matter systems over a wide range of time scales using Coherent X-ray Scattering (CXS) in general and X-ray Photon Correlation Spectroscopy (XPCS) in particular. The X-ray Correlation Spectroscopy (XCS) instrument at the LCLS allows the study of equilibrium- and non-equilibrium dynamics in disordered or modulated materials.The XCS instrument is located in Hutch 4 of the Far Experimental Hall of LCLS. The XCS instrument is located about 420m away from the source.Characteristic timescales ranging from 10 milliseconds to thousands of seconds can be explored using a sequential mode of operation.Ultrafast timescales ranging from 100 femtosecond to several nanoseconds can be probed using a novel “Split and Delay” mode of operation.

Contacts

Name: Aymeric ROBERTXCS group Leader

Phone: (650) 926-2747 Email: [email protected]

Name: Chiara CaronnaPhone: (650) 926-3767Email: [email protected]

Name: Sooheyong LeeVisiting Scientist (DESY)

Phone: (650) 926-8915Email: [email protected]


X-ray Photon Correlation Spectroscopy TechniqueCoherent X-rays are particularly well suited for investigating disordered system dynamics down to nanometer and atomic length scales, using X-ray Photon Correlation Spectroscopy [1].When coherent light is scattered from a disordered system, the scattering pattern present a peculiar grainy appearance also known as speckles, as illustrated in the figure (Top) [2]. These speckles originate from the exact position of all scatterers within the system under investigation.XPCS characterizes the temporal fluctuations in speckle patterns, produced when coherent light is scattered by a disordered system. From these fluctuations, insight in the dynamic behavior of the system can be revealed.XPCS, as currently performed on 3rd generation synchrotron light sources (Advanced Photon Source, European Synchrotron Radiation Facility. PETRA III) is complementary to Dynamic Light Scattering (DLS) or Photon Correlation Spectroscopy (PCS) with visible coherent light, techniques that probe slow dynamics (<106 Hz) but can only access the long wavelength regime (Q < 4 ∙10-3 Å-1). Neutron-based techniques (inelastic, quasi-elastic neutron scattering, neutron spin-echo) and Inelastic X-ray Scattering (IXS) can access the same Q range as XPCS, but these techniques probe the dynamic properties of matter at high frequencies (from typically 108 Hz to about 1017 Hz), as illustrated in the figure (bottom).The XPCS areas (in transmission/diffraction and grazing incidence (GI) geometry) are defined from existing measurements performed at the existing 3rd generation synchrotron light sources instruments.The peak coherent flux of the LCLS radiation will be 9 orders of magnitude larger than 3rd generation synchrotron light sources. This will allow for the first time the studies of dynamics up to about 1013 Hz at large Q’s using the fine pulse structure of the LCLS beam. The unique capabilities of the XCS instrument at LCLS enables to address a wide variety of scientific questions, such as coherent optical properties of hard X-rays, phase transition dynamics, dynamics of glassy materials, surface XPCS, non- equilibrium dynamics, local ordering in disordered materials.

[1] G. Grübel, A. Madsen, A. Robert, Soft Matter Characterization, edited by R. Borsali & R. Pecora, pp. 954-995. Heidelberg : Springer.[2] A. Robert, J. Appl. Cryst. 40, (2007), pp. s34-s37


Example of Scientific Programs (I)Phase Transition DynamicsAtomic-scale fluctuations occur at equilibrium near many phase-transitions. These fluctuations are the basis for the new phases that form when the phase transition point is crossed by changing relevant physical parameters, such as temperature, electric or magnetic field, pH, etc …. XPCS is an ideal tool for observing the equilibrium dynamics of these fluctuations, and understanding the mechanisms that control microstructure formation in materials.For example transitions in magnetic, ferroelectric and ferroelastic materials exhibit time scales spanning from sub-picoseconds to many seconds in the vicinity to the transition point.

