On the Calculation of an X-Ray Natural Circular Dichroism Signal

The sequential derivation of an expression that describes an X-ray natural circular dichroism (XNCD) signal in the most general case is performed. The obtained expression is used as a basis to consider XNCD signals and their structural origins in copper metaborate CuB₂O₄ crystals at an incident radiation energy near the K absorption edge of copper and lanthanum–gallium silicate La₃Ga₅SiO₁₄ crystals at an incident radiation energy near the K absorption edge of gallium. It has been demonstrated that the measurements of circularly polarized X-ray absorption spectra in combination with corresponding first-principle calculations provide an efficient method for studying the contributions of crystallographically non-equivalent atoms to the X-ray optical activity of crystals.     

X-ray natural circular dichroism in copper metaborate

The local electronic structure of copper ions in a copper metaborate CuB₂O₄ crystal is studied on the ESRF synchrotron using X-ray absorption polarization-dependent spectroscopy. The X-ray natural circular dichroism near the K absorption edge of copper is measured in the direction that is perpendicular to crystal axis c. The data obtained indicate the presence of hybridized p–d electronic states of copper. Theoretical calculations are used to separate the contributions of the two crystallographically nonequivalent positions of copper atoms in the unit cell of CuB₂O₄ to the absorption and X-ray circular dichroism spectra of the crystal.     

Multiple scattering approach to Co K-edge XMCD and XANES spectra in Gd–Co alloys

We have measured and analyzed Co K-edge X-ray absorption near edge structure (XANES) and X-ray magnetic circular dichroism (XMCD) near edge spectra in crystalline and amorphous GdCo₂ alloys. We have used a semi-relativistic full multiple scattering approach to the analyses of the XMCD spectra. A general formula is obtained which is applicable to randomly oriented systems in space. Useful information is obtained on both the electronic and geometric structure around a Co atom. The difference in XANES and XMCD spectra between crystalline and amorphous GdCo₂ is well explained by models referring to the anomalous X-ray scattering result, where three Gd atoms in the second shell are removed away. The calculated XANES are not so sensitive to the electronic structure, whereas the calculated XMCD spectra are rather sensitive to the spin polarization on Co atoms. The result shows that the spin polarization on Co atoms in GdCo₂ alloys is smaller than that in Co metals.     

Density functional study on chirospectra of hydrogen-bonded systems X-（H2O）3（X=F,Cl,Br,I）

This paper calculates the molecular structures,infrared,Raman,circular dichroism spectra and optical rotatory powers of some hydrogen-bonded supramolecular systems as a cyclic water trimer,(H2O)3 and its pyramidal halide complexes,X-(H2O)3(X = F,Cl,Br,I) with the gradient-corrected density functional theory method at the B3LYP/6-311++G(2d,2p) and B3LYP/Aug-cc-pVTZ levels.It finds that the complexation of halide anions with the water trimer can efficiently modulate the chirally optical behaviors.The calculated vibrational circular dichroism spectrum illuminates that the vibrational rotational strength of S(+)-(H2O)3 mostly originates from the O-H rocking modes,whereas chirality of S(-)-X-(H2O)3(X = F,Cl,Br,I) has its important origin in the O-H stretching modes.The calculated optical rotatory power demonstrates that S(+)-(H2O)3 and S(+)-F(H2O)3 are positively chiral,whereas S(-)-X-(H2O)3(X = Cl,Br,I) are negatively chiral.With the polarizable continuum model,calculated bulk solvent effect in the solvents water and carbontetrachloride and argon shows that the positive chirality of S(+)-(H2O)3 is enhanced and the negative chirality of S(-)-X-(H2O)3(X = Cl,Br,I) and the positive chirality of S(+)-F(H2O)3 are reduced with an augmentation of the solvent dielectric constant.     

