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TECHNIQUES alphabetical listA wide variety of scientific techniques and methods are available at the SRS. Listed below are those most commonly available to users. Click a letter to navigate through the alphabetical list below. ANGLE RESOLVED PHOTOELECTRON SPECTROSCOPY ( ARPES )Gas phase photoelectron spectroscopy (PES) involves the illumination of a molecular target species with photons of sufficient energy to cause photoionisation and the corresponding emission of photoelectrons. The photoelectrons are collected by an energy analyser and the measured kinetic energy distribution provides information on the electronic structure, or partial density of states, of the molecule under investigation. Where the incident radiation has a high degree of linear polarisation, it becomes possible to use the technique of angularly resolved photoelectron spectroscopy (ARPES) to map the distribution of photoelectrons as a function of both energy and angle. This provides information about the symmetry of the molecular orbital from which the electron has been ejected. AnomAlous diffraction / ANOMALOUS SCATTERINGThe term anomalous diffraction is used to describe the interaction of X-rays with matter when the X-ray energy is close to an absorption edge (or resonant energy) of an element contained within it. This so-called anomalous scattering (in fact, in physical terms, there is nothing anomalous about it) provides additional information and this can be essential in distinguishing different crystal structures. Bio-EXAFSExtended X-ray Absorption Fine Structure (EXAFS) is important to the study of biological molecules as a powerful tool for probing the atomic environment around the active metal sites in metalloprotein molecules which either elude crystallisation or yield poor crystal diffraction. EXAFS is element specific and requires tuneable X-ray energies. These macromolecular systems have low concentrations of metal elements thus EXAFS data are acquired in fluorescence mode. The biological samples can be in any physical state: solution, amorphous as well as single crystal. Structural information around the excited metal atoms from EXAFS include bond distances (accuracy better than 0.1 Å for first-shell neighbouring atoms in macromolecules), coordination numbers, element identities, disorder, chemical valence state and spin state. Circular DichroismCircular Dichroism (CD) 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. Confocal Scanning Microscopy (SYCLOPS)Confocal microscopy is a powerful technique that can be used to reveal fine details of many important biological processes. In confocal microscopy signal to noise is enhanced in two stages: first, by allowing focused incident wavelength light to fall on only a small area of the specimen the illuminating intensity is concentrated in the area of interest and decreases rapidly out of the plane of focus, and secondly, an appropriately placed aperture prevents fluorescent or scattered light travelling back from other parts of the sample to the detector. The sample is scanned in two dimensions and an image (e.g. a fluorescence map) is reconstructed from the detector response. Diffraction Enhanced ImagingDiffraction Enhanced Imaging (DEI) is an X-ray phase contrast technique which is showing great promise for a number of medical and materials imaging problems. It relies on having a source of highly collimated monochromatic X-rays, which is currently only available at synchrotron radiation facilities. The phase shifts that occur as the X-ray waves pass through the object under investigation are made visible using Bragg diffraction X-ray optics in the form of a post-sample analyser crystal. The DEI system being developed on the bending magnet beamline 7.6of the SRS at Daresbury Laboratory has been used successfully to image a number of small medical specimens. It proved possible to record contrast between regions of different tissue type varying only slightly in density, and hence it promises to become a powerful new research and diagnostic tool. X-ray energies from 10 keV to 20 keV have been used so far with maximum beam dimensions of 40 mm by 1 mm: an image up to 40 mm by 150 mm is made possible by scanning the sample through the beam. DILUTE EXAFS / DILUTE XAFSIt is possible to obtain X-ray absorption spectra from very dilute systems (i.e. containing low concentrations of the absorbing element) in the energy range 7 - 30keV. A combination of extremely high flux from sagittaly focused beams, together with a 30-element solid-state fluorescence detector capable of count-rates in excess of 200kHz per channel, make Station 16.5 the station of choice on the SRS for EXAFS measurements of dilute systems. With multiple scanning, good quality spectra from concentrations down to tens of ppm are routinely obtained. Minimum sample size required is 6mm x 3mm. Drug Design / Rational Drug DesignObtaining accurate details of the structure of a protein or another biological macromolecule allows the design of a small molecule medicine that can alter its function, activity or shape in a favourable manner. The prior knowledge defines the overall shape and size of the medicine and gives insight of the type of non-bonding interactions required so that the design process can be short-circuited so avoiding time-consuming empirical methods used to select potential chemicals by trial and error.