Dynamics and Local Ordering of Glassy MaterialsMany liquids, when rapidly cooled below their freezing point, form meta-stable glasses or amorphous phases. This applies for a wide variety of materials including metallic alloys, oxides such as silica, polymeric materials, and many others (e.g. colloidal dispersions for which the volume fraction is the analogous of the inverse temperature driving the phase transition). In general, glassy materials are among the least understood at a fundamental level. Conventional equilibrium statistical mechanics does not predict the existence of the amorphous state, whereas it can explain most other states of matter, even quite exotic ones like superconductivity and superfluidity. Understanding the dynamics and local ordering in glassy materials is thus an important topic of fundamental research.

Surface and Interfaces DynamicsSurface XPCS experiments at 3rd generation storage ring sources have been performed successfully in the past, and surface dynamics have been investigated in a variety of systems, such as membranes, polymer films, and liquids. Surface XPCS in combination with the X-ray standing wave technique also allows the study of fluctuations of buried interfaces. However, surface dynamics at the nanometer length scale is not accessible today due to insufficient coherent photon flux. The XCS instrument at LCLS will provide exciting new possibilities in this science area.


Example of Scientific Programs (II)Non-equilibrium dynamicsWhen a disordered homogeneous material is rapidly brought to a new set of conditions, corresponding to the coexistence of equilibrium phases, a spatial pattern of domains of each phase develops. For example, such a change of conditions can be accomplished by a rapid quench from high to low temperature, below the miscibility gap. This results in the creation of a microstructure of interconnecting domains. These domains grow in order to minimize the areas of the domain walls that separate the phases. Out-of-equilibrium behavior has been observed in soft condensed matter glassy systems as well (i.e. attractive and repulsive colloidal glasses), where the dynamics are slowed down as a function of time and are described as aging. The XCS instrument at LCLS is ideal for studying these non-equilibrium systems where the dynamic behavior needs to be probed on all length scales simultaneously.

Coherence properties of hard X-raysCoherence is one of the most prominent features of the novel radiation produced by the LCLS. A comprehensive understanding of the X-ray laser coherence properties is not only of fundamental interest but a necessity for properly interpreting coherence-based experiments such as Coherent X-ray scattering in general and XPCS in particular. The characterization of the coherence properties of the source in terms of correlation functions and photon statistics was successfully used for characterizing 3rd generation sources XPCS dedicated instruments. The study of first and higher order correlation functions will allow not only determine the spatial and temporal coherence parameters of the source (i.e. the longitudinal and transverse coherence lengths as well as the coherent flux) but it will also provide insight into the mode structure, photon statistics (bunching/anti-bunching) and possible non-Gaussian properties of the source.


Operation Modes (I)Sequential Mode (timescales from 10-2 to 103 seconds)The shortest time scale of this mode of operation is limited by the LCLS repetition rate (i.e. up to 120Hz). Important dynamical phenomena at mesoscopic lengthscales can occur on relatively long time scales, e.g. longer than 10-1 s .

This operation mode of operation is illustrated in the frequency-wavector graph. It employs the very large time-averaged coherent X-ray flux from the LCLS, averaged over 120 Hz repetition rate, to investigate dynamics by means of 2D-XPCS data collection.

These experiments consist of collecting time-resolved sequences of speckle patterns on an area detector as shown in figure 2 [1]. From an analysis of these sequences, correlation times from a few inter-pulse periods up to many minutes can be obtained. The data analysis techniques are similar to those used until now at 3rd generation sources.

The XCS instrument at LCLS will provide higher signal rates than currently available, thus allowing to probe dynamics at much larger wavevectors than currently possible. It will also offer the possibility to investigate weakly scattering samples.

Sequential Mode

[1] G.B. Stephenson, G. Grübel and A. Robert, Nature Materials 8, 2010,703


Operation Modes (II)Split-Delay Mode- (timescales from 10-12 to 10-8 seconds)The short pulse duration of the LCLS hard X-ray radiation (< 250 fs ) allows the extension of XPCS studies to much faster time scales than currently possible.

In order to probe time scales between 10-12 and 10-8 s (as presented in the frequency-wavevector phase space diagram), a split-delay technique shown schematically in Figure 2 is employed and takes advantage of the unprecedented peak brilliance of the LCLS beam.

