An unconventional method for measuring the Tc L3‐edge of technetium compounds

Tc L₃‐edge XANES spectra have been collected on powder samples of SrTcO₃ (octahedral Tc⁴⁺) and NH₄TcO₄ (tetrahedral Tc⁷⁺) immobilized in an epoxy resin. Features in the Tc L₃‐edge XANES spectra are compared with the pre‐edge feature of the Tc K‐edge as well as other 4d transition metal L₃‐edges. Evidence of crystal field splitting is obvious in the Tc L₃‐edge, which is sensitive to the coordination number and oxidation state of the Tc cation. The Tc L₃ absorption edge energy difference between SrTcO₃ (Tc⁴⁺) and NH₄TcO₄ (Tc⁷⁺) shows that the energy shift at the Tc L₃‐edge is an effective tool for studying changes in the oxidation states of technetium compounds. The Tc L₃‐edge spectra are compared with those obtained from Mo and Ru oxide standards with various oxidation states and coordination environments. Most importantly, fitting the Tc L₃‐edge to component peaks can provide direct evidence of crystal field splitting that cannot be obtained from the Tc K‐edge.     

Finite lifetime broadening of calculated X‐ray absorption spectra: possible artefacts close to the edge

X‐ray absorption spectra calculated within an effective one‐electron approach have to be broadened to account for the finite lifetime of the core hole. For methods based on Green's function this can be achieved either by adding a small imaginary part to the energy or by convoluting the spectra on the real axis with a Lorentzian. By analyzing the Fe K‐ and L_2,3‐edge spectra it is demonstrated that these procedures lead to identical results only for energies higher than a few core‐level widths above the absorption edge. For energies close to the edge, spurious spectral features may appear if too much weight is put on broadening via the imaginary energy component. Special care should be taken for dichroic spectra at edges which comprise several exchange‐split core levels, such as the L₃‐edge of 3d transition metals.     

Crystal field effect on X-ray magnetic circular dichroism spectra at the L<Subscript>2,3</Subscript> absorption edges of mixed-valence Ce and Yb compounds in high magnetic fields

A theoretical study is presented, with an extended single impurity Anderson model, for the crystal field effect on the X-ray magnetic circular dichroism (XMCD) spectra at L_2,3 edges of mixed-valence Ce and Yb compounds in high magnetic fields. The crystal field acting on the 4f electrons is assumed to have cubic symmetry. Due to the competition among the effects of crystal field, mixed valency, and external magnetic field, the magnetic-field-dependence of XMCD spectra exhibits a variety of features; for instance, the branching ratio, R(L₂/L₃), of L₂ and L₃ XMCD intensities of Ce compounds can take R(L₂/L₃) > 1.0 and <1.0, and that of Yb compounds can take R(L₂/L₃) > 0 and <0. It is shown that the magnetic-field-dependence of the total XMCD intensity I(L₂ + L₃) is proportional to the magnetization curve, but that of R(L₂/L₃) gives more precise information on the ground state wavefunction in magnetic fields. A new and useful method to correlate the XMCD spectra, the 4f magnetization and the ground state wavefunction is proposed and used to discuss the relation between I(L₂ + L₃) and the magnetization curve and that between R(L₂/L₃) and the ground state wavefunction.     

Spectroscopic investigation on the electronic structure of a 5d band insulator SrHfO3 in proximity to ferroelectric instability: Comparison with SrTiO3 and SrZrO3

We investigated the high-energy electronic structure of a 5d perovskite SrHfO₃ by using optical spectroscopy and O 1s x-ray absorption spectroscopy. From the combined spectra the values of electronic structure parameters are estimated properly. In particular, the crystal field splitting energy, which is closely associated with the p–d hybridization strength, is as high as ～5 eV, and the Sr 4d bands appear to be strongly mixed with the Hf 5d bands. These findings are discussed in relation to a possible ferroelectric instability in SrHfO₃, and are compared with electronic properties of similar compounds, 3d SrTiO₃ and 4d SrZrO₃.     

