Article

Low-energy d–d excitations in MnO studied by resonant X-ray fluorescence spectroscopy

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Abstract

We measured the Mn {ital L}{sub {alpha},{beta}} x-ray fluorescence spectra of MnO excited by selected photon energies near the {ital L}{sub 2,3} absorption edges. The resulting resonant inelastic x-ray scattering spectra probe low-lying electronic excited states, due to {ital dd} and charge-transfer excitations. Using a two-step model and a purely atomic approximation, we reproduce both energies and varying intensities of {ital dd} excitations relative to the electronic recombination peak. Our results show that strongly varying line shapes in resonant x-ray emission need not be due to channel interference effects. {copyright} {ital 1996 The American Physical Society.}

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... The RIXS cross section is described by the Kramers-Heisenberg equation [70,71]. The possibilities in the study of correlated systems by means of RIXS go from Coulomb interactions on high energy scales over charge transfer excitations to lower excitation energies, especially regarding optically inaccessible bands, such as dd transitions [72,73,74]. ...
... The 2p transition metal (TM) oxides belong to the most interesting materials for RIXS since the interpretation of the TM 2p → 3d → 2p RIXS data is controversial and leads to the limitations of both the band model and the Anderson impurity model. Butorin et al. reported RIXS spectra of MnO to be due to low-lying dd and charge transfer excitations, which can be described by an atomic multiplet calculation [72]. Furthermore the Mn 2p → 3d → 2p RIXS spectra of La 0.5 Ca 0.5 MnO 3 have been interpreted within a charge transfer multiplet approach [189]. ...
... We present here, for the first time a detailed x-ray spectroscopic study of La 1−x Ba x MnO 3 (0.2 ≤ x ≤ 0.55) and the mentioned A-and B-site doped manganites using of a number of complementary spectroscopic techniques, namely XPS, XES, XAS and RIXS. The results are analyzed along the questions stated above, compared with measurements on undoped LaMnO 3 and discussed in the light of both available band structure calculations [186,187,188,193], as well as cluster model calculations [72,104]. [181]. ...
... The figure does not show energy gains (anti-Stokes Raman), but in principle this is possible if the system before the X-ray excitation is already in an excited state. In the earlier days of RIXS, Butorin showed advantages of the RIXS technique, such as clear band showing allowed dd excitations compared to EELS and UV-VIS absorption spectra where dd transitions often appear as weak structures [37]. RIXS spectroscopy gained increased interest with the advances on the brilliance of synchrotrons and X-ray free-electron lasers as well as advances in more efficient detectors. ...
... In the early days, soft X-ray RIXS with moderate resolution was performed a lot on standard compounds such as manganese oxide (MnO) [37,40], cobalt oxide (CoO) [40], and nickel oxide (NiO) [40][41][42][43]. Interestingly, in the soft X-ray regime when the SAXES spectrometer [44] became available with a much better energy resolution, and the general improved resolution was established on MnO [45], CoO [46], and NiO [47], high-energy resolution RIXS was focused mostly on the Cu L-edge of high-temperature superconductors (HT-SC) and representative HT-SC model compounds and as well with some measurements at the oxygen K-edge to gain understanding of superconducting properties related to phonon, magnon, and electronic structure, for example, Refs [48][49][50][51][52] for Cu L-edge and Refs. ...
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... In RIXS, the resonant absorption of a photon by a core electron leads to a radiative de-excitation which leaves the system either in its ground or in a neutral excited state [29,30]. Thus dd excitations yield characteristic spectral losses dis- persing with incident photon energy (Raman regime) and information is projected on the cation site [31,32]. Nowadays, high energy resolutions (25 meV) can be achieved by RIXS spectrometers in the soft and hard x-ray regime [33][34][35][36][37][38]. ...
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... To compare the intensity change, the spectra are normalized to one, which can demonstrate the number of scattering photons or difficulty in scattering these photons. The elastic peaks located at 0 eV energy loss are attributed to the refilling of the 2p core holes by the incident electron, the yellow area represents the d-d excitation between 1 and 5 eV, which is caused by the refilling of 2p core holes by electrons occupying the 3d valence or conduction band, and the blue region means the charge transfer (CT) excitation [47][48][49] . First, the CT intensity is related to the extent of hybridization of the Mn-O bond. ...
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... The energy difference between incident and emitted photons, i.e., the energy transfer (Ω-ω), corresponds to the energy of the fundamental process that is probed, e.g., a core, valence or charge-transfer excitation. [10][11][12][13] An important advantage of RIXS is that the lifetime broadening in the energy transfer direction only depends on the lifetime of the final state, not the lifetime of the short-lived core hole in the intermediate state. [14][15][16][17] This makes it possible to achieve high-resolution energy transfer spectra while still keeping the advantages of the X-ray probe. ...
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... 7,15,16 Combining bright light sources with high resolution X-ray emission spectrometers approaching 10 meV resolution 17,18 has allowed the dispersion of many elementary low-energy excitations to be resolved, including spin excitations and magnons, 19,20 and even vibrational manifold mapping onto nuclear wavepackets. 21 However a more modest X-ray emission resolution of about 1 eV is often sufficient to access orbital excitations, such as d-d transitions, 22 and charge transfer excitations. 23 L-edge RIXS can yield more rich chemical insights into crystal field, charge transfer, and valency than XAS alone, even at moderate resolution. ...
