Polarization anisotropy of anomalous scattering in lithium iodate and effect of K-level width

Abstract

The anomalous scattering tensor, measured using synchrotron radiation with lithium iodate near the iodine K absorption edge, shows polarization anisotropy similar to that in the bromate ion, but lesser in magnitude: about 1 electron/atom at most. The reduction is explained by a greater natural width of the K level. Equally small or smaller anisotropy is predicted for any other absorption edge above 33 keV. I. IntroductionX-ray dichroism occurs in some molecules near absorption edges as a result of transitions to electronic states which have symmetry that reflects the direc-0108-7673/89/010039-04503.00 tional character of the chemical bonding. The anomalous scattering also depends on the direction of polarization of the radiation and needs to be represented by a tensor rather than a scalar function.Having found large effects of this kind for the pyramidal bromate ion near the K edge of bromine (Templeton & Templeton, 1985a) we were eager to test them in the iodate ion, which has analogous electronic structure and the same pyramidal shape. Lithium iodate, which crystallizes with two molecules per cell in the non-centrosymmetric space group P63 (Rosenzweig & Morosin, 1966; de Boer, van Bolhuis, Olthof-Hazekamp & Vos, 1966), is a suitable material for observation of the dichroism because the threefold axes of all the iodate ions are parallel. This molecular orientation and the lack of a center of inversion permit O 1989 International Union of Crystallography 40 POLARIZATION ANISOTROPY OF ANOMALOUS SCATTERING one to measure the complex scattering tensor by a diffraction technique used earlier for the tetrachloroplatinate ion (Templeton & Templeton, 1985b). Access to 33 keV radiation at the iodine K absorption edge was possible when a diffractometer (Nielsen, Lee & Coppens, 1986) was installed on a wiggler beam line at the Cornell High Energy Synchrotron Source (CHESS).as a first-order correction for 88% polarization. This correction is valid for the scattering by lithium and oxygen and for the spherical average of the iodine tensor, but not for the anisotropy terms. Because the anisotropy is a small perturbation of the total scattering, we believe that the result of this approximation is to move f,~ and f,, toward each other by an amount that is a few percent of their difference. Method of the experimentThe components of the scattering tensor were determined as the variables in least-squares adjustments of observed and calculated structure-factor magnitudes, with the atomic coordinates and displacement parameters held constant. The values found by Svensson, Albertsson, Liminga, Kvick & Abrahams (1983) by neutron diffraction at 295 K were used. The atomic scattering tensors of all the iodine atoms can be represented by the matrix f= f~ b in a Cartesian coordinate system with z in the direction of the crystallographic c axis. For s-polarized radiation an azimuthal angle (the rr setting) can be found for any Bragg plane such that the scattering factor reduces to f~=fo+f'+if'~ (Templeton & Templeton, 1985b). F...

XANES, EXAFS and Kbeta spectroscopic studies of the oxygen-evolving complex in Photosystem II

“…This is possible because the EXAFS amplitude is proportional to cos 2 θ, where θ is the angle between the X -ray E vector and the absorber-backscatterer vector. 282,287,289,291,292 This study was able to confirm the results from an earlier EXAFS study using isotropic samples 293 that the S(Met) ligand is not detectable by EXAFS, possibly due to uncorrelated vibrational motion of Cu and S; this reflects a weakly bonded situation. The contributions from t he remaining ligands, two histidine ligands and a cysteine ligand, were shown to be dichroic, and the experimentally determined dichroism in the EXAFS spectra was consistent with the dichroism pattern calculated from the X -ray crystal structure.…”

“…The Cr=O complexes were particularly interesting because the dichroism measurements showed that the intense 1s → 3d pre-edge transition was almost completely polarized along the Cr=O bond. 284 This was interpreted by Templeton et al 287,288 in similar systems to be a transition between the 1s core level and an antibonding molecular orbital with metal d z 2 and oxygen p z character.…”

“…This intense pre-edge transition was shown to occur in similar high-valent metal complexes, and was assigned to a transition between the 1s core level and an antibonding molecular orbital with metal d z 2 and ligand p z character. 287,288 The intensity of this transition is minimized at the φ = 45°, χ = 90° orientation, where the long axis of the crystal, and hence the Mn-N vectors, is oriented vertically and is orthogonal to the X-ray E vector. The non-zero amplitude of the pre-edge features is due to contributions from the other dipole-forbidden 1s → 3d transitions and the fact that a where θ is the angle between the Mn-backscatterer vector and the X-ray E vector.…”

“…Presumably, this pre-edge feature arises from transitions from the 1s core level to an antibonding molecular orbital with metal d z 2 and ligand p z character. 287,288 Furthermore, the shape of the pre-edge peak changes significantly between the two orientations. Because of the short time of exposure to X -rays, it is unlikely that this difference is due to photoreduction effects.…”

K-edge anomalous scattering in rubidium hydrogen tartrate

“…The structural parameters were redetermined by Templeton & Templeton (1989) [abbreviated below as TT (1989)]. This case exhibits relatively diffuse fine structure and a relatively weak resonance line.…”