Its concept is to split each LCLS X-ray pulse into two equal-intensity pulses separated in time, but propagating along the same path [1]. The scattering from the two pulses will then be collected during the same exposure of an area detector.

If the sample is static (i.e. does not present any dynamics on the time scale of the time delay between the two pulses), the contrast in the summed speckle pattern will be identical to the one of a single pulse. If on the other hand the sample evolves on this time scale, then the summed speckle pattern will present a lower contrast [2].

By performing the contrast analysis of sets of ” single shot” summed speckle pattern as a function of time delay, the correlation time of the system can be measured on time scales down to the pulse duration. Such contrast function (i.e. contrast as a function of time delay) can also be linked to the intermediate scattering function (i.e. the normalized dynamic structure factor in the time domain f(Q,t)), as usually performed in the sequential mode with the second order intensity autocorrelation function [2].

A split and delay unit with a path length difference variable from 3x10 -6 to 3m provides delay times from about 10-14 to 10-8 seconds [1].

[1] W. Roseker, H. Franz, H. Schulte-Schrepping, A. Ehnes, O. Leupold, F. Zontone, A. Robert and G. Grübel, Optics Letters 34 (12), (2009), pp. 1768-1770.

[2] C. Gutt, L. –M. Stadler, A. Duri, T. Autenrieth, O. Leupold, Y. Chushkin and G. Grübel , Optics Express 17

(1), (2009), pp. 55-61

Split and Delay Mode


XCS Instrument Description IIThe XCS instrument is a dedicated LCLS instrument for the use of Coherent X-ray Scattering techniques in general and X-ray photon Correlation Spectroscopy in particular. It can operate in the hard X-ray range (5.5-25keV) on any of the harmonics of LCLS.

MonochromaticityThe XCS instrument operates mainly in monochromatic beam but it can also operate in pink beam if scientifically required. By default the XCS instrument will be providing Si(111) monochromaticity. In order to provide enhanced longitudinal coherence lengths, XCS will also provide monochromatic beam with Si(220) and Si(511) in the near future.

Grazing Incidence and local harmonic rejectionXCS is also providing local harmonic rejection by means of two silicon mirrors located 1.5 and 2.1m upstream the sample location. These also offer the possibility to provide the beam with a grazing angle to the sample. Both mirrors can rotate 360°, and can thus deflect the beam downwards of upwards depending on the required scattering geometry.


XCS Instrument Description IIFocusingThe beam can be focused by inserting Beryllium compound refractive lenses in the beam path. The focal length can be adjusted for a given X-ray energy by selecting an appropriate number of individual lenses (up to 10) and stacking them. One can switch from one stack to another (up to three) remotely. Such a unit is located 6.8 meters upstream from the sample. XCS intend to provide later a second unit at 3 meters from the sample, capable to provide micron size focus size. Each unit has the capability to be translated longitudinally ±0.3m. This allow some tunability in the beam size at the sample when not working at the focus.

DiffractometerThe XCS instrument is providing a horizontal scattering 4-circle Huber diffractometer. It has a local 2θ detector arm to easy crystal alignments. It can be removed from the beam path for accommodating large sample environment that are not compatible with the diffractometer. The top surface of the diffractometer is 300x300mm2. The distance between the top surface of the diffractometer and its center or rotation is 270mm.

Sample EnvironmentsThe XCS instrument will try to provide a complete suite of sample of environments to the user community. This will however happen along the operation of XCS. At the beginning of the operation of XCS, no sample environment may be provided and the users are expected to provide their own favorite sample environment. Any integration issue should be discussed with the XCS instrument team.


XCS Instrument Description III

Large Angle Detector MoverThe XCS instrument has a Large Angle Detector Mover as a long sample-detector 2θ arm. It was build by FMB/OXFORD. The LADM provides two sample detector distances : 4 and 8m. It can rotate up to 55° scattering angle in the horizontal plane and up to 1° in the vertical plane for Grazing Incidence scattering geometries. It provides an evacuated fly path between the diffractometer and the detector. The Kapton exit window can be as large as 250mm Ø. It also provides three different in vacuum beamstops upstream the exit window.The LADM provides Small Angle X-ray Scattering capabilities for 2θ=0°.At the end of the LADM, two vertical and horizontal translations allow to move a 2-dimensional detector to be located at a position of interest.