Electronic structure and X-ray spectra of transition metal sulfides NiS,CuS, and ZnS

A numerical electronic band structure calculations for sulfides NiS, CuS, and ZnS are carried out. Using the results a detailed analysis of a valence states is performed; obtained partial densities of states are compared with X-ray SL _2,3 and $ SK_{\beta _{1,3} } $ SK_{\beta _{1,3} } -emission spectra. We showed that spectrum lineshape depends on hybridization strength between various Me(3d)-orbitals and 3p-states of sulfur. The hybridization strength and the symmetry of hybrid Me(3d)-orbitals are defined by crystal lattice structure. Finally a well splitted in energy bonding and antibonding states Me(3d)-S(3p) appear while weakly hybridized Me(3d)-states mainly contribute to spectra intensity in the energy between them. A good agreement between the theoretical and the experimental spectra of valence band for considered sulfides is obtained.     

Effect of Ga incorporation on valence band splitting of OVC CuIn3Se5 thin films

Polycrystalline thin films of CuIn_2.95Ga_0.05Se₅ produced by the incorporation of Ga into the ordered vacancy compound CuIn₃Se₅ by a two-stage vacuum evaporation process were structurally, compositionally and optically characterized using X-ray diffraction, energy dispersive analysis of X-rays and optical absorbance measurements. From the X-ray diffraction data of the films, the structural parameters like lattice constants, tetragonal deformation, bond lengths and anion displacement were evaluated and their effect on the optical behavior of films was discussed. The Hopfields quasi-cubic model adapted for chalcopyrites with tetragonal deformation was used to elucidate the crystal field and spin orbital splitting parameters in the uppermost valence band of the compound, using the three energy gaps 1.649, 1.718 and 1.92 eV corresponding to the threefold absorption in the fundamental absorption region of the optical spectra of these films. The percentage contributions of Se p and Cu d orbitals to p-d hybridization in this compound were calculated using linear hybridization of orbitals model and the effects of p-d hybridization on the band gaps were studied.     

Strong spin-dependent absorption at theL 2,-edges of 5d-impurities in iron

Large spin-dependent absorption effects have been observed with circularly polarized photons of energies between 10.8 keV and 13.8 keV at theL _2,3-edges of Os, Ir, Pt, and Au impurities (3 at.%) in iron. The largest value of the spin-dependent absorption coefficient (µ _c /µ _o +0.22) is found at the PtL ₂-edge. From the energy dependence of the spin-dependent absorption at theL ₂- andL ₃-edges the spin-density profiles of thed _3/2-andd _5/2-projected states populated in the absorption process are derived. Thed _5/2-spin densities deduced from theL ₃ spin-dependent absorption spectra agree well with spin-polarized band-structure calculations for the ferromagnetic ground state. A comparison of thed _3/2- andd _5/2-spin densities shows a strong contribution of the spin-orbit effects to the exchange splitting in Os, whereas only small spin-orbit effects are indicated for Ir, Pt and Au impurities.     

Vanadium valency and hybridization in V-doped hafnia investigated by electron energy loss spectroscopy

The valency of vanadium, and thus indirectly the oxygen stoichiometry, of V-doped hafnia synthesized under different atmospheres have been investigated on a nanometer scale by means of electron energy loss spectroscopy (EELS). The EELS V L_2,3 spectra are compared with the results of crystal field multiplet calculations and experiments on reference vanadium oxides. The EELS spectra indicate that V-doped hafnia prepared under reducing (H₂) and neutral (Ar) atmosphere are unambiguously substituted with trivalent vanadium atoms leading to the creation of oxygen vacancies in the structure. On the contrary, stoichiometric (Hf, V)O₂ compound (i.e. V⁴⁺) is more likely to be stabilized under oxidative (air) atmospheres. We also show that the amount of hybridization alters for the different compounds studied but may in part be analyzed by high spatially resolved EELS. The crystal field multiplet calculations particularly indicate that a simple reduction of the Slater integrals gives a good account of the spectral modification induced by hybridization for the case of tetravalent vanadium atoms. Received 17 November 2000 and Received in final form 17 April 2001     