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... Here, it has been seen that the increase of temperature increases the concentration of Mn atoms that can be discussed in further subsections. [20]. Similar RIXS origin is predictable for the Ca modified LaMnO 3 system due to the considerable degree of localization of Mn 3d states. ...
... Here, it has been seen that the increase of temperature increases the concentration of Mn atoms that can be discussed in further subsections. [20]. Similar RIXS origin is predictable for the Ca modified LaMnO 3 system due to the considerable degree of localization of Mn 3d states. ...
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Spectroscopic probes of the electronic structure of transition metal-containing materials are invaluable to the design of new molecular catalysts and magnetic systems. Herein, we show that 2p3d resonant inelastic X-ray scattering (RIXS) can be used to observe both spin-allowed and (in the V(III) case) spin-forbidden d-d excitation energies in molecular vanadium complexes. The spin-allowed d-d excitation energies determined by 2p3d RIXS are in good agreement with available optical data. In V(acac)3, a previously undetected spin-forbidden singlet state has been observed. The presence of this feature provides a ligand-field independent signature of V(III). It is also shown that d-d excitations may be obtained for porphyrin complexes. This is generally prohibitive using optical approaches due to intense porphyrin π-to-π* transitions. In addition, the intensities of charge-transfer features in 2p3d RIXS spectroscopy are shown to be a clear indication of metal-ligand covalency. The utility of 2p3d RIXS for future studies of complex inorganic systems is highlighted.
Article
Resonant Inelastic X-ray Scattering (RIXS) is a powerful probe for studying electronic excitations in materials. Standard high energy RIXS measurements do not measure the polarization of the scattered x-rays, which is unfortunate since it carries information about the nature and symmetry of the excitations involved in the scattering process. Here we report the fabrication of thin Si-based polarization analyzers with a double-concave toroidal surface, useful for L-edge RIXS studies in heavier atoms such as the 5-d transition metals.
Article
For first-row transition metals, high-resolution 3d electronic structure information can be obtained using resonant inelastic X-ray scattering (RIXS). In the hard X-ray region, a K pre-edge (1s → 3d) excitation can be followed by monitoring the dipoleallowed Kα (2p → 1s) or Kβ (3p → 1s) emission, processes labeled 1s2p or 1s3p RIXS. Here the restricted active space (RAS) approach, which is a molecular orbital method, is used for the first time to study hard X-ray RIXS processes. This is achieved by including the two sets of core orbitals in different partitions of the active space. Transition intensities are calculated using both first- and second-order expansions of the wave vector, including, but not limited to, electric dipoles and quadrupoles. The accuracy of the approach is tested for 1s2p RIXS of iron hexacyanides [Fe(CN)6]n- in ferrous and ferric oxidation states. RAS simulations accurately describe the multiplet structures and the role of 2p and 3d spin-orbit coupling on energies and selection rules. Compared to experiment, relative energies of the two [Fe(CN)6]3- resonances deviate by 0.2 eV in both incident energy and energy transfer directions and multiplet splittings in [Fe(CN)6]4- are reproduced within 0.1 eV. These values are similar to what can be expected for valence excitations. The development opens up for the modeling of hard X-ray scattering processes for both solution catalysts and enzymatic systems.
Article
This review is comprised of four main sections and is organized as follows: the first part of this paper (sections 1 and 2) provides the reader a motivation for the need to address the most pressing energy-related concerns we face today and the available transformative technologies for efficiently harvesting solar energy. The second part of this paper (sections 3 and 4) presents a survey comprised of mainly theoretical studies on the correlation between phase, morphology, and photoelectric properties in TiO2. Significant emphasis has been given herein to detail (on the basis of theoretical prediction) the evolution in the band structure of TiO2 as a function of phase transition by virtue of introduction of impurity elements into the host oxide lattice and manipulation of its morphological properties. The third part of this paper (section 5) presents a detailed summary on recent experimental studies on the alteration of the optical and photoelectric properties of TiO2 via the manipulation of compositional and structural properties of the oxide that enables alternative pathways for the photoelectrons to propagate to reduce the probability for electron−hole recombination. Section 5 is a follow-up to the theoretical work presented on the same in section 4. Finally, the fourth part of this review details the principles of synchrotron-based X-ray spectroscopy techniques and their importance/contribution to the field of materials research and related energy applications.
Chapter
Soft x-ray absorption and emission are important techniques for the study of new designer materials. The methods coupled with high-brightness third generation synchrotron light sources are being used to study cutting edge designer materials in the form of nanoparticles and buried thin films. The electronic and magnetic properties are being elucidated at specific chemical and atomic sites with rapidly developing technology that allows these materials to be studied in the presence of magnetic fields of magnitude of up to 5T. This paper provides a few examples that illustrate the rapid development of this field.