Energy Dispersive DiffractionInstead 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. The wavelength (or energy) of the X-rays contributing to any given peak is related to a d-spacing in the sample that generated it and to the 2-theta angle of the detector system collecting it. The diffracted intensities are recorded using an energy discriminating detector in conjunction with multi-channel analyser electronics which sorts the data into bins according to their energy. 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. ENERGY DISPERSIVE EXAFSEnergy dispersive EXAFS (EDE) is a technique for recording X-ray Absorption Fine Structure in which the whole spectrum of interest is obtained at the same time so allowing time-resolved experiments to be performed. Extended X-ray Absorption fine structure ( EXAFS )Fibre DiffractionNatural and synthetic polymers are long chain molecules made up of repeating units. The number of repeats is usually very large, and could be up to 20,000 or more. Moreover, natural fibres tend to have helical symmetry, where the repeat units arrange themselves in a continuous helix with an irrational repeat ratio, which can only be approximated by factors like '213 repeats in 71 turns'. Crystalline material made of fibres will never have exact symmetry to produce single crystal-type diffraction intensities. Diffraction is observed in quasi-continuous smears arranged in layer lines, with no clear separation between one diffraction order and another. Fluorescence Lifetime Imaging Microscopy (FLIM)Fluorescence Lifetime Imaging Microscopy relies on the time-resolved measurement of the decay in fluorescence intensity in confocal microscope images. The fluorescence lifetime is a function of the fluorophore's local environment, so FLIM measurements allow the simultaneous acquisition of information on the location of proteins in live cells and, for example, the characteristics of their biophysical microenvironment. High Pressure Experiments using Diamond Anvil CellsA diamond anvil cell (DAC) is a device for subjecting materials to high pressure. Samples are squeezed between two brilliant cut diamonds and constrained laterally within a small hole (< 100 microns diameter) in a metal gasket located between them. The diamond’s shape applies a large force over a small unit area. Because diamonds are transparent to visible and infrared light, and transmit high energy X-rays, the contents of the DAC can be examined using a wide variety of experimental techniques. DACs are used on the SRS for X-ray diffraction and spectroscopy experiments, carried out on crystalline powders or single crystals. High Resolution Powder Diffraction (HRPD)Infrared microspectroscopy and imagingInfrared (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 Spectroscopy of SurfacesReflection Absorption Infrared Spectroscopy (RAIRS) is a technique to probe the vibrations of adsorbed species with high resolution. The SRS extends the spectral range of RAIRS to longer wavelengths than can be accessed by conventional thermal sources. This allows the low frequency modes of adsorbates, including the substrate-adsorbate stretch, to be probed.
Macromolecular CrystallographyMagnetic scatteringMagnetic diffraction is used to get information on the magnetic properties of 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. By analysing the X-ray polarisation after scattering, the magnetic diffraction may be distinguished from the normal charge diffraction. MICROMOTT POLARIMETER
multidimensional single molecule spectroscopyMultidimensional single-molecule fluorescence microscopy is a method to image the fluorescence from a population of single fluorophores discriminating in colour and polarisation. See also: Fluorescence Lifetime Imaging Microscopy (FLIM) Multiple Isomorphous ReplacementPhase information has to be obtained or deduced to solve crystal structures. This information can be obtained largely ab initio for small molecules that diffract to very high resolution. Proteins tend to diffract to moderate resolution, and the phase information has to be obtained independently. One method is Multiple Isomorphous Replacement. Multiwavelength Anomalous Diffraction ( MAD )In general, the diffracted intensity from one side of a set of crystal planes is equal to that diffracted from the other side, when Friedel’s Law is said to be obeyed. However, when anomalous scattering occurs, an additional phase change is introduced and a difference in both amplitude and phase of the two reflections results. The overall effect is small but it varies significantly around an X-ray absorption edge for one or more of the constituent elements. Making a series of diffraction measurements close to the absorption edge allows phase information to be extracted which in turn helps to locate the position of the absorbing species in the structure. It can lead to determination of the absolute configuration of molecules that crystallise in non-centrosymmetric arrangements and is particularly useful in protein crystallography. Near-Edge X-ray Absorption Fine Structure (NEXAFS)
