Synchrotron Technique Glossary

Angle Resolved Photoelectron Spectroscopy (ARPS)

Angle resolved photoelectron spectroscopy is used to investigate the electronic structure of solids, solid surfaces and interfaces. Atoms in solids contain electrons in a range of energy levels, or bands. By detecting photoelectrons emitted from a surface at different emission angles, the energy of the electrons may be determined. This process is known as "band mapping".

Attenuated Total Internal Reflection (ATR)

Attenuated Total Internal Reflection (ATR) is an IR technique where the infrared beam samples a surface by internal reflection of the light. It is based on the measurement of the supercritical internal reflectance of a sample-ATR crystal interface. Often combined with FTIR.

See also: Fourier Transform Infrared Spectroscopy

Auger Electron Spectroscopy (AES)

Auger electron spectroscopy uses a beam of electrons to knock electrons out of inner-shell orbitals. After removal of a core electron, the atom relaxes from a high-energy state to a low-energy state, by having one of the electrons from a high-energy shell collapse to the low energy vacant shell. Concomitant with this is either the release of flourescence or an Auger electron. AES is based upon the measurement of the kinetic energies of the emitted electrons. The technique is used for determining the composition of the top few layers of a surface. It cannot detect hydrogen or helium, but is sensitive to all other elements, being most sensitive to the low atomic number elements.

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Circular Dichroism (CD)

Circular dichroism is the difference in absorption between left and right circularly polarised light by an asymmetric or chiral sample. In the ultraviolet part of the spectrum, CD reports on the secondary structure content of proteins and other macromolecules. Synchrotron radiation provides orders of magnitude more photon flux than conventional CD instruments providing high signal-to-noise data over a wide wavelength range, allowing the much more accurate quantification of secondary structure content, such as alpha helix, beta strand, beta turns, and other structure types.

See also: X-ray Magnetic Circular Dichroism

Crystal Truncation Rod Scattering (CTR)

Crystals, by their very nature, are finite in size. This leads to diffuse scattering streaks, or rods, around Bragg peaks. Crystal truncation rod (CTR) analysis is uniquely sensitive to the atomic structure of the surface of a sample crystal.

See also: Surface X-ray Diffraction

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Deep Etch X-ray Lithography (LIGA)

Deep etch X-ray lithography (LIGA) is a technique for the production of structures with exceptionally fine scales (~1 micron) with high quality characteristics (optically smooth edges, perpendicular faces). A conducting substrate is coated with an X-ray resist and exposed to synchrotron radiation through a mask. The resist is developed and the exposed areas are dissolved. In the electroforming step, the new voids are electroplated and filled with metal. After dissolving the remaining resist, a metal structure with geometric pattern of the mask remains. In the molding step, the metal structure can be used as a tool to replicate the microscopic structure in softer materials such as polymers. Procedures have also been developed to produce more complex structures or to produce deep microstructures in metals and ceramics. LIGA has been applied to the creation of microelectromechanical systems (MEMS).

See also: Extreme Ultra-Violet Lithography, Microelectromechanical Systems

Diffraction Anomalous Fine Structure (DAFS)

A combination of diffraction and EXAFS or XANES. In the extended fine structure region, DAFS provides the same short-range structural information as EXAFS: the bond lengths, coordination numbers, neighbor types, and bond disorders for the atoms surrounding the resonantly scattering atoms. In the near-edge region, DAFS provides the same structural and spectroscopic sensitivities as XANES: the valence, empty orbital and bonding information for the resonant atoms. In DAFS atoms of the same element, but which occupy inequivalent sites in a crystal, may be independently studied.

See also: Extended X-ray Absorption Fine Structure, X-ray Absorption Near Edge Structure

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Extended X-ray Absorption Fine Structure (EXAFS)

This technique that uses X-ray absorption measurements in regions past the absorption edge to yield information about the local coordination environment of the atoms under study. This provides information about bond lengths, coordination numbers, and structural disorder parameters.

Extreme Ultraviolet Lithography

Extreme ultraviolet (EUV) lithography is similar to deep etch X-ray lithography (LIGA) with the notable exception that it uses short (13nm) wavelengths instead of the longer optical wavelengths (~248nm in LIGA). Currently this technique is being developed; however, it is expected to be useful in microelectronics and other small-scale manufacturing processes.

See also: Deep Etch X-ray Lithography

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Fourier Transform Infrared Spectroscopy (FTIR)

A non-destructive method which is used as one of the main analytical techniques in chemistry, physics and biology. The technique identifies types of chemical bonds in a molecule by producing an infrared absorption spectrum, akin to a molecular "fingerprint". It is particularly powerful by its ability to identify different chemical functional groups. IR absorption is concentration dependant; as a result, FTIR can also be used as a quantitative tool.

