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.     

Calculation of X-ray natural circular dichroism

A method is presented to calculate the natural circular dichroism recently discovered in the X-ray range (XNCD). The basic formula represents XNCD as an odd second-rank tensor and leads to a sum rule that relates XNCD to the mixing of odd and even orbitals in the ground state. A multiple-scattering theory of XNCD is presented, and calculated spectra for the L-edges of iodine in LiIO₃ compare favorably with the experiments. Received: 20 October 1997 / Revised: 18 February 1998 / Accepted: 31 March 1998     

Precision measurement of L X-ray wavelenghts and line widths for 74 ≦ Z ≦ 95 and their interpretation in terms of nuclear perturbations

Techniques for the precision measurement of x-ray lines using the two-crystal spectrometer are discussed, and then applied to measurement of the L x-ray spectra of the transuranic elements uranium, neptunium plutonium, and americium. In all, 52 emission lines and 4 L_III absorption edges were measured, all of them with higher precision than has heretofore been obtained and many of them for the first time. Using these data, the binding energies of the electrons for these atoms were computed. The L_II-L-_III level splitting was computed and compared with theory and it was found that current theory is not sufficient to account quantitatively for the observed data. Certain features of the emission line widths are discussed and given qualitative explanations in terms of the Coster-Kronig transition L_III-L_IIIM_V and hyperfine structure. The observed hyperfine structure is due to the large magnetic moment of Np²³⁷ which leads to increased widths of certain of the Np lines. It is believed that this is the first experimental observation of hyperfine structure in x-ray spectra.     

DasKLL- Auger-Spektrum von Chlor

TheKLL Auger spectrum of chlorine in CCl₄ has been investigated with an electrostatic energy analyzer. The ionization in theK shell of chlorine was caused by electron impact. Absolute energies and relative intensities of transitionsKL ₁ L ₁(¹ S,KL ₁ L _2,3(¹ P),KL ₁ L _2,3(³ P) andKL _2,3 L _2,3(¹ D) have been determined. Via the width of theK level of chlorine also experimental values of absolute transition probabilities have been calculated. While the relative intensities deviate strongly from theoretical values, the absolute transition probabilities agree better with theoretical values, especially with those calculated byRubenstein. — In an appendix the natural line widths of theKLL components of argon have been calculated theoretically.     

X-ray emission and photoluminescence spectroscopy of nanostructured silica with implanted copper ions

Quartz glass samples and compacted SiO₂ nanopowders have been studied by x-ray emission (CuL _{2, 3} transition 3d4s → 2p _{1/2, 3/2}) and photoluminescence spectroscopy following pulsed Cu⁺ ion implantation (energy, 30 keV; pulse current up to 0.5 A; pulse duration, 400 μs; irradiation doses, 10¹⁵, 10¹⁶, and 2 × 10¹⁷ cm^?2). It has been established that ion irradiation gives rise to the formation of glassy and compacted SiO₂ samples of nanosized metallic and oxide phases in the structure. An analysis of CuL x-ray emission spectra has shown that copper nanoparticles are thermodynamically metastable and chemically active because ion beam bombardment transfers them readily to the oxide form. This results from the radiation-stimulated fracture of regular Si-O-Si bonds in amorphous SiO₂ and the formation of defective Si-Si bonds, followed by capture of oxygen by copper atoms. The enhanced degree of oxidation of copper ions in SiO₂ nanostructured pellets can be reduced by coimplantation and thermal annealing. Optical spectroscopy studies suggest that, in glasses and SiO₂ nanostructured pellets, there exist metallic Cu _n ⁰ nanoclusters, which at low temperatures exhibit quantum-confined photoluminescence with a characteristic stepped excitation spectrum.     