Article
The electronic structure of MnS was studied using resonant inelastic soft X-ray scattering (RIXS) from 2p(6)3d(5)-> 2p(5)3d(6)-> 2p(6)3d(5) channel. The d-d excitation and charge-transfer ( CT) excitation were obtained which were resonant at excitation energies corresponding to L absorption edge and the satellites, respectively. RIXS and XAS spectra were simulated by Hartree-Fock method under atomic SO3 and cubic O-h symmetries. The crystal field 10Dq value was evaluated to 0.80 eV-0.85 eV. The stronger CT excitation of MnS than that of MnO originates from its narrower bandwidth.
Article
A new multi-atom resonant photoemission (MARPE) effect between core levels of neighboring atoms in multi-element samples has been observed. Experimental evidence for the effect and several possible applications are considered. In the metal oxides MnO, Fe2O3, and La0.7Sr0.3MnO3, we have observed a significant enhancement in the core-level photoelectron peak intensity associated with one element in the sample (e.g. O1s) when the excitation energy is tuned through an energetically deeper absorption edge of a second element (e.g. Mn2p or Fe2p or La3d). The effects observed are up to a 105% increase in peak intensity, and a 29% increase in energy-integrated intensity. A generalization of the theory of single-atom resonant photoemission has also been applied to MARPE, and is found to be in semiquantitative agreement with our experimental results.
Article
Two manganese porphyrin complexes, manganese tetraphenylporphyrin chloride (MnTPP-Cl) and manganese octaethylporphyrin chloride (MnOEP-Cl), exhibit distinctive spectral features of metal-to-ligand charge-transfer (MLCT) when dissolved in dichloromethane, characterized by resonant inelastic X-ray scattering at the Mn L-edge and N K-edge. The metal-ligand orbital mixing that mediates the MLCT is analyzed with the help of density functional theory/restricted open-shell configuration interaction singles calculations. On the basis of experimental and theoretical analyses, the distinctive MLCT is argued to originate from alteration of the porphyrin outer ligands: phenyl groups in MnTPP-Cl and ethyl groups in MnOEP-Cl.
Article
Growing environmental concerns have renewed the interest for light induced catalytic reactions to synthesize cleaner chemical fuels from syngas. This, however, requires a sound understanding for the dynamics taking place at molecular level as a result of light – matter interaction. We present herein the principles of soft X-ray resonant emission spectroscopy (RXES) and resonant inelastic scattering (RIXS) and the importance of these spectroscopic techniques in materials science in light of their unique ability to emanate characteristic fingerprints on the geometric structure, chemical bonding charge and spin states in addition to chemical sensitivity. The addition of in situ/operando RXES and RIXS capability offers new opportunities to project important material properties and functionalities under conditions nearly identical to the operational modes.
Chapter
Resonant inelastic X-ray scattering (RIXS) measurements at the actinide 5d threshold provide an opportunity to study elementary excitations in actinide systems in detail. It has turned out that the technique is very sensitive to the valency and the chemical state of actinide in contrast to X-ray absorption spectroscopy, being hampered by the substantial smearing of spectral structures due to a large core-hole lifetime broadening. In this situation, the virtually unlimited resolution (defined by the response function of the instrument) of the RIXS technique and its ability to enhance transitions to low-lying excited states are especially useful. RIXS spectroscopy provides good signatures in terms of new distinct transitions, representing electronic excitations within the 5f shell and having a characteristic profile. This helps to distinguish between actinide species with different oxidation states, especially in the case when one of the species has a much lower concentration than another. Experimental data for systems of light actinides, such as U, Np, and Pu, are presented and discussed along with the results of model calculations.
Article
The local electronic structures of Zn 0.97Mn 0.03O, Zn 0.67Mn 0.33O thin films prepared by a molecular beam epitaxy (MBE) at 200°C were investigated by soft X-ray fluorescence spectrometer of beamline 8.0.1 of the Advanced Light Source (ALS). The special interest can be given to find the relationship between the electronic structure of Mn and magnetic properties of our samples. Analysis of the integral intensity ratio of Mn L 2 to L 3 emission lines (I(L 2)/I(L 3)) from resonant and nonresonant Mn L 2,3 X-ray emission spectra (XES) indicates that ferromagnetism (FM) is related to the free d charge carriers in the film. For ferromagnetic Zn 0.97Mn 0.03O sample, the majority of Mn atoms are incorporated at Zn substitutional sites and the film shows strong Coster-Kronig (C-K) transitions due to a large amount of free charge carriers available around Mn atoms. Both non-localized d charge carriers as itinerant electrons and 4s electrons from interstitial Mn obtained by Ruderman-Kittel-Kasuya-Yosid (RKKY) calculations can induce the ferromagnetic exchange interaction. However, the disappearance of FM in Zn 0.67Mn 0.33O sample can be explained in terms of the existence of MnO clusters leading to a reduction in the number of free charge carriers.
We have studied resonant V Lα-fluorescence spectra of vanadium oxides with V in several different oxidation states. The spectra are dominated by the O 2p-contribution centered at about 6 eV below the top of the valence band (VB-top). The V 3d-contribution, found close to the VB-top, increases with decreasing valency of the vanadium atoms. Resonant inelastic (Raman) x-