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Grazing Incidence X-ray Scattering (GIXS)

Grazing incidence X-ray scattering or diffraction refers to a method where the incident X-ray beam makes a small (typically about 1 degree) angle to the sample surface. This has an advantage, which is particularly important for thin films, of limiting the penetration depth of the X-rays into the sample with consequently low background scattering from the substrate.

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High-Pressure Research

Synchrotron stations are equipped with equipment for studying samples at higher than normal pressures. Use of a diamond anvil cell for these studies can generate pressures higher than 360 GPa (3.6 Mbar), higher than the pressures found at the centre of the Earth.

High-Resolution X-ray Scattering (HRX)

Use of specialized equipment and monochromators allows for the resolution of incident energies with resolvable differences of ~1meV for fine analysis of samples. Development of specialised optical equipment: monochromators, detectors, software, and methodology is necessary for these resolution ranges to be attainable. Nuclear resonant scattering and inelastic X-ray scattering techniques are some of those designed to utilise these developments.

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Inelastic X-ray Scattering (IXS)

Inelastic X-ray scattering can occur as either resonant (RIXS) or non-resonant (nrIXS). The technique probes the electronic structure of compounds in an element specific manner. One of the major strengths of the inelastic scattering technique is the possibility of controlling both the energy and the momentum transfers.

Infrared Microspectroscopy

Infrared (IR) microspectroscopy allows chemical information to be obtained, with high spatial resolution, from a wide range of materials. With synchrotron radiation as the source of infrared light, experiments can be run at higher spatial resolution and in a shorter time than is otherwise practical.

Infrared Vibrational Spectroscopy

Specific bond energies resonate with infrared frequencies, so by measuring absorption levels of different frequencies it is possible to detect the presence of particular molecular compounds and bonds.

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Laser/Compton Backscattering

Compton backscattering of the laser light from relativistic electrons can produce a quasi-monochromatic photon beam. The energetic photon beam can be used for the investigation of photonuclear reaction, the calibration of the detectors, and electron beam diagnostics.

Liquid X-ray scattering

Scattering studies of liquid samples, either bulk or liquid-liquid interfaces.

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Magnetic Linear Dichroism (MLD)

While circular dichroism involves the use of circularly polarized light, MLD uses linearly polarized light in the VUV and x-ray region, which is more easily accessible.

Magnetic X-ray Scattering

A scattering technique that is applied specifically to magnetic materials. X-ray magnetic diffraction intensity is usually very small but at resonant energies, close to absorption edges, enhancements of the magnetic signal by many orders of magnitude may occur.

Microdiffraction Imaging

Microdiffraction is the determination of crystalline tilt, mosaic, and/or lattice spacings using a sub- to several- micron-sized x-ray beam.

Microelectromechanical Systems (MEMS)

Microelectromechanical systems (MEMS) are devices which integrate microscopic mechanical parts, sensors, actuators, and electronics on a common substrate using microfabrication technology. The electronics are fabricated using standard integrated circuit techniques, while the micromechanical components are shaped by "micromachining" processes that either selectively etch away parts of the substrate or add new structural layers. The micromachining processes may require synchrotron-based techniques such as LIGA, especially if high aspect ratios or unusual materials are necessary. Areas of application for MEMS include micromechanics, micro-optics and microfluidics.

See also: Deep Etch X-ray Lithography

Microflourescence

A nondestructive technique for detection and identification of organic compounds on the surfaces of polymeric materials.

Microtomography

X-ray microtomography has been developed for use in various fields such as biology, materials science, petrology, and medical diagnosis. This method's setup commonly consists of a simple projection geometry with a high-resolution imaging detector and a highly collimated beam. It provides nondestructive characterization of the microstructure of a sample.

Multi-wavelength Anomalous Diffraction (MAD)

A technique used in X-ray crystallography that accelerates the determination of protein structures. Taking advantage of a synchrotron's tunable X-ray wavlengths, anomalous signals from absorption peaks can be determined for certain atom types (commonly Se, Zn, Hg, or other heavy atoms) from a single crystal, allowing for the determination of phases without having to use isomorphous replacement. The technique typically involves collecting two or three datasets at specific wavelengths around absorption edges.

See also: Protein Crystallography, Single-wavelength Anomalous Dispersion

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Photo Emission Electron Microscopy (PEEM)

PEEM operates by shining linearly or circularly polarised X-rays onto the sample surface. The distribution of electrons being emitted from the surface can be imaged.

