Synchrotron sources introduction

X-ray diffraction

The energy dispersive x-ray diffraction (EDXD) has a fixed scattering geometry with white beam, which is especially suitable for in situ studies in special environments (e. g. very low or high temperatures and/or pressures. The EDXD allows a rapid structure analysis and able to study materials that are unstable and only exist for short periods of time. However, EDXD techniques suffer from relatively poor resolution, unavoidable contaminant features such as sample fluorescence lines, and poor powder averaging due to high degree of collimation required between the sample and the detector. This has resulted in unreliable intensity and has made it difficult to detect structural subtleties and to perform structure refinement.

By introduction of two dimensional area detectors (like Image Plate, CCD) with monochromatic synchrotron beam, the high-resolution diffraction patterns can be acquired with the angle dispersive x-ray diffraction (ADXD). The area detectors are highly sensitive to the short wavelength (<0.7 Å), which provides much better signal to noise with very low noise background from diamond anvils. Moreover, the two dimensional patterns can provide the stress-stain information under the non-hydrostatic pressure to study the texture, which is more convenient and accurate to perform radial diffraction experiments than EDXD configuration.

Beamlines:

APS: 13-BM-C,D, 13-ID-C,D,E, 16-BM-B,D, 16-ID-B

https://beam.aps.anl.gov/pls/apsweb/beamline_display_pkg.technique_dir

NSLS: X17C

http://beamlines.ps.bnl.gov/bytechnique.aspx

SPRING8: BL04B2, BL10XU

http://www.esrf.eu/cms/live/live/en/sites/www/home/UsersAndScience/Experiments.html

ESRF: ID09A, ID27

http://www.spring8.or.jp/en/facilities/bl/list/

Total x-ray scattering/Pair distribution function (PDF)

Total scattering or pair distribution function (PDF) analysis utilises not only Bragg scattering from a material but also the diffuse scattering in order to look beyond the average structure to examine the local or short-range structure, providing a powerful tool to characterize disordered, nano-crystalline and amorphous materials. Due to ~6 orders of magnitude less than Bragg diffraction's intensity, the total scattering prefers to the high-energy (> ~60keV) synchrotron mono beam with high flux (> 1010 photons/sec). Accompany by utilizing large area detector, high Q range (scattering vector, ~20 Å-1 or higher) can be obtained.

Beamlines:

NSLS: X17B3, X17A

APS: 6-ID-D, 1-ID-B,C,E

SPRIN8: BL14B1

X-ray absorption

X-ray absorption spectroscopy (XAS) is a widely used technique for determining the local geometric and/or electronic structure of matter. The synchrotron radiation with high energy and brightness can penetrate the diamond anvil to generate XAS data. There are three main regions found on a spectrum generated by XAS data. 1) The "absorption threshold" determined by the transition to the lowest unoccupied states : a) the states at the Fermi energy in metals giving a "rising edge" with an invtan shape; b) the bound core excitons in insulators with a Lorentzian line-shape (they occur in a pre-edge region at energies lower than the transitions to the lowest unoccupied level); 2) The X-ray Absorption Near-Edge Structure XANES are dominated by core transitions to quasi bound states (multiple scattering resonances) for photoelectrons with kinetic energy in the range from 10 to 150 eV above the chemical potential, called "shape resonances" in molecular spectra since are due to final states of short life-time degenerate with the continuum with the Fano line-shape. In this range multi-electron exitations and many-body final states in strongly correlated systems are relevant; 3) In the high kinetic energy range of the photoelectron the scattering cross-section with neighbor atoms is weak and the absorption spectra are dominated by EXAFS (Extended X-ray Absorption Fine Structure) where the scattering of the ejected photoelectron off neighboring atoms can be approximated by single scattering events.

However, the white beam at lower energy range (<10keV) has much less transmission rate, which needs really thin or perforated diamond anvils. Sometimes the fluorescence lines of diamonds might disturb the absorption spectrum. By rotating the DAC, these lines could be reduced partially or even removed completely.

Beamlines:

APS: 13-BM-D, 13-ID-C,D, 9-BM-B,C

SPRIN8: BL12XU, BL12B2, BL28XU

ESRF: ID26, BM23, ID24

Nuclear Resonance scattering (NRS)

Nuclear resonant scattering of synchrotron radiation describes the whole process of absorption and reemission of synchrotron photons by the nuclear levels of resonant atoms. During the scattering process, the properties of the radiation drastically change due to the interaction with the scattering centers. By studying the properties of the scattered radiation, accurate and detailed information about the local magnetic and electronic environment of the resonant nuclei can be obtained.

Compared to radioisotopes in conventional Mössbauer spectroscopy, synchrotron radiation has the advantage of a higher brilliance and energy tunability making easily accessible a large variety of Mössbauer nuclei. Electric charges, charge gradients and magnetic moments surrounding the nucleus may shift and split the nuclear energy levels (hyperfine interaction). The shifts and splittings of the energy levels can be used to probe the environment in which the nucleus is embedded. In this way the valence state, the local symmetry, the distributions of charges and magnetic moments can be determined locally around the probe nucleus. The dynamics (relaxation, fluctuation) of these properties can also be observed in the corresponding time window.