Application of symmetry coupling coefficients in zero-field splitting of d5 ion complexes

Symmetry coupling coefficients and symmetry reduced matrix elements are used in calculating the minimum required matrix elements required to determine the analytical expressions for the three ⁶ A ₁ ground-state energy levels for a d⁵ ion in a weak crystal field of tetragonal symmetry. Expressions for the spin hamiltonian parameters a, D, F and G are derived, from which the effect of spin-orbit coupling and the tetragonal component of the crystal field are examined. The parameters describe the form of the hamiltonian and consequently the spin eigenfunctions and eigenvalues of the ground state where a is a measure of the isotropic component whereas D, F and G measure the tetragonal component of the interaction giving rise to the zero-field splitting. Some experimental results are examined.     

Extended soft X-ray emission spectroscopy: quantitative assessment of emission intensities

Soft X‐ray emission spectroscopy (SXES) in the energy range between 150 eV and 1500 eV has typical attenuation lengths between tens and a few hundred nanometres. In this work the transmission of soft X‐rays in synchrotron‐based SXES has been quantitatively analysed using specially prepared layer samples. The possibility of extending the standard qualitative analysis of SXES by exploiting the information underlying the emission intensity was examined for thin layer structures. Three different experiment series were accomplished with model layer systems based on different sulfur‐containing substrates: (i) MoS₂, (ii) CuInS₂, (iii) Cu(In,Ga)(S,Se)₂. The absorption of the S L_2,3 emission line by ZnO cover layers of up to 80 nm thickness was monitored and compared with theoretical expectations. By comparison with a reference intensity recorded from a bare substrate, the attenuation of the S L_2,3 emission could be used to accurately determine the ZnO overlayer thickness up to a critical thickness, depending on the set‐up and the net S L_2,3 emission intensity. The results from these local energy‐resolved spot measurements were compared with spatially resolved scans of the integral S L_2,3 emission intensity over areas of several mm². In the scan images the attenuation of the S L_2,3 emission intensity clearly reflects the local ZnO layer thickness. From the attenuation the ZnO layer thicknesses were calculated and compared with ellipsometric measurements and were found to be in excellent agreement. These results demonstrate the benefits of a quantitative analysis of SXES, making it an even more powerful tool for examining buried interfaces and for monitoring lateral inhomogeneities.     

Improved charge transfer multiplet method to simulate M‐ and L‐edge X‐ray absorption spectra of metal‐centered excited states

Charge transfer multiplet (CTM) theory is a computationally undemanding and highly mature method for simulating the soft X‐ray spectra of first‐row transition metal complexes. However, CTM theory has seldom been applied to the simulation of excited‐state spectra. In this article, the CTM4XAS software package is extended to simulate M_2,3‐ and L_2,3‐edge spectra for the excited states of first‐row transition metals and also interpret CTM eigenfunctions in terms of Russell–Saunders term symbols. These new programs are used to reinterpret the recently reported excited‐state M_2,3‐edge difference spectra of photogenerated ferrocenium cations and to propose alternative assignments for the electronic state of these cations responsible for the spectroscopic features. These new programs were also used to model the L_2,3‐edge spectra of Fe^II compounds during nuclear relaxation following photoinduced spin crossover and to propose spectroscopic signatures for their vibrationally hot states.     

Atomic absorption background of Ba in EXAFS analysis of BaFe12O19 nanoparticles

The approximate barium X‐ray atomic absorption in the energy region of L‐edges is reconstructed from the absorption spectrum of an aqueous solution of BaCl₂. The result is corroborated by comparison with pure atomic absorption spectra of neighbour elements Xe and Cs. The application of the atomic absorption signal as a proper EXAFS background is demonstrated and discussed in the analysis of Ba hexaferrite nanoparticles with a very weak structural signal. The essential gain is found in the decrease of uncertainty intervals of structural parameters and their correlations. A simple analytical model of the absorption background for the practical EXAFS analysis is demonstrated.     