Photoacoustic Spectroscopy

In photo-acoustic spectroscopy, modulated radiation is absorbed by the sample and causes a periodic temperature fluctuation within the optical absorption depth, and periodic heat transfer to an ambient gas occurs. The photo-acoustic spectroscopy signal results from the periodic pressure fluctuations in this gas that are associated with the temperature modulation produced by the heat transfer from the sample.

Photoelectron Diffraction

Photoelectron diffraction is a method to determine the adsorption geometry of molecules or atoms which are physisorbed or chemisorbed on single crystal surfaces.

Photoionisation Spectroscopy

The technique of photoelectron-photoion coincidence spectrometry enables study of the unimolecular decomposition of polyatomic ions with precisely known internal energies.

Powder Diffraction (PD)

Powder Diffraction is a very widely used X-ray technique to study the structure and related properties of polycrystalline materials. Although the main application is crystal structure determination, the technique is applicable for a wide range of studies including phase identification, qualitative analysis, thermal expansion, preferred orientation (texture), line profile analysis (crystallite size/shape, micro-strain and defects), phase transformation (pressure and/or temperature dependence) and stress/strain mapping.

Protein Crystallography (PX)

An X-ray diffraction technique for determining the three dimensional structures of macromolecular structures. Of particular importance at synchrotrons is the tunability of the wavelength to increase the anomalous signal to facilitate the structure determinations (via MAD or SAD). For industry, this is particularly important in biotech/pharmaceutical research.

See also: Multi-wavelength Anomalous Diffraction, Single-wavelength Anomalous Diffractionion

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Reflection Absorption Infrared Spectroscopy (RAIRS)

A technique to probe the vibrations of adsorbed species with high resolution.

Resonant X-ray Scattering (RXS)

A technique for probing the electronic structure of orbitals in atoms. RXS is now rapidly becoming a crucial complementary technique to neutron scattering for investigating the subtleties of microscopic magnetism.

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Scanning Transmission X-ray Microscope (STXM)

In a scanning transmission X-ray microscope, a zone plate produces a small x-ray probe through which the sample is mechanically scanned in order to produce a 2-dimensional image. Point NEXAFS spectra and image sequences at many energies can also be acquired.

Single Crystal Diffraction

This is a general term to describe diffraction studies involving a single well-formed crystal. It includes protein crystallography and small-molecule crystallography.

See also: Protein Crystallography, X-ray Diffraction

Single-wavelength Anomalous Diffraction (SAD)

Single-wavelength anomalous diffraction, like multi-wavelength anomalous diffraction (MAD), is a technique used in X-ray diffraction to determine phasing information from a single crystal. Unlike MAD, however, it involves the collection of a single dataset at a wavelength appropriate for the element to collect anomalous signals. This technique is particularly useful for crystals that suffer radiation damage during data collection, as it minimises the time spent in the beam.

See also: Multi-wavelength Anomalous Diffraction

Small-angle X-ray Scattering (SAXS)

Small and wide-angle X-ray scattering is a useful and complementary method for determining the size, size distribution and structure of a wide range of disordered (non-crystalline or semi-crystalline) materials. Examples include polymers, liquid crystals, oils, suspensions and biological samples like fibers or protein molecules in solution.

See also: Wide-angle X-ray Scattering

Spin-Resolved Photoelectron Spectroscopy

In spin resolved photoemission photoelectrons are analyzed not only for their emission angles and energies, but also for their spin states.

Surface X-ray Diffraction, X-ray Crystal Truncation Rod (SXRD, CTR)

Surface X-ray Diffraction is a technique for locating the positions of atoms at surfaces and interfaces using X-rays. Crystal truncation rod (CTR) scattering deals with the changes in scattering patterns at the edges of crystals (on surfaces). Suited for finding surface non-homogeneities.

Synchrotron Laser Raman Spectroscopy

raditional Raman Spectroscopy involves using specific wavelengths of light to excite atomic structures at specific wavelengths. Use of synchrotron radiation expands the possible incident wavelengths not currently accessible by lasers.

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Three-Dimensional Scanning

In this developing technique, 3D image is reconstructed after scanning by a MEMS scanning micromirror. This has been used in biomedical engineering to construct a detailed model of a human ear canal, but could presumably be expanded to other imaging applications.

Time Resolved X-ray Scattering / Spectroscopy

The goal of such experiments is to correlate the microscopic structure in a material with its macroscopic physical properties. This may include a number of diffraction/scattering techniques (powder diffraction, SAXS, WAXS, XAS) with the data being collected very rapidly.

