Synchrotron radiation is electron magnetic radiation emitted by (near speed of light) electrons travelling through magnetic fields (bending magents, wigglers, undulators) in a storage ring. Synchrotron radiation is emitted over the entire range of the electromagnetic spectrum, tangential to the orbit of the electrons and is collected by a beamline. The beamline includes optical devices which control the wavelength, photon flux, beam dimensions, focus, and collimation of the rays. The optical devices include slits, attenuators, crystal monochromators, and mirrors. At the end of the beamline is the experimental end-station, where samples are placed in the path of the radiation, and detectors are positioned to measure the resulting absorption, diffraction, scattering or secondary radiation.
X-ray Absorption Fine Structure:
X-ray absorption fine structure (XAFS) is a specific structure observed in X-ray absorption spectroscopy. XAFS is a spectroscopic technique that uses X-rays to probe the physical and chemical structure of matter at an atomic scale. By analyzing the XAFS, information can be acquired on the local structure and on the unoccupied electronic states. XAFS is element-specific, in that X-rays are chosen to be at or above the binding energy of a particular core electronic level of a particular atomic species therefore an energy-tunable X-ray source like a synchrotron is needed for XAFS measurements.
The X-ray absorption spectra exhibit a steep rise in the absorption coefficient at the core-level binding energy of X-ray absorbing atoms and attenuate gradually with the X-ray energy. The XAFS spectra are usually divided into three energy regions: 1) the edge region, 2) the X-ray Absorption Near Edge Structure (XANES); 3) the Extended X-ray Absorption Fine Structure (EXAFS). The absorption peaks at the absorption edge region ~5eV are due to electronic dipole transitions from a core-level to an unoccupied orbital or band above the Fermi level. The oscillatory structure extending for hundreds of eV past the absorption edge is the EXAFS, resulting from the interference in the single scattering process of the excited photoelectron scattered by neighbouring atoms and provides information on the local structure. The energy region of XANES (extending over a range of about 100 eV) between the edge region and the EXAFS region has been assigned to multiple scattering resonances and provide information on the geometry of the local structure. In the case of organic molecules this energy region has been later called near-edge X-ray absorption fine structure (NEXAFS), but NEXAFS is synonymous with XANES.
X-ray Excited Optical Luminescence:
X-ray excited optical luminescence (XEOL) monitors the luminescence from a light emitting material, by measuring a specific de-excitation channel associated with the energy redistribution by a system upon the absorption of an energetic photon. The absorption of an X-ray photons and the decay leads to the production of photoelectrons, Auger electrons, and fluorescence X-ray photons. These processes and associated secondary processes result in the formation of thermalized holes in the valence band and electrons in the conduction band of the luminescent solid. The radiative recombination of holes and electrons produces luminescence. Optical photons are the product of electron-hole recombination between the conduction and valence bands, or from defect energy levels in the band gap. Essentially, XEOL is an X-ray photon in, optical photon out technique. XEOL has the added advantage of being element, site and excitation channel specific, which is achieved by tuning the photon (excitation) energy to a particular absorption edge of an element of which the local electronic structure is effectively coupled to the luminescence channel, thereby exciting preferentially, those sites responsible for the optical emission. The optical or photoluminescence yield (PLY), in turn, can be used to monitor the absorption; this technique is sometimes called optical-XAFS.