DetectorXCS intend to provide a dedicated detector fulfilling all requirement to perform CXS and XPCS experiments at LCLS.It is for now unclear whether or not that detector will be available for the first run of operation of XCS. That detector intend to provide 100% DQE, 102 dynamic range, very low noise ( << 1 photon) 55x55 μm2, 1k x 1k pixels, 120 fps.

At a minimum XCS will provide a standard direct illumination CCD (Princeton Instruments, LCX) providing 50% DQE at 8keV (30% at 10keV) , 50 photon dynamic range, very low noise ( << 1 photon) 20x20 μm2, 1.3k x 1.3k pixels, 0.3 fps.


Beam DiagnosticsPop-in Profile MonitorsThe spatial profile of the LCLS beam can be measured at various locations along the XCS beamline using a scintillating screen and a high resolution camera-lens combination. The screen is mounted on a translation stage to insert it into the beam.

Pop-in Intensity MonitorThe integrated intensity of the LCLS beam is measured at various locations along the XCS beamline using a photodiode that can be inserted in the beam path.

Intensity-Position MonitorThe Compton backscattering of a thin silicon nitride foil (i.e. allowing most of the beam to be transmitted) is used to measure the incident intensity on a shot-to-shot basis. The back-scattering is measured using a quadrant diode located right upstream of the foil. The integrated intensity of all the diodes provides a measurement of the beam intensity for each. The relative signal from each tile can be used to get the beam position.


XCS Capabilities and ParametersScientific Applications

X-ray Photon Correlation Spectroscopy (XPCS)

Coherent X-ray Scattering (CXS)

Scattering Geometries

Small Angle X-ray Scattering (SAXS)Grazing incidence Scattering (GI-SAXS, GID)Wide Angle X-ray Scattering/Diffraction

Source Parameters

Photon Energy5.5-10keV , 1st harmonic10-25 keV , 2nd and 3rd harmonic

Source Size (e-) ~ 60 x 60 µm2 (HxV) FWHM @ 8.3 keV

Source Divergence (e-)

~ 1.1 x 1.1 µrad2 (HxV) FWHM @ 8.3 keV

Repetition Rate 120* , 60, 30, 10 Hz

Pulse Duration70 - 300 fs (high charge mode)~ <10 fs (low charge mode)

Pulse Energy~ 1-3 mJ (high charge mode)~ 0.2 mJ (low charge mode)

Photons per Pulse~1 x 1012 (high charge mode @ 8.3 keV)~1 x 1011 (low charge mode @ 8.3 keV)

* LCLS will operate at 120Hz in the near future. For now it routinely operates up to 60Hz


XCS Capabilities and Parameters

Focusing Capability

A selection (up to 10) of compound refractive lenses can be inserted in the beam to provide focusing at a given energy. By locating the sample at or out of focus, the beam size at the sample can be tuned from a couple of microns to several tens of microns.

Beam Size at Sample (8 keV)

Up to 750 x 750 µm2 FWHM @ 8.3keV

Energy Range5.5-10 keV (1st harmonic)10-25 keV (2nd and 3rd harmonic)

Energy Resolution ΔE/E

Silicon (111) 1.4 x 10-4

Silicon (220) 6.1 x 10-5 *Silicon (511) 1.1 x 10-5 *Pink beam 2-3 x 10-3 *

* Si(220) and (511) may not be available at start.* Si(220) and (511) may not be available at start. Operation in

pink beam is possible but is a non standard mode of operation and therefore requires prior discussion.


XCS Beamline Schematic

LCLS Beam Direction

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XCS Alcove

xcs overview

Documents

dynamic light

disordered system dynamics

visible coherent light

coherent xray scattering

linac coherence light

nonequilibrium dynamics

scattering pattern

slow dynamics