Non-crystalline Diffraction
Normal Incidence X-ray standing wavefield (NIXSW)Normal Incidence X-ray Standing Wavefield (NIXSW) is a surface measurement technique to locate adsorption sites with the added advantage that it is technically also feasible to determine the site distribution for simple surfaces. Optical pump-probe X-ray absorption spectroscopyOptical pump-probe studies combine the use of both synchrotron radiation and laser light. The methods are ideally suited to conjugated systems, where analysis of individual phases is needed, but where in normal synchrotron studies several phases are being probed simultaneously. Optically Detected X-ray ray absorption spectroscopy (OD-XAS)Optically Detected X-ray ray absorption spectroscopy (OD-XAS) is one of the few methods available for directly linking the optical and structural properties of matter. It is useful for probing both the bulk atoms of a solid, and defects contained within them. Phase information in crystallographySingle crystal diffractionis a physical phenomenon resulting from the interference between X-rays scattered by the regular array of repeating units or cells in the crystal. Crystal structures containing large molecules have large unit cells and a large number of diffracted beams are produced. An image of the molecule that produced the diffraction pattern can be reconstructed by projecting all the diffracted beams back to the same origin, and arranging them so that the peaks and troughs of the diffracted waves are correctly aligned relative to each other, i.e. the phase relationship between them is correctly identified. Phasing with Sulphur Anomalous DiffractionPhase information is necessary for solving macromolecular crystal structures. This information can be obtained largely ab initio for small molecules that diffract to very high resolution, but protein crystals tend to diffract to moderate resolution, and the phase information has to be obtained independently. One method is by harnessing the inherent anomalous diffraction from the sulphur present in 90% of proteins, in cysteine or methionine residues. The anomalous diffraction effect due to S is very weak at the wavelengths commonly employed in diffraction experiments, but Station 10.1 was designed to access longer wavelengths, up to 2.5 Å, easily. At a wavelength of 2.0 Å, the anomalous effect from S is doubled, and becomes useful for phasing.
Photo-double-ionisationPhoto-double-ionisation involves the simultaneous ejection of two photoelectrons, from a target species, by a single photon of light. This process requires a strong interaction between the two ejected electrons: photo-double-ionisation experiments therefore provide unique information about electron correlations. PHOTOELECTRON SPECTROSCOPYGas phase photoelectron spectroscopy (PES) involves the illumination of a molecular target species with photons of sufficient energy to cause photoionisation and the corresponding emission of photoelectrons. The photoelectrons are collected by an energy analyser and the measured kinetic energy distribution provides information on the electronic structure, or partial density of states, of the molecule under investigation. Vacuum-ultraviolet and soft X-ray sources, with photon energies of up to 100eV, are suitable for valence band PES studies. However, such low photon energies are insufficient to initiate photoemission from core levels, the study of which requires hard X-ray sources with photon energies of typically 1500eV or more. Photo-ionisation co-incidence techniquesThe technique of photoelectron-photoion coincidence spectrometry enables study of the unimolecular decomposition of polyatomic ions with precisely known internal energies. A monochromatic photon source is used to photoionise the target molecule: an energy dispersive hemispherical analyser permits energy analysis of the photoemitted electron whilst a time-of-flight mass spectrometer, time-gated against the detection of the photoelectron, is used to identify the coincident cation. Powder Diffraction and High Resolution Powder Diffraction (HRPD)Powder Diffraction (PD) is a very widely used X-ray technique to study the structure and related properties of polycrystalline materials. The naturally high collimation