Beamlines:

APS: 16ID-D, 3ID-B,C,D

SPRIN8: BL09XU, BL11XU

ESRF: ID26

Inelastic x-ray scattering

Inelastic x-ray scattering (IXS) is an advanced technique to study the dynamic behavior of matter, which measures the energy and momentum change of the scattered photon. The energy and momentum lost by the photon are transferred to intrinsic excitations of the material under study and thus IXS provides information about those excitations. Using IXS method we can get electronic band structure, excitions, plasmons, and their dispersions under high pressure.

However, due to a very small scattering cross section (only 1 of 108 photons are inelastically scattered), the high flux synchrotron source has been needed, especially 3rd generation of synchrotron at APS, ESRF, SPRING-8 and SSRF. When the energy of the incident photon is chosen such that it coincides with, and hence resonates with, one of the atomic x-ray absorption edges of the system. The resonance can greatly enhance the inelastic scattering cross section, sometimes by many orders of magnitude. Moreover, the incident beam hits the sample, scattered x-ray is collected by spherically-bent single-crystal analyzers and focused to the solid state detector in a nearly backscattering geometry. During IXS experiments, both incident x-ray energy and scattering angle need to be scanned to obtain the dielectric function and the dynamic structure factor.

Beamlines:

APS: 3-ID-B,C,D, 9-ID-B,C, 30-ID-B,C, 13-ID-C,D

SPRIN8: BL35XU, BL08W

ESRF: ID16, ID28

X-ray image

Synchrotron X-ray image is the use of synchrotron beam to view a non-uniformly composed material (i.e. of varying density and composition). A heterogeneous beam of X-rays is projected toward an object. The density and composition of each area determines how much of the ray is absorbed. The X-rays that pass through are captured behind the object by a detector (either photographic film or a digital detector). The detector gives a 2D representation of all the structures superimposed on each other. In tomography, the X-ray source and detector move to blur out structures not in thefocal plane. Computed tomography (CT scanning), unlike plain-film tomography, generates 3D representations used computer-assisted reconstruction. The X-ray imaging technique supports not only traditional physical, medical, materials science and engineering subjects, but also new areas such as environmental, archaeological, palaeontological and biological studies.

Beamlines:

APS: 13-ID-C,D,E, 20BM-B, 20-ID-B,C, 32-ID-B,C

SPRIN8: BL17SU, BL19B2, BL20XU, BL20B2

ESRF: ID16A, ID19, ID21, ID 22

Raman scattering

When photons are scattered from an atom or molecule, most of photons are elastically scattered (Rayleigh scattering), such that the scattered photons have the same energy (frequency and wavelength) as the incident photons. However, a small fraction of the scattered photons (approximately 1 in 10 million) are scattered by an excitation, with the scattered photons having a frequency different from, and usually lower than, that of the incident photons. In a gas, Raman scattering can occur with a change in energy of a molecule due to a transition. The spectrum of the Raman-scattered light depends on the molecular constituents present and their state, allowing the spectrum to be used for material identification and analysis. Raman spectroscopy is used to analyze a wide range of materials, including gases, liquids, and solids. Highly complex materials such as biological organisms and human tissue can also be analyzed by Raman spectroscopy. For solid materials, Raman scattering is used as a tool to detect high-frequency phonon, magnon excitations, the force constant and bond length for molecules.

Beamlines:

APS: 16-ID-D, 20-ID-B,C,

SPRIN8: BL12XU

Infrared spectroscopy

Infrared spectroscopy exploits the fact that molecules absorb specific frequencies that are characteristic of their structure. These absorptions are resonant frequencies, i.e. the frequency of the absorbed radiation matches the transition energy of the bond or group that vibrates. The energies are determined by the shape of the molecular potential energy surfaces, the masses of the atoms, and the associated vibronic coupling.

In particular, in the Born–Oppenheimer and harmonic approximations, i.e. when the molecular Hamiltonian corresponding to the electronic ground state can be approximated by a harmonic oscillator in the neighborhood of equilibrium molecular geometry, the resonant frequencies are associated with the modes corresponding to the molecular electronic ground state potential energy surface. The resonant frequencies are also related to the strength of the bond and the mass of the atoms at either end of it. Thus, the frequencies of the vibrations are associated with a particular normal mode of motion and a particular bond type.

Beamlines:

SPRIN8: BL43IR

NSLS: U2A

Neutron scattering

X-ray photons scatter by interaction with the electron cloud of the material, neutrons are scattered by the nuclei. This means that, in the presence of heavy atoms with many electrons, it may be difficult to detect light atoms by X-ray diffraction. In contrast, the neutron scattering lengths of most atoms are approximately equal in magnitude. Neutron diffraction techniques may therefore be used to detect light elements such as oxygen or hydrogen in combination with heavy atoms. The neutron diffraction technique therefore has obvious applications to problems such as determining oxygen displacements in materials like high temperature superconductors and ferroelectrics, or to hydrogen bonding in biological systems. As neutrons also have a magnetic moment, they are additionally scattered by any magnetic moments in a sample. In the case of long range magnetic order, this leads to the appearance of new Bragg reflections. However, due to smaller interception, neutron scattering requests larger volume (~mm) compared to x-ray scattering, which results in a much lower-limit by using the piston-cylinder press.

Beamlines:

NIST: high resolution powder diffrctometer-BT1

http://www.ncnr.nist.gov/instruments/bt1/

SNS: Beamline3 (SNAP) http://neutrons.ornl.gov/snap/