Synchrotron source polarization

The advent of hehcal magnetic insertion devices at newer generation synchrotron sources has greatly improved this position, providing highly pure, symmetric left- and right-circular polarization that can be rapidly switched. 

An interesting feature of polarized IR spectroscopy is that rapid measurements can be performed while preserving molecular information (in contrast with birefringence) and without the need for a synchrotron source (X-ray diffraction). Time-resolved IRLD studies are almost exclusively realized in transmission because of its compatibility with various types of tensile testing devices. In the simplest implementation, p- and s-polarized spectra are sequentially acquired while the sample is deformed and/or relaxing. The time resolution is generally limited to several seconds per spectrum by the acquisition time of two spectra and by the speed at which the polarizer can be rotated. Siesler et al. have used such a rheo-optical technique to study the dynamics of multiple polymers and copolymers [40]. 

The polarization factor, Pol, takes into account the variation in scattering intensity caused by the beam polarization, as can be anticipated because elastic scattering is forbidden along the polarization direction of the incident beam. Synchrotron sources typically produce X-ray beams that are linearly polarized in the horizontal plane. Most synchrotron-based scattering measurements are performed with the scattering angle measured in the vertical plane, so that P= 1. For horizontal scattering spectrometers with a horizontally-polarized beam, the polarization factor is P = cos(2 ). A number of spectrometers scan the detector in both the vertical and horizontal planes. 

The XSW technique is not exclusively a synchrotron-based technique it can be performed with use of a conventional fixed-tube X-ray source or rotating anode. However, several practical considerations make it far more advantageous, and often essential, to perform these experiments at a synchrotron source. Some of these considerations are generally true for any X-ray experiment, while others are specific to the XSW techniques. These considerations are often of great practical importance in carrying out XSW experiments. We briefly discuss some of these factors, which include X-ray source brightness, tunability, and polarization. 

By using circularly polarized X-rays from a synchrotron source the intensity of photoemission becomes dependent on the magnetisation of the sample. This can be used to provide element specific magnetic contrast for imaging magnetic domains whose behaviour can then be studied in real time. 

Better resolution but stiU controlled by diffraction can be obtained with a synchrotron IR source in a confocal arrangement. The intrinsically brighter synchrotron IR source allows areas as small as 3-4 p,m to be probed [248-252, 266], which is very important for improving the quality of maps (vide infra). An additional advantage of synchrotron sources in orientational measurements is that the probing radiation is 100% polarized in the plane of the storage ring. 

Fig. 5. A schematic illustration of an angle-resolved photoemission experiment An incident photon, with wavevector p and polarization E, strikes the sample with polar incidence angles (61p, p) relative to the crystal axes. In practice the light source is generally fixed relative to either the crystal or the detector. However, the ability to vary the photon polarization from synchrotron sources provides a powerful tool for obtaining information on the symmetries of electronic states. By moving the analyzer or the sample (depending on the details of the experimental apparatus), photoelectrons leaving the surface at polar angles (6, ) are collected by the spectrometer the component of their crystal momentum, k, parallel to the sample surface is strictly conserved, allowing accurate determination of the two-dimensional band structure.

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