Temperature dependence of the Ho L2‐edge XMCD spectra of Ho6Fe23

An X‐ray magnetic circular dichroism (XMCD) study performed at the Ho L_2,3‐edges in Ho₆Fe₂₃ as a function of temperature is presented. It is demonstrated that the anomalous temperature dependence of the Ho L₂‐edge XMCD signal is due to the magnetic contribution of Fe atoms. By contrast, the Ho L₃‐edge XMCD directly reflects the temperature dependence of the Ho magnetic moment. By combining the XMCD at both Ho L₂‐ and L₃‐edges, the possibility of determining the temperature dependence of the Fe magnetic moment is demonstrated. Then, both μ_Ho(T) and μ_Fe(T) have been determined by tuning only the absorption L‐edges of Ho. This result opens new possibilities of applying XMCD at these absorption edges to obtain quantitative element‐specific magnetic information that is not directly obtained by other experimental tools.     

X-ray magnetic circular dichroism at IrL2,3 edges in Fe100−x Ir x and Co100−x Ir x alloys: Magnetism of 5d electronic states

The formation of induced 5d magnetic moment on Ir in Fe_100−x Ir _x (x=3, 10 and 17) and Co_100−x Ir _x (x=5, 17, 25 and 32) alloys has been investigated by X-ray magnetic circular dichroism (XMCD) at Ir L_2,3 absorption edges. Sum rule analysis of the XMCD data show that the orbital moment of Ir is in the range of −0.071(2)μB to −0.030(1)μB in Fe-Ir alloys and −0.067(2)μB to 0.024(1)μB in Co-Ir alloys. We find that the total moment of Ir in Fe-Ir alloys is approximately 1/5 of the total 3d moment on Fe at all the three compositions. In contrast, the total moment on Ir in Co-Ir alloys varies between 1/6 to 1/16 of the 3d moment on cobalt. The observed trends of Ir moments and the role of interatomic exchange interactions in 5d moment formation are discussed.     

L-subshell fluorescence and coster-kronig yields atZ=92 and the decay of235Np

TheK andL x-rays emitted in the decay of²³⁵Np have been studied with high resolution Ge(HP) and Si(Li) detectors in coincidence, in order to obtain theL ₂ andL ₃ subshell fluorescence and Coster-Kronig yields atZ=92. The results are:v ₂=0.630±0.036, ω₂=0.560±0.033,f ₂₃=0.147±0.010, and ω₃=0.481±0.029. Results for theL ₁ subshell were derived from singles spectra, by assuming a value off ₁₃ of 0.67, and are:v ₁=0.54±0.04, ω₁=0.21±0.04, andf ₁₂<0.07. With the same assumption, theL ₁/K electron capture ratio and decay energyQ _ec for²³⁵Np were found to be 29.0±3.6 and 123.6±0.7 keV, respectively. RelativeL x-ray intensities forZ=92 also were measured and are compared with the theory of Scofield and with recent diffraction experiments. Electron ejection from theL shell during²³⁵Np decay has been studied byL x-ray-L x-ray coincidences and found to occur with a probability of (1.3±1.0) × 10^?4 per disintegration.     

Inner shell excitation and ionization of the monohalobenzenes

Energy loss (excitation) spectra of the gaseous monohalobenzenes, C ₆H ₅X (X = F, Cl, Br and I), were obtained by fast electron impact in the regions of the respective carbon 1 s, chlorine 2 p and 2 s, bromine 3 d and iodine 4 d edges. Gas phase X-ray PES measurements of the binding energies of these levels are also reported. Structure observed below the ionization limits has been interpreted with the aid of term values derived from the two sets of measurements.     

The crystal structure of bis-(L-threonine) copper(II).H2O

The crystal structure of bis-(L-threonine) copper (II).H₂O, Cu (C₄H₈NO₃)₂.H₂O has been determined by heavy atom and Fourier methods and refined by least-squares using visually estimated three-dimensional x-ray data of 893 reflections. The blue crystals are monoclinic, space groupP2₁ withα=11·02,b=4·90,c=11·16Å andβ=93·5°,Z=2. The finalR is 0·10. Coordination of copper is distorted square pyramidal with ligands intrans configuration. The conformation of one of the aminoacid ligand is identical with L_s-Threonine while the other has a conformation with torsional angleχ ^{1, 2}=?74(1)°.     