Magneto-optical activity of crude oil and its heavy fractions

We have experimentally studied optical and magneto-optical spectra of solutions of crude oils of different origin and their heavy fractions in the visible spectral range. Magnetic circular dichroism of oil in the wavelength range ??550 nm has been revealed. We show that the shape of the spectra of this dichroism depends on the origin of crude oil, with the magnetic dichroism magnitude being proportional to the concentration of the oil in the solution. A comparison of the data of magneto-optical spectroscopy with electron paramagnetic resonance spectra and chemical composition of samples has allowed us to conclude that the observed magneto-optical activity is determined by the occurrence of VO²⁺ complexes in the oil samples. The revealed magneto-optical activity of crude oil can form the basis of a unique method of analysis of the composition and properties of oils of different origin and heavy fractions thereof.     

Magneto-optical properties of [Li]0 defects in MgO

Magnetic circular dichroism and optical absorption spectra are reported for MgO crystals containing [Li]⁰ defects. The spectra are identified by optical detection of the [Li]⁰ ground-state paramagnetic resonance spectrum. The spectra are interpreted for both the ion and polaron models of the defect. Either model gives a qualitative interpretation of the spectra, but neither gives quantitative agreement.     

Magneto-optical and optical properties of polycrystalline Со–Р films with nanometer thickness

Features of the optical and magneto-optical properties of polycrystalline Со–Р films with nanometer thickness are established experimentally. It is found that the spectra of optical parameters n and k and magnetic circular dichroism depend largely on film thickness, suggesting that magneto-optical activity is governed by different mechanisms in thin films and bulk materials.     

Relativistic multiple scattering theory of K-edge X-ray magnetic circular dichroism

We discuss the angular dependent K-edge X-ray magnetic circular dichroism (XMCD) spectra based on the non-relativistic Green’s function expansion of the relativistic 4×4 Green’s function developed by Gesztesy et al. [Ann. Inst. Henri Poincaré 40 (1984) 159]. For the core functions we use the Dirac equation solutions whereas the relativistic effects for photoelectrons are automatically taken into account in the Gesztesy expansion. Analyses of the angular anisotropy provides us useful information on local symmetry violation around X-ray absorbing atoms even though we include relativistic effects. We explicitly show three different types of the symmetry which give rise to the suppression of the sin β-dependence in XMCD spectra. We also present explicit formulas of XMCD for randomly oriented and spatially fixed systems. Discussion on Debye–Waller factors is given in the present theoretical framework. Some illustrative calculations are also shown to understand the relativistic effects on the XMCD. The results are given for the Gd L₁-edge and Fe K-edge XMCD.     

Optical activity of trigonal coordination compounds

The optical activity of trigonal coordination compounds is investigated using the crystal field theory. The ionic model is shown to account for the circular dichroism of Co(en)₃ ⁺³. The absolute configuration of Cr(C₂O₄)₃ ^-3 is assigned on the basis of the ionic model.     

Stress-induced optical dichroism of V−-centers in MgO

The stress-induced optical dichroism and optical absorption spectra of the V^?-centers in electron irradiated MgO are reported. The dichroism spectrum can be largely attributed to a paraelastic alignment of inherently-dichroic V^?-centers. A curve-fit analysis of the lower-energy portions of the dichroism and absorption spectra yields the ratio of the transition dipole strengths, $\text{D}{}_{\mathrm{\parallel }}\text{D}{}_{\mathrm{\perp }}\text{= 1.55 ± 0.2}$, and the energy splitting, E_∥ ? E_⊥ = 0.22 ± 0.07 eV, for light polarized parallel (∥) and perpendicular (⊥) to the defect symmetry axis. The spectra also have considerable intensity in their high-energy tails, and it is speculated that a portion of this intensity is due to the V-center. The implications of the results with regard to the electronic structure of the center are discussed.     

Optically detected electron nuclear double resonance of anion antisite defects in GaP

Anion antisite defects in semi-insulating GaP single crystals (doped with group V atoms) are investigated with optically detected electron paramagnetic resonance and electron nuclear double resonance (ODEPR and ODENDOR) ODEPR and ODENDOR are measured as microwave- and microwave- and rf-induced changes of the magnetic circular dichroism of the optical absorption (MCDA), respectively. In GaP: V two P antisite defects are found in about equal concentration which within ENDOR precision have tetrahedral site symmetry with respect to the nearest P ligands. The two P_GaP₄ defects have only slightly differentg-factors and³¹P superhyperfine interactions, but differ by one order of magnitude in their spin-lattice relaxation times. Their optical properties are also different from those of the P_GaP₄ defects reported earlier in GaP: Cr. It is concluded that there is no experimental prove yet. that any of the P_GaP₄ defects hitherto observed in GaP in an isolated P_GaP₄ defect which is not associated with another defect to form an antisite related defect complex.     

