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Polarized Cu K x-ray absorption near-edge structure spectra of CuO are measured and analyzed with the aim of extracting quadrupole and dipole partial spectral components. Theoretical spectral components are calculated within the framework of real-space multiple-scattering technique, relying on a non-self-consistent muffintin potential. A local coordinate system xЈyЈzЈ suitable for polarization analysis is defined by the sides of the CuO 4 quasirectangle rather than by directions of the Cu-O bonds. An exclusively quadrupole nature of the prepeak is established both experimentally and theoretically. By analyzing the experimental data, we find that those states which give rise to the prepeak lie within the CuO plane, and have mainly a d x Ј y Ј character. The theory correctly reproduces gross features of the polarized structure generated by dipole transitions, apart from a spurious peak at 8983 eV for the p z Ј component. A partial agreement between theory and experiment in the shoulder region suggests that this structure may arise partly from one-electron states and partly from manybody processes.

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Two model compoundsvaline C 5 H 11 NO 2 (L-Val) and isoleucine C 6 H 13 NO 2 (L-Ile)were chosen to study isomorphism of the compounds formed by enantiomeric molecules having similar shapes, but differing in the number of chiral centers. The X-ray powder diffraction method was applied to investigate samples with various ratios of valine and isoleucine crystallized from aqueous solutions by cooling and isothermal methods. Limited solid state miscibility between valine and isoleucine was established. A non-equimolar discrete compound with the molecular ratio Val:Ile = 2:1 (V 2 I) has been discovered, and its crystal structure was determined. Accordingly, the corresponding regions of solid solutions and two-phase areas in-between were distinguished. Solubility data in water support the phase behavior in the system.

We studied the anisotropy of the unoccupied electron states of three titanium dichalcogenides by means of polarized x-ray-absorption spectroscopy. The experimental spectra of TiS 2 , TiSe 2 , and TiTe 2 at the Ti K edge and of TiTe 2 at the Te L 1 edge were compared with the results of real-space multiple-scattering calculations. The local angular momentum projections of the densities of states at the titanium sites are presented for energies up to 25 eV above the Fermi level. The Ti K-edge spectra of all three dichalcogenides are quite similar to each other provided the differences in their lattice constants are suppressed by comparing the spectra in equivalent energy scales. However, the correspondence between the Te L 1 edge of TiTe 2 and the S K edge of TiS 2 is much less pronounced. For most of the spectra good agreement between experiment and theory was found. ͓S0163-1829͑98͒01336-8͔

Using polarized x-ray-absorption spectroscopy along with band-structure and real-space multiple-scattering calculations, we have studied unoccupied states of TiS 2 . With respect to titanium and sulfur sites, the local angular momentum projections of the density of states are presented for states up to 25 eV above the Fermi level. Good agreement between experiment and theory suggests that quadrupole (1s→d) transitions in Ti K absorption are negligible compared to dipole (1s→p) transitions. We have applied a new technique for construction of pseudopotentials adapted to the chemical bond and found that previous calculation of electronic states with energies higher than 10 eV above Fermi level was not correct.

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Polarized Ni K XANES of single crystal NiO is studied experimentally and theoretically for two crystal orientations relative to the polarization and wave vectors of incident X-ray beam. The spectra calculated by the full multiple scattering method for a spin-dependent cluster potential and incomplete screening of the core hole appear to be in an excellent agreement with the experiment. The low energy pre-edge peak is shown to be caused by the quadrupole transitions of the Ni 1s electron to an exciton-type state shifted down by $4 eV from the empty e g high-energy conduction band under the influence of the core hole potential. The feature caused by the Ni 1s ! e g band dipole transitions is also recognized in the spectrum. It is shown that the same mechanisms, which are responsible for the formation of the pre-edge fine structure in the oxides of early 3d-metals in 3d 0 state, work in the considered case as well. A comparative analysis outlining the significance of different mechanisms for the pre-edge structure formation in cases of early and late 3d-metal spectra is presented. The antiferromagnetic order of the absorber is taken into account within oneelectron approximation.Die polarisierte Ni K XANES von einkristallinem NiO wurde experimentell und theoretisch fü r zwei Kristallorientierungen relativ zum Polarisations-und Wellenvektor des einfallenden Rö ntgenstrahls untersucht. Die Spektren wurden nach der Full-multiple-scattering Methode fü r ein spinabhä ngiges Clusterpotential und fü r unvollstä ndige Abschirmung des Rumpfloches in exzellenter Ûbereinstimmung mit dem Experiment berechnet. Die niederenergetische Vorkantenabsorption wird durch Quadrupolü bergä nge von Ni 1s Elektronen in einen exzitonä hnlichen Zustand erklä rt, der durch das Rumpflochpotential um $4 eV gegenü ber dem unbesetzten e gartigen Leitungsband abgesenkt ist. Die aus den Ni 1s ! e g Dipolü bergä ngen berechneten Strukturen sind auch in den experimentellen Spektren zu erkennen. Es wird gezeigt, da im vorliegenden Fall die gleichen Mechanismen wirken, die auch fü r das Auftreten der Vorkantenabsorption bei den frü hen 3d-Metalloxiden im 3d 0 -Zustand verantwortlich sind. Eine vergleichende Analyse der Bedeutung der verschiedenen Mechanismen fü r das Entstehen der Vorkantenstruktur bei den Verbindungen der 3d-Metalle niedriger und hö herer Ordnungszahl wird durchgefü hrt.