L-subshell ionization cross sections for Au,Pb, and Bi by 60–600 keV electron impact

TheL ₁-,L ₂- andL ₃-subshell ionization cross sections of Au, Pb and Bi by 60–600 keV electron impact have been determined from the measured x-ray production cross sections. The experimental results have been compared with the relativistic PWBA, the Binary Encounter Approximation (BEA) and the Kolbenstvedt Approximation (KA) calculations. These calculations, especially the relativistic PWBA, give good agreement with the experimental data for theL ₃-subshell. For theL ₁- andL ₂-subshells there are some deviations and these can explain the disagreement observed for theL _α/L _χ- andL _γ/L _χ intensity ratios.     

The origin of an elastic line in the L 3 x-ray emission spectrum of metallic manganese

A strong elastic line has been found in the L ₃ x-ray emission spectrum of metallic Mn. To unravel its physical nature, the ground-state properties of α-Mn were studied in comparison with those of 3d metals (Cr and Fe in bcc structure), as well as the properties of the L ₃ absorption final states of these three transition metals. To model the electronic structure, LSDA band-structure calculations of Cr, Mn, and Fe were carried out, and L ₃ absorption spectra of these metals were computed in the atomic approximation. A joint analysis of the properties of the ground state and of the final absorption state excited by an x-ray-produced core hole suggests that the elastic line in the α-Mn spectra should be assigned to the specific character of the absorption final-state multiplet.     

Analysis of the structure and magnetic properties of an interface in multilayered (Fe/Si) N nanostructures with the surface-sensitive XMCD method

The structural and magnetic properties of (Fe/Si) _N nanostructures obtained by successive deposition on the SiO₂/Si(100) surface at a temperature of the substrate of 300 K have been studied. The thicknesses of all Fe and Si layers have been determined by transmission electron microscopy measurements. The magnetic properties have been studied by the X-ray magnetic circular dichroism (XMCD) method near the Fe L _{3, 2} absorption edges. The orbital (m _l ) and spin (m _S ) contributions to the total magnetic moment of iron have been separated. The thicknesses of magnetic and nonmagnetic iron silicide on the Si/Fe and Fe/Si interfaces have been determined with the surface sensitivity of the XMCD method and the model of the interface between the nonmagnetic and weakened magnetic phases.     

Chromium doped manganites: evidence for electronically phase-separated systems

X-ray absorption spectroscopy (XAS) and soft-X-ray magnetic circular dichroism (SXMCD) at the Mn L_2,3-, Cr L_2,3- and O-K edges of Sm_0.5Ca_0.5Mn_1-xCr_xO₃ () bulk polycrystalline samples have been performed at T=20 K below the ferromagnetic Curie temperature. We show the existence of a magnetic sublattice on each of the probed cations. Considering an electronically phase-separated system, results are compared with magnetization and resistivity measurements and a tentative correlation with magnetoresistance properties on such doped compounds is discussed. Received 7 January 2000     

Electron capture decay of181W: L subshell fluorescence and Coster-Kronig yields in Ta

The TaL x-ray spectrum from the 140 d electron capture decay of¹⁸¹W was studied with high resolution semiconductor detectors and fast coincidence techniques. Measurement ofL x-ray-K x-ray andL x-ray—γ-ray coincidence rates yielded the followingL shell fluorescence and Coster-Kronig yields and radiative decay branching ratios: ω₂=0.250±0.013, ω₃=0.228±0.013 (corrected for angular correlation effects),f ₂₃=0.180±0.007,v ₁=O.218±0.016,s ₃=0.205+-0.010,s ₂=0.215+-0.010,f ₁₃+f ₁₂ f ₂₃=0.36+-0.02, and ω₁+f ₁₂ω₂=0.14±0.02, from which upper limits were obtained off ₁₃<0.36 and ω₁<0.14.     