Steady‐State and Time‐Resolved Emission Studies of 6‐Methoxy Quinoline

Abstract

Steady‐state absorption, fluorescence excitation, and emission spectra of 6‐methoxy quinoline (6‐MQ) were measured at room temperature in cyclohexane, dioxane, ethanol, acetonitrile, water, and water–dioxane solvents. Absorption spectra of cyclohexane, n‐hexane, and isopentane solutions show resolved vibronic structure at room temperature. However, the excitation spectrum of cyclohexane solution is structureless and is found to be emission wavelength dependent, indicating the formation of at least two distinct species in the ground state. Similar behavior was observed in dioxane and water–dioxane solutions. For all other solutions, the fluorescence excitation spectrum of 6‐MQ was found to be the same for different emissions. Emission of 6‐MQ in all solvents consisted of two bands with their maxima around 355 nm (I) and 430 nm (II), the actual positions and the relative intensities being dependent on the solvent used. The bands I and II were respectively attributed to normal and protonated/H‐bonded species of either ¹L_a or ¹L_b states or mixed (¹L_a/¹L_b) state of ππ* character. Fluorescence decay of this dye in all solvents monitored over each emission maximum showed biexponential behavior, and the analysis yielded two different lifetime components for each emission band. The short and long fluorescence decay components were respectively in the range of 0.30–3.00 ns and 18–20 ns. The observed emission characteristics coupled with the nature of the fluorescence polarization spectra and two different decay components for each emission suggest the existence of two different conformers having two different excited electronic states.     

X-ray magnetic circular dichroism sum rule correction for the light transition metals

Distinguishable L₂ and L₃ edges and a clear separation into j _3/2 and j _1/2 excitations are necessary for the application of L_2,3 edge X-ray magnetic circular dichroism (XMCD) sum rules, which provide element-specific information about spin and orbital magnetic moments. This separation is present for the heavy transition metals (TM), like Co and Ni, due to their large L_2,3 spin–orbit splitting. However, for the light TM, the 2p spin–orbit splitting is strongly reduced and quantum mechanical mixing of j _3/2 and j _1/2 excitations is present. This mixing reduces the observed XMCD related spin and magnetic dipole term contributions and prevents the direct application of XMCD spin sum rules. A large number of 2p?→?3d absorption spectra have been fitted nearly perfectly by a simple and phenomenological model, which takes into account lifetime effects and provides quantitative information about jj-mixing at the light TMs. On the basis of this mixing coefficient, sum rule correction factors have been determined. The proposed model results in renormalized magnetic projected XMCD spin moments, verified for different compounds of V, Cr, and Mn. A comparison with complementary methods gives consistent results. This or a similar fitting procedure and the estimated correction factors can be used in the future as a light element XMCD spin renormalization technique.     

Anomalies in resonant absorption line profiles of atoms with large hyperfine splitting

We examine a monochromatic absorption line in the velocity-nonselective excitation of atoms when the components of the hyperfine stricture of the electronic ground states are optically pumped. We show that the absorption lines possess unusual substructures for some values of the hyperfine splitting of the ground state (which exceed the Doppler absorption linewidth severalfold). These substructures in the absorption spectrum are most apparent if the hyperfine structure of the excited electronic state is taken into account. We calculate the absorption spectra of monochromatic light near the D ₁ and D ₂ lines of atomic rubidium ^85,87Rb. With real hyperfine splitting taken into account, the D ₁ and D ₂ lines are modeled by 4-and 6-level diagrams, respectively. Finally, we show that atomic rubidium vapor can be successfully used to observe the spectral features experimentally. Zh. éksp. Teor. Fiz. 111, 93–106 (January 1997)     

Influence of crystal structure,ligand environment and morphology on Co L‐edge XAS spectral characteristics in cobalt compounds

