Synchrotron radiation polarization studies

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. 

Extension of these edge studies (21) to the blue copper site in plastocyanin indicates that there is less than 1% p orbital mixing into d 2 2 and in particular that this involves the p and p orbitals. The Is to 3d 2 2 transition at 8979 eV occurs only wheX the polarization vector of the synchrotron radiation is in the xy plane. This can be seen from the Is to 3d transition reproduced in Figure 15 which appears only with the electric vector perpendicular to the Cu-methionine bond. As emphasized above, this p, p mixing cannot account for the small copper hyperfine coupling constant of... 

The initial state valence electrons of a ferromagnet are unequally populated by majority (t) and minority (J electrons, which result from electron exchange interactions. This can result in fairly substantial spin polarization. For instance, studies of the amorphous alloys FegoB2o and Co77B23 reveal polarization as high as 20% when excited by 35-eV synchrotron radiation [36]. A recent report of Co(100)... 

In the case of structural studies with x-rays, sample size is rarely a serious problem since single crystals as small as about 0.001 mm3 (sealed x-ray tube) or 10-9 mm3 (synchrotron radiation) will do. In the case of x-ray diffuse scattering measurements with the assembly described in Section VI.D, the crystal must be 2 mm long at least but can be relatively thin. With neutrons, however, sample size is more critical. Indeed, for structure measurements, 0.1 mm3 is a minimum, with the highest neutron fluxes available nowadays, and more usual sizes are in the range of several cubic millimeters to several tens of cubic millimeters. For polarized neutrons or inelastic scattering studies, much larger samples are necessary. 

The results of Pandey et al. [249] applied toa lll 7x7 surface, but the more detailed results of Schliiter et al. [247] and Larsen et al. [250] were almost certainly obtained from a 111 2x1 reconstructed surface, although simultaneous LEED measurements were not reported. The precise form of reconstruction was, however shown to be important by Rowe et al. [248], who studied Cl2 adsorption on 111 7x7, 111 2x1 and 100 2xl surfaces by photoemission polarization effects with synchrotron radiation. The first point is an apparent difference between their LEED results and those of Florio and Robertson [246] and Pandey et al. [249]. Rowe et al. [248] found that the saturation coverage of chlorine on Ill 7x7 and lll 2xl surfaces produced a primitive lxl pattern, whereas the other workers observed that the 7x7 periodicity... 

Polarization is a very important property of synchrotron radiation. It provides the capability for the study of magnetic and optical circular and linear dichroism, for polarization dependent EXAFS, and a variety of other experiments. On-axis, (in the case of a single electron in the orbit plane, i.e., zero emittance) the photon is 100% polarized with the E vector parallel to the plane. Above and below the plane the radiation is elliptically polarized and the degree of linear polarization is defined as ... 

The characterization of the unoccupied and partially occupied states of TTF-TCNQ has recently been the object of a combined X-ray absorption near edge spectroscopy (XANES) and SIESTA study [234]. The use of synchrotron radiation allowed the determination of the partial DOS of S, N, and C because of the element selectivity of X-ray absorption and the determination of the spatial distribution of the molecular orbitals due to the polarization of the synchrotron hght. The SIESTA computations were used to calculate the DOS and PDOS into the carbon, nitrogen, and sulfur of TTF, TCNQ, and TTF-TCNQ. As a consequence, an in-depth discussion of the origin of the different features of the unoccupied DOS of the three materials, TTF, TCNQ, and TTF-TCNQ, has been possible leading to the assignment of most of the experimentally obtained features to specific molecular orbitals. 

Multilayers of glycine, adsorbed on Ti02 (110), were studied with synchrotron radiation-based UV light [428]. Ultraviolet photoemission spectroscopy (UPS) and XPS data showed the multilayers as formed by glycine molecules in polar zwit-terionic form (NHs CH2COO ). Photon induced damage of multilayers occurs fast (produces a first order desorption of zwitte-rionic molecules with total cross section). 

TIME RESOLVED POLARIZATION STUDIES USING SYNCHROTRON RADIATION... 

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