Computational study of titania-ceria hybrid clusters for electrochemical applications

First principles calculations were carried out to investigate the electronic and optical properties with spotlight on excitation spectra and circular dichroism (CD) spectra of Ce₂O₄, Ti₂O₄ and the hybrid cluster CeTiO₄. The calculated UV/Vis spectrum and CD spectrum for Ce₂O₄ and Ti₂O₄ clusters found in respective IR and UV regions is shifted to visible region in case of the hybrid cluster. The major singlet-singlet-allowed transitions for the structures are discussed in detail. The hybrid cluster is optically active in the visible region and simultaneously contains titania’s appealing catalytic properties as well as ceria’s attractive properties for solid state electrolytes. The results point to possibility of adjustable band edges for potential applications in water splitting, coupling semiconductors applicable in dye-sensitized solar cells and other electrochemical devices.     

Characterization of the 2D antiferromagnets Rb2MnCl4 and Rb3Mn2Cl7 : Optical,Raman and magnetic circular dichroism studies

Below T_N, the site symmetry at the Mn²⁺ ion is centrosymmetric (Rb₂MnCl₄) and non-centrosymmetric (Rb₃Mn₂Cl₇) respectively. As a result, one expects the appearance of magnetic dipole or electric dipole exciton origins in the optical spectra. These were clearly seen via polarized absorption and magnetic circular dichroism measurements through the 4T₂(D) band. The zone edge magnon frequencies are found to be 80 cm^?1 (Rb₂MnCl₄) and 90 cm^?1 (Rb₃MnCl₂in7). The two compounds are also easily distinguished through their room temperature axial absorption (⁴T₂ (G) band) and Raman spectra. Low temperature data indicate that the tetragonal field plays an important role.     

Co0.9Fe0.1薄膜面内元素分辨的磁各向异性

利用X射线磁性圆二色技术对Co_0.9Fe_0.1薄膜面内元素分辨的磁各向异性进行了研究，通过剩磁模式测量不同磁化方向的样品组分原子单位空穴磁矩的变化，发现除了在生长的磁诱导方向存在易磁化轴外，在与该轴垂直的方向还存在一个类似易轴的软磁化轴；面内的两个难磁化轴与易磁化轴取向大约成66°夹角，从而构成了面内双轴磁各向异性；对不同组分元素，其单位空穴磁矩随磁化方向的变化趋势基本相同，不同磁化方向Fe原子单位空穴的磁矩值约为Co的对应值的87%，反映了Fe原子和Co原子之间存在着强烈的铁磁性耦合. 关键词： 磁各向异性 X射线磁性圆二色 铁磁耦合 CoFe合金薄膜     

X-ray dichroism in biaxial gyrotropic media: Differential absorption and fluorescence excitation spectra

The differential absorption and the differential change in the polarization state of an X-ray beam propagating inside a gyrotropic crystal are described using a Müller matrix, the 16 elements of which are related to the anisotropic components of the multipolar polarizability tensors at the absorbing site. Analytical expressions are given up to third order for X-ray linear and circular dichroism, X-ray optical rotation and X-ray circular polarimetry in transmission. The same formalism is extended to discuss Fluorescence detected dichroism spectra with or without polarization analysis of the fluorescence. Fluorescence detected dichroism is strictly proportional to dichroism measured in the transmission geometry only for uniaxial crystals. In biaxial crystals, the tiny effects of X-ray gyrotropy are swamped by large linear dichroism signals due to the imperfect polarization transfer function of Bragg monochromators. Second order effects should also be taken into consideration. Our general formulation of linear and circular dichroism includes terms of odd parity with respect to the action of the time reversal operator: such terms cannot contribute to natural dichroism but can be activated by a magnetic field. The terms responsible for X-ray magnetic circular dichroism are well known but non-reciprocal X-ray gyrotropy effects are also predicted in magnetic crystals of appropriate symmetry. Received 3 March 1999     

