Polarization, of synchrotron

Figure 2.1. Polarization of synchrotron radiation in a hypothetic non-relativistic case. In real ity the toroidal radiation pattern is degenerated to a narrow forwar... 

X-ray fluorescence analysis has a long history as a standard technique for analysis of mm to cm specimens using a hot-cathode X-ray source. However, the X-ray flux from a hot-cathode source is too divergent (isotropic emission) to permit efficient focusing. Thus, the niche of the synchrotron X-ray beam is in microbeam applications of the XRF technique, the inherent collimation and polarization of synchrotron radiation is well suited to use in an XRF microprobe. Details of synchrotron radiation generation are given in an accompanying chapter (Sham and Rivers, this volume). 

Using the linear polarization of synchrotron radiation is fundamental to describe unoccupied states in molecular inner-shell spectroscopy. Because the core level electron is localized on a definite atom, spectroscopy based on core excitation into unoccupied valence MOs appear simplified with respect to valence electrons excitation spectroscopy. 

The diffraction mechanisms in XPD and AED are virtually identical this section will focus on only one of these techniques, with the understanding that any conclusions drawn apply equally to both methods, except where stated otherwise. XPD will be the technique discussed, given some of the advantages it has over AED, such as reduced sample degradation for ionic and organic materials, quantification of chemical states and, for conditions usually encountered at synchrotron radiation facilities, its dependence on the polarization of the X rays. For more details on the excitation process the reader is urged to review the relevant articles in the Encyclopedia and appropriate references in Fadley. ... 

The use of synchrotron radiation overcomes some of the limitations of the conventional technique. The high brilliance of up to 10 ° photons s mm mrad /0.1% bandwidth of energy, and the extremely collimated synchrotron beam lead to a large flux of photons through a very small cross section (0.1-1 mm ). This allows measurements with samples of small volume if isotopi-cally enriched (with the relevant Mossbauer isotope, e.g., Fe). Measurements that were described earlier [4] and that require a polarized Mossbauer source now become experimentally more feasible by making use of the polarization of the synchrotron radiation. Additionally, the energy can be tuned over a wide range. This facilitates measurements with those Mossbauer nuclei for which conventional sources are available but with life times that are too short for most experimental purposes, e.g., 99 min for Co —> Ni and 78 h for Ga —> Zn. 

Another very important property of synchrotron radiation is its very high degree of polarization. The radiation is predominantly polarized with the electric field vector parallel to the acceleration... 

Polarization is a relevant issue, because we are dealing with transversal waves (Guinier [6], p. 10-11). Polarization correction should be carried out for MAXS and WAXS data. It is less important for SAXS and USAXS patterns. In particular, if synchrotron radiation is used, the polarization correction is quite involved and based on the degree of polarization. For the purpose of reliable correction it is thus recommended to let a polarization monitor measure the actual degree of synchrotron beam polarization. 

Synchrotron light is, in general, polarized in horizontal direction ([10], p. 9-13). Nevertheless, the polarization of the beam is never perfect. In order to be able to carry out a quantitative polarization correction, the quality of polarization should be monitored by means of a polarization monitor [11] that is positioned in the primary beam. The polarization monitor is registering the horizontally polarized component, 4, and the vertically polarized component, 4- From these two intensities the quality... 

An alternate method for obtaining angular information is to make use of the plane polarized nature of synchrotron radiation. It has long been known that XAS should exhibit a polarization dependence for anisotropic samples (18) however it is only recently that attempts have been made to exploit this effect. Early attempts to observe anisotropic XAS suffered from the low intensity and incomplete polarization of conventional x-ray sources. This work has been reviewed by Azaroff (19). 

In this chapter, we briefly discuss the theoretical background of polarized x-ray absorption spectroscopy (PXAS). Many of the recent applications of synchrotron radiation to polarized absorption edge structure and to EXAFS are discussed, with particular emphasis being given to the study of discrete molecular systems. We present here some indication of the potential applications of PXAS to systems of chemical and biological interest. 

The CD of saccharides has been less documented than those of proteins and nucleic acids, because of the difficulty of the CD in the VUV region. Snyder et al.247,248) reported the useful application of synchrotron orbital radiation (SOR) for VUV CD measurements. SOR from modem electron storage rings is highly linearly polarized and more intense than conventional VUV continuum sources. These properties make SOR ideal for CD measurements because of the better signal-to-noise ratio resulting... 

R. Schinke Photodissociation Dynamics 2. L. Frommhold Collision-Induced Absorption in Gases 3. T. F. Gallacher Rydberg Atoms 4. M. Auzinsh and R. Ferber Optical Polarization of Molecules 5.1. E. McCarthy and E. Weigold Electron-Atom Collisions 6. V. Schmidt Electron Spectrometry of Atoms using Synchrotron Radiation 7. Z. Rudzikas Theoretical Atomic Spectroscopy... 

A further important property of synchrotron radiation concerns its polarization characteristics. The radiation is completely polarized, and the kind of polarization depends on the direction of the circulating electron beam as well as on the direction of photon emission. In order to understand these polarization properties, it is useful to recall the result for the emission of electromagnetic radiation from an electron moving with non-relativistic velocity in a circle the electric field vector follows the same shape and orientation as the projection of the electron s path onto a plane perpendicular to the observation direction. 

Another important aspect of electron spectrometry of free atoms using synchrotron radiation concerns the polarization of the monochromatized light. 

Due to the mixed polarization of monochromatized synchrotron radiation, the angle dependence of photoelectron emission as expressed in equ. (1.30) for completely linearly polarized light requires modification. This is considered in detail in Section 9.1, but the implication for the corresponding appropriate experimental set-up is treated in the next section. 

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