and intensity of a synchrotron X-ray beam can be exploited for High (angular) Resolution Powder Diffraction (HRPD) studies. Although the main application is crystal structure determination, the technique is also 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 / Macromolecular CrystallographyCrystalline samples of proteins, DNA, RNA or other large molecules interact with X-rays to produce diffraction patterns. Refocusing of the diffraction pattern is done mathematically, to produce an average picture of the constituent molecules of the crystal. Reflected EXAFS ( REFLEXAFS )Using an optically flat (i.e. flat on a near atomic scale) surface, EXAFS can be obtained by keeping the sample tilted at an angle to the X-ray beam which is less than the critical angle. The incident beam is reflected off the sample and EXAFS (see XAFS) spectra can be collected that have a high degree of surface sensitivity. The penetration depth is ca 50 Å. Reflection-absorption infra-red spectroscopy ( RAIRS)
reflectivityX-ray reflectivity is a powerful technique for investigating the structure of surfaces, thin solid films and multilayered structures. The technique can return information about layer thickness and surface roughness, while in multilayered samples properties of the interface between layers can also be investigated. Due to the typically low number of atoms interacting with the beam, the high flux and very small vertical beam divergence of a synchrotron source are essential for this type of work Single Crystal DiffractionSingle crystal X-ray diffraction is the use of X-rays to examine a sample in which the constituent atoms or molecules are well ordered such that all crystal planes of the same type are aligned. Often it is only possible to see whether the sample is a single crystal by looking at its diffraction pattern. The high brightness of synchrotron radiation allows investigation of small single crystals, e.g. less than 0.15 mm3 for protein crystallography and typically 0.0001 mm3 for small molecule crystallography. The diffraction measurements are generally carried out to enable the internal three-dimensional structure to be deduced at atomic resolution. Much larger crystals are sometimes studied ( by X-ray Diffraction Topography ) to investigate such things as growth history, growth defects, and the physical properties of highly perfect material. Single wavelength Anomalous Diffraction ( SAD )In general, the diffracted intensity from one side of a set of crystal planes is equal to that diffracted from the other side, when Friedel's Law is said to be obeyed. However, when anomalous scattering occurs, an additional phase change is introduced and a difference in both amplitude and phase of the two reflections results, and this additional phase information helps to locate the position of the absorbing species in the structure. The overall effect is generally small but it varies significantly around an X-ray absorption edge for one or more of the constituent elements. When X-rays pass through any sort of material a proportion of them will be absorbed and for crystals containing a range of elements it is quite likely that measurable anomalous differences will arise without being very close to an absorption edge. Small and wide angle X-ray scattering ( SAXS/WAXS )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 fibres or protein molecules in solution. The chemical and physical behaviour of these materials is influenced by the structural properties on a typical length scale of 0.1-200 nm. Small-angle X-ray scattering (SAXS) covers the range 2-200 nm and occurs at low scattering angles (1-10°) whilst wide-angle X-ray scattering (WAXS) routinely covers the angular range 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. Soft X-Ray SpectroscopyUse can be made of low energy, long wavelength X-rays in XAFS studies of local atomic structure around low atomic number centres. Typically C, O, N, Na, Mg, Al, Si, P, S & Cl are the target atomic species, and measurements are similar to those used in higher energy EXAFS (or XANES), but the lower penetrating power of soft X-rays used means that experiments have to be conducted in vacuum or in helium. Spin Polarised PhotoemissionSpin polarised photoemission is utilised to establish the energy, angle and spin of photoemitted electrons. It is the most direct means of probing spin-split electronic structures and it is a key technique in the study of magnetic materials. Daresbury Laboratory is home to the only research equipment in the UK designed and built for this purpose. The photon characteristics depend on the beamline chosen and can vary from a few eV to around 1keV.