Optical and magneto-optical properties of the endohedral La@C82 crystal

The magneto-optical properties of solids are theoretically described by the circular dichroism (CD) and birefringence coefficient (θ). Using the Su-Schrieffer-Heeger (SSH) model in conjunction with the local field method, the optical dielectric tensor, CD and θ of the simple cubic phase of the La@C₈₂ and C₈₂ crystals are calculated. The results obtained from the La@C₈₂ and C₈₂ crystals are compared with those of the C₇₀ and C₆₀ crystals. It is shown that La@C₈₂ has a richer optical spectrum than C₈₂, C₇₀ and C₆₀. In the La@C₈₂ crystal, absorption bands are shown by spectrum in the 0.58 to 7.0?eV region with sharp structures in each band which indicate the localized molecular structure coupled with long-range crystalline order. Results show that the circular dichroism and birefringence coefficient of the La@C₈₂ crystal due to a single spin localized on the C₈₂ cage are very larger than those of the C₆₀, C₇₀ and C₈₂ crystals. Also, results show that the circular dichroism and birefringence coefficient of the C₈₂ crystal are similar to those of the C₆₀ and C₇₀ crystals.     

Specific features of magneto-optical spectra of Tb<Subscript>3</Subscript>Ga<Subscript>5</Subscript>O<Subscript>12</Subscript>

The spectra of magnetic circular dichroism in the range of the ⁷ F ₆→⁵ D ₄ absorption band and the spectra of magnetic circular polarization of luminescence in the range of the ⁵ D ₄→⁷ F ₅ band in the terbium-gallium garnet Tb₃Ga₅O₁₂ are studied at a temperature of 80 K. The optical transitions between the Stark sublevels of the ⁷ F ₆, ⁷ F ₅, and ⁵ D ₄ multiplets are identified based on the analysis of the magneto-optical and optical spectra. It is shown that the experimentally determined symmetry and energy of the Stark sublevels of these multiplets confirm the results of numerical calculations of the energy spectrum of the Tb³⁺ rare-earth ion in terbium-gallium garnet.     

木榄醇手性光谱的密度泛函研究

手性光学理论研究有助于解释手性分子的旋光机理和设计新的手性药物.采用B3LYP方法,计算了木榄醇A—C的旋光度和圆二色谱.从分子结构、正则振动和电子结构方面,探索了分子手性微观起源,分析了旋光度和电子圆二色谱的溶剂效应.表明OH的引入可调节分子的手性,甲基和苯环加强了分子的手性.发生在手性骨架上的振动和电子跃迁,加强了分子的手性.溶剂效应减小旋光度、削弱电子圆二色谱. 关键词： 木榄醇 光学活性 密度泛函理论 圆二色谱     

Soft X-ray magnetic circular dichroism of Heusler-type alloy Co2MnGe

Co and Mn 2p core absorption (XAS) and X-ray magnetic circular dichroism (XMCD) spectra have been measured for the ferromagnetic ternary alloy Co₂MnGe. The observed Co 2p XAS spectrum can be understood on the basis of the unoccupied Co 3d partial density of states, whereas the overall features of the Mn 2p XAS and XMCD spectra have been partly reproduced by the Mn 2p⁵3d⁶ final state multiplets. We have found that the orbital polarization of the Co 3d and even the Mn 3d states are recognizable, which suggests that a spin-orbit coupling should be taken into account in the energy band structure in order to reproduce the half metallic nature of this alloy.     

Linear and quadratic magneto-optical effects observed in La0.7Sr0.3MnO3 thin films in transmitted light

Magnetic linear dichroism (MLD) is observed in polycrystalline La_0.7Sr_0.3MnO₃ films in transmitted light. The spectral and temperature dependences of the MLD are determined for the energy E of the light wave ranging from 1.5 to 3.5 eV. The experimental spectra are fitted by a sum of Lorentzian curves. The MLD spectrum exhibits more features in comparison to the spectra of magnetic circular dichroism (MCD), which we measured earlier using the same film samples. Various temperature behaviors of the intensities of the Lorentzian components centered at different E values have been revealed. This can be related to the effect of the surface states on the magneto-optical characteristics of the samples.