The electronic structure of a material plays an important role in its functionality for different applications which can be probed using synchrotron‐based spectroscopy techniques. Here, various cobalt‐based compounds, differing in crystal structure, ligands surrounding the central metal ion and morphology, have been studied by soft X‐ray absorption spectroscopy (XAS) at the Co L‐edge in order to measure the effect of these parameters on the electronic structure. A careful qualitative analysis of the spectral branching ratio and relative intensities of the L₃ and L₂ peaks provide useful insight into the electronic properties of compounds such as CoO/Co(OH)₂, CoCl₂.6H₂O/CoF₂.4H₂O, CoCl₂/CoF₂, Co₃O₄ (bulk/nano/micro). For further detailed analysis of the XAS spectra, quantitative analysis has been performed by fitting the spectral profile with simulated spectra for a number of cobalt compounds using crystal field atomic multiplet calculations.     

Microwave absorption spectra and the problem of the crystal field in tetragonal compounds HoBa2Cu3O x (x = 6.0, 6.3)

The resonance microwave absorption at wavelengths 871, 406, 305, and 118 μm in tetragonal layered perovskites HoBa₂Cu₃O _x (x = 6.0, 6.3) in pulsed magnetic fields up to 40 T has been detected. This absorption is caused by the electronic transitions between low lying levels of the Ho³⁺ ion in the crystal field. The positions and intensities of the basic resonance absorption lines for the crystal with x = 6.0 in the magnetic field oriented along the tetragonal axes are adequately described in terms of the tetragonal crystal field with known interaction parameters. To explain the weaker absorption lines, it is necessary to take into account the effect of the inhomogeneous orthorhombic and monoclinic components of the crystal field, which are due to disorder in the oxygen subsystem. This effect is more pronounced in the absorption spectra of the crystal with x = 6.3 for which the oxygen disorder is more pronounced.     

Electronic structure of strontium titanate

The spectra of three sets of optical functions for SrTiO₃ crystals are determined in a broad energy range of fundamental absorption. The calculations are carried out using the experimental reflectivity spectrum in the range 1–35 eV and two theoretical permittivity spectra in the ranges 0–30 eV and 0–14 eV. The special features of these spectra have been determined. The theoretical spectra of the optical functions are compared with the spectra determined using the experimental reflectivity spectrum.     

Thickness and angular dependence of the L‐edge X‐ray absorption of nickel thin films

We report on the near‐edge X‐ray absorption fine structure spectroscopy of the L₃ (2p_3/2) and L₂ (2p_1/2) edges for ferromagnetic pure nickel transition metal and show that the L_2,3 edge peak intensity and satellite feature at ~6 eV above the L₃ edge in nickel increase with increasing nickel film thickness both in the total electron yield and transmission modes. The absorption spectra of nickel metal, however, exhibit strong angular‐dependent effects when measured in total electron yield mode. In addition, we calculated the mean electron escape depth of the emitted electrons (λ_e), which was found for pure nickel metal to be λ_e=25 ± 2 Å. We point out the advantages of the total electron yield technique for the study of the L‐edge of 3d transition metals. Copyright © 2011 John Wiley & Sons, Ltd.     

Zeeman effect in the absorption spectrum of EuMg-nitrate crystals

Large single crystals of EuMg-nitrate were grown of optical quality. The polarized absorption spectra were investigated with high optical resolution at helium temperatures and in external magnetic fields up to 6.6T parallel as well as perpendicular to the trigonal axis. The energy, symmetry quantum number,g-factor, and Zeeman shift of the various crystal field levels were determined, as well as first order matrix elements of the crystal field energy. The second order Zeeman effect and the temperature independent van Vleck paramagnetic susceptibility of the ground level⁷ F ₀ were calculated from these constants, in excellent agreement with the spectroscopic and magnetic measurements.Project of the Sonderforschungsbereich Festkörperspektroskopie Darmstadt-Frankfurt/Main