TheL 1 subshell fluorescence and Coster-Kronig yields forZ=96

High resolution Si(Li)-Si(Li) and Si(Li)-Ge(Li) detector arrangements have been used to studyL x-ray spectra in coincidence withL-shell internal conversion electrons, gamma rays, andK x-rays emitted in²⁴⁹Cf decay. TheL ₁ subshell fluorescence and Coster-Kronig yields determined forZ=96 from these spectra are ω₁=0.25±0.06,f ₁₂≦0.10, andf ₁₃=0.69 ±0.08. The presence of theL ₁-L ₃ radiative transition is also further confirmed in this work.     

Temperature dependence of the interband absorption edge of copper

The optical absorption spectra of bulk copper measured by Pells and Shiga have been fitted by means of a model calculation in the interband edge region. The optical gap and the dipole matrix element of the L₃-L'₂(E_F) transition have been determined as a function of the temperature and a comparison with similar results obtained for silver and gold is presented.     

L 2 andL 3 subshell fluorescence and Coster-Kronig yields forZ=96 from the decay of249Cf

TheL ₂ andL ₃ subshell fluorescence and Coster-Kronig yields forZ=96 have been measured by theK-L x-ray coincidence method with high resolution Si(Li) and Ge(Li) x-ray detectors. The results ν₂=0.650±0.036, ω₂=0.552±0.032, ω₃=0.515±0.034, andf ₂₃=0.188±0.019 agree fairly well with theory. New measurements of theL x-ray singles spectra of²⁴⁴Cm and²³⁸Pu with a high resolution Si(Li) detector permitted reevaluation of the values of ω₂ andf ₂₃ forZ=92 and 94 reported by Byrneet al. The revised values atZ=92 are ω₂=0.529±0.035 andf ₂₃=0.23±0.12, and atZ=94, ω₂=0.523±0.023 andf ₂₃=0.24±0.08. These revised values are in much better agreement with theory and with current experiments.     

Atomic level energies in hafnium

The photo electron method for obtaining electron binding energies is described. The method has been applied to theL _I,L _II, andL _III levels in hafnium. The HfK α₁ and HfK α₂ x-ray emission lines have been measured with a crystal spectrometer. The electron binding energies of theK toO _III shells in hafnium are obtained from photo electron and x-ray emission data. A new type of Moseley diagram is described and is demonstrated for theK andL levels in theZ region 57 to 75. From these diagrams accurate binding energies for the element Pm (Z=61) have been obtained by interpolation.     

First-principles study of the sulfur K and L2,3 edges of transition metal disulfide monolayers,MS2 (M=Mo,W and Re)

By means of the energy loss near edge structure (ELNES) analysis, the electronic structures of layered transition metal disulfides were studied. In the framework of full potential linearized augmented plane wave method, ELNES spectra of sulfur K and L_2,3 edges of layered MoS₂, WS₂ and ReS₂ have been calculated at magic angle conditions, and compared with those of bulks and the only existing experimental fine structure. Compared to the bulks, the energy differences between the main peaks in sulfur K and L_2,3 edges of monolayers decrease due to the slightly larger bond lengths that it can be used as a fingerprint for monolayers. Sulfur K edges in monolayers include some main features originated from electron transition to p_z (π) and p_x+p_y (σ) states and their hybridization. The overall dispersions of the sulfur L_2,3 edges in all cases are similar to the d-symmetry density of states. The first two features in L_2,3 edge of bulks and monolayers can be attributed to electron transition of sulfur 2p to the both unoccupied 3s-like states of sulfur and 4d states of transition metal atoms. Due to the considerable amount of s states at the energy position of a shoulder like structure in L_2,3 edge of both bulks and monolayers, these structures can be assigned to the sulfur 2p electron transition to unoccupied sulfur 3s states. The other features at higher energies are due to the transition of sulfur 2p electrons to the d-symmetry states of sulfur. In addition, due to the considerable energy band gaps, it seems that the use of core–hole approximation is essential for accurate reproduction of ELNES features of transition metal disulfides.