Strain scanningIn strain scanning, a small X-ray beam is directed at a polycrystalline sample. The diffraction peaks from the sample are recorded as the sample is scanned on an X-Y grid to build up a two dimensional map of the sample. The angular position and width of the diffraction peaks give information on the strain at each spatial position within the sample. Measurements may be carried out in energy dispersive or angle dispersive mode. The samples studied are typically of engineering interest, for example a weld may be scanned to reveal regions of high tensile strain liable to crack in use. High energy X-rays are required in order to penetrate thick samples. Structural Genomics and ProteomicsFollowing elucidation of the human genome, structural genomics aims to determine a large number of protein structures and thus requires a high throughput methodology. The discipline of proteomics has been established to significantly increase our knowledge of biochemical and physiological mechanisms at the functional molecular level, and there are now a number of initiatives worldwide. (A proteome is the PROTEin complement to a genOME.) Structure solution and Structure refinementCrystal structure solution is the derivation of the three dimensional co-ordinates of atoms within the repeating units or cells in the crystal using experimental X-ray (or neutron) diffraction data (the angular positions and intensities of diffraction maxima). An initial solution is used to produce a starting model for structure refinement, and calculated diffraction data are regenerated from this model. In structure refinement an iterative process is adopted to minimise the difference between the calculated values and those measured in the experiment. This is done by repeatedly making small adjustments to the model structure, i.e. to the atomic positions, until the model with the smallest differences is obtained. This model is then used to examine the detail in the crystal structure. This methodology is applicable to both single crystal and powder diffraction. Surface Extended X-Ray Absorption Fine Structure ( SEXAFS )Surface Extended X-ray Absorption Fine Structure (SEXAFS) refers to the modulation in the X-ray absorption coefficient of a photo-emitter. The oscillations observed in the absorption spectrum have a period characteristic of interatomic distance while the amplitude of the oscillations relates to the number of scattering atoms in the immediate environment of the emitter atom. Thus, the technique yields two important structural parameters: bond lengths and coordination numbers. Surface X-ray Diffraction ( SXRD )Surface X-ray Diffraction is a technique for locating the positions of atoms at surfaces and interfaces using X-rays typically in the wavelength range 0.8 Å to 2.5 Å. An incident beam of focused monochromated radiation is incident at a macroscopically flat surface and diffracted beams are mapped for position and intensity with a 5-axis diffractometer. Threshold photoelectron spectroscopy (TPES)Threshold photoelectron spectroscopy (TPES) is an extremely high-resolution method for the study of the ionic states of gas-phase molecules. In comparison with conventional photoelectron spectroscopy, the spectra generated by TPES are inherently more complex due to the sensitivity of the method to autoionisation of neutral Rydberg states, as well as direct ionisation processes. Time resolved MicrofluorimetryThis method combines fluorimetry with optical microscopy to investigate process in cell biology. Membrane transport mechanisms play a crucial role in allowing selected molecules to pass through otherwise impermeable cell membranes without which cells cannot function. One of the most important membrane transport mechanisms is receptor mediated endocytosis (RME), responsible for triggering and regulating many fundamental cell processes, such as nutrition and growth. Prior to observation in the microfluorimeter, the cell receptors and/or ligands (small charged chemical groups) of interest are ‘labelled’ by covalently attaching highly fluorescent ‘donor’ and ‘acceptor’ molecules (fluorophores) to particular proteins and introducing these into the cells. Time-resolved microfluorimetry has been specifically designed to measure. receptor/ligand interactions during signalling and endocytosis. Time resolved XRDThe goal of such experiments is to correlate the microscopic structure in a material with its macroscopic physical properties. The main synchrotron techniques available to do this are powder diffraction (XRPD) (or wide-angle scattering (WAXS)), small angle scattering (SAXS) and X-ray absorption spectroscopy (XAS). These techniques have traditionally been carried out on different beamlines specialising in just one technique, but a new facility has been constructed to allow data to be collected very fast using the techniques in combination. TOF-SPIN POLARIMETERSEE: SPIN POLARISED PHOTOEMISSION
Tomographic Energy Dispersive Diffraction Imaging ( TEDDI )Although there are many forms of tomographic imaging, virtually all rely on absorptive or spectroscopic responses of a material object to invading radiation. By contrast, TEDDI is unique in using both diffraction and absorption or diffraction and spectroscopic data. The basis of the method is that a white (polychromatic) beam of X-rays from the SRS is collimated to the desired spatial resolution (typically with a cross section diameter ~50 micron) to allow a small sample volume to be probed. This small diffracting sample volume (called the diffracting lozenge) is defined by the track of the incident and scattered beams through the sample the latter being constrained by a collimating aperture placed at an angle in front of a detector. The smaller the collimator size and the larger the angle, the greater the spatial resolution. The detector angle (generally < 30°) is selected by consideration of the type of material under study and the desired spatial resolution. See also: Energy Dispersive Diffraction TOPOGRAPHY / X-RAY DIFFRACTION TOPOGRAPHY ( XRDT )X-ray Diffraction Topography is used to examine highly perfect crystals to reveal the presence of crystal growth defects such as dislocations inclusions and low angle grain boundaries. The X-ray beam is set up to illuminate a large area of the crystal and a direct image is formed after Bragg diffraction from crystal planes. The image is often recorded on X-ray sensitive photographic film or plates with spatial resolution down to about 1 micron. Total Internal Reflection Fluorescence (TIRF) MicroscopyWhen total internal reflection occurs in one medium at the boundary with another, there is a disturbance in the second medium in the form of a wave which propagates along the interface – the evanescent wave. TIRF illumination makes use of the evanescent wave formed between two surfaces of different refractive index (e.g. a glass slide coverslip and the cell culture medium) under glancing-angle illumination. The evanescent wave decays exponentially away from the interface, vanishing in a few hundred nanometres, therefore being ideal to illuminate plasma membrane proteins without illuminating the cell cytosol or the culture medium, which results in a substantial reduction in the fluorescence background. X-ray Absorption Fine Structure (XAFS)Local information on the electronic and structural properties of a particular element in a substance (whatever the phase) can be obtained by measuring the variation in its X-ray absorption coefficient in the vicinity of an absorption edge. The technique is particularly useful for amorphous solids and liquids or solutions where there is a high degree of disorder. X-ray absorption near-edge structure ( XANES )When X-rays pass through any sort of material, a proportion of them will be absorbed. Measuring the amount of absorption with increasing X-ray energy reveals so-called edge structures where the level of absorption increases suddenly. This happens when an X-ray has sufficient energy to free or excite a bound (or core) electron within the material. Usually, small oscillations can be seen superimposed on the edge step and these gradually die away as the X-ray energy is further increased. The oscillations, which occur relatively close to the edge (within about 40 eV) are known as XANES (X-ray Absorption Near Edge Structure) or NEXAFS (Near Edge X-ray Absorption Fine Structure). X-RAY ABSORPTION SPECTROSCOPY ( XAS )This is a generic term covering 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. The amount of absorption (or the associated fluorescence or photoelectron yield following absorption) is measured as a function of the incident photon energy. At lower energies (below 1 keV) electronic or magnetic information about the material is obtained; above about 1 keV, as well as oxidation state and core levels, site symmetry, bond length, co-ordination number and other local structural information can be provided about the excited element. X-ray DiffractionX-ray diffraction results from the interference of X-rays (short wavelength electromagnetic waves) scattered by the electrons surrounding atoms. When these are in phase constructive interference occurs, i.e. the peaks of the waves are superimposed and add up to give a diffraction maximum. When the atoms form a regular array of repeating units, as in crystalline material, diffraction maxima are usually distinct. The angles (2-theta) between these maxima and the incident beam direction depend on the X-ray wavelength (l) and the spacing (d) between the atoms (Bragg’s Law: l = 2d sin(theta) ). Measuring the diffraction angles gives information on the spacing between atomic planes and the symmetry of the crystal lattice of regular repeating units. Measurements of the intensity of the maxima can be related to the atomic arrangement within each repeating unit. For disordered matter such as amorphous materials, glasses, liquids etc. the maxima are broad and relatively weak. X-ray magnetic circular dichroism (XMCD)X-ray magnetic circular dichroism (XMCD) is now a well-established technique for determining the element specific moments in magnetic materials. The technique utilises the differences in X-ray photoabsorption that occur near certain absorption edges when circularly polarised light is incident on a magnetised sample, and the relative alignment of the light helicity and magnetisation is reversed. On the SRS this reversal is achieved by 'flipping the magnetisation of the sample using an electromagnet (maximum field ± 0.6T). By tuning the X-ray energy through the specific absorption edges that couple to the valence band responsible for magnetism, the moments in different elements can be probed. Thus the technique is element and indeed site specific, as for example cations occupying different lattice positions in magnetic oxides result in slightly shifted absorption edges. Furthermore, through the application of sum rules, the separate spin and orbital moment for a given element in a material can be obtained. X-ray Spectroscopy ( XRS )SEE: X-RAY ABSORPTION SPECTROSCOPY (XAS) |
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