Topography Measurements of Optical Surfaces

Use of instruments for the characterization of high quality optical surfaces working in the photon energy range from Infrared to Hard X-rays. (i.e. Mirrors, Diffractometers, Lenses)

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White X-ray diffraction / Energy Dispersive Diffraction

Instead of fixing the wavelength and recording diffracted intensity from a crystalline sample over a wide range of angles, in this technique the diffraction angle (2-theta) is fixed and white (or polychromatic) radiation is used. With this technique all data are recorded simultaneously minimising data collection time and making the technique ideally suited to the real time study of chemical reactions and phase transitions using time slices of < 1 minute.

Wide-Angle X-ray Scattering / Diffraction (WAXS)

While SAXS covers the range 1-10°, WAXS routinely covers the angular range from 7-60°. The technique is used beneficially when applied simultaneously with methods that influence and/or change the samples' structural characteristics (e.g. during temperature changes, under shear forces or upon the application of electric/magnetic field stimuli). The high flux of synchrotron X-rays allows us to follow these changes in a time-resolved manner.

See also: Small-Angle X-ray Scattering

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X-ray Absorption Fine Structure (XAFS)

XAFS probes the physical and chemical structure at an atomic scale in an element-specific manner, and is sensitive to local chemical and atomic states surrounding the selected atom species. Nearly all elements can be studied with XAFS, though the emphasis has traditionally been on the heavier elements.

X-ray Absorption Near Edge Spectroscopy (XANES)

XANES can provide information about vacant orbitals, electronic configuration and site symmetry of the absorbing atom.

X-ray Absorption Spectroscopy (XAS)

XAS is a general term, applying to all techniques involving the excitation of atoms of a particular element in a material by X-rays of energy close to an absorption edge of that element.

X-ray Diffraction (XRD)

X-ray diffraction results from the interference of X-rays scattered by the electrons surrounding atoms. When the atoms form a regular array of repeating units, as in crystalline material, diffraction maxima are usually distinct. For disordered matter such as amorphous materials, glasses, liquids etc. the maxima are broad and relatively weak.

X-ray Excited Optical Luminescence (XEOL)

X-ray Excited Optical Luminescence (XEOL) is a technique developed by T.K. Sham where tunable X-rays are used to excite optical luminescence in a sample. When absorption edges are encountered the luminescence is sensitive to both the site and the channel of excitation, allowing for the determination of the source of luminescence.

X-ray Flourescence Spectroscopy

The use of synchrotrons can increase the ability to analyze fluorescence of samples through the larger range of wavelengths and brightness at the synchrotron source.

X-ray Magnetic Circular Dichroism (XMCD)

XMCD is a technique for determining the element specific moments in magnetic materials. Materials that have been studied to date using XMCD include multilayers and heterostructures, magnetic oxides (e.g. spinels), dilute magnetic semiconductors, magnetic nanostructures and clusters, and spin transport systems (e.g. spin valves, tunnel junctions).

X-ray Microprobe (XMP)

X-ray electron probe microanalysis is based on quantitative measurements of the characteristic radiation and background white radiation from the local spot where the X-ray beam falls on the surface of the sample. The method uses focused electron beams that permit both studies of large surface areas and analysis of local particles with dimensions comparable with the X-ray emitting zone.

X-ray Photoabsorption Spectroscopy

This spectroscopy technique is used to probe the electronic structures of compounds by measuring the absorption of incident photons. The high flux and tunability of undulator beamlines contributes to this technique significantly.

X-ray Photoelectron Spectroscopy (XPS)

XPS provides surface and interface information on solid samples to a depth of 200 Å. This includes quantitative information on the elements present, their chemical state and bonding. The technique can be carried out under conditions close to those occurring during material use or processing and is useful in the surface analysis of polymers, coatings, fibres, composites, ceramics, pharmaceuticals, medical and biological materials.

X-ray Reflectivity

A technique for investigating the near-surface structure of many materials. It probes the electron density with a depth resolution of less than one nm for depths of up to several hundred nm. The method involves measuring the reflected X-ray intensity as a function of X-ray incidence angle (typically small angles are used). The method is used for studies of thin films and multilayers of metals, semiconductors and polymers. It can accurately determine films thickness, density, average roughness, and the roughness correlation function.

X-ray Refraction Contrast Imaging

A technique of imaging that utilizes differences in the refraction indices of the sample to construct an image, in contrast to absorption (as used in standard medical imaging). Relies on synchrotrons to be effective because of the higher flux. Also known as "refraction contrast radiography", "phase dispersion introscopy", and "diffraction imaging".

X-ray Standing Waves (XSW)

On a sample the incident and Bragg-reflected X-rays set up a standing-wave pattern throughout the region penetrated by the X-rays, and above it. A surface technique is then used to monitor the surface as the standing-wave pattern is varied by changing the angle of the sample relative to the incident X-ray beam.

X-ray Topography

X-ray topography is a technique that uses x-ray scattering to provide direct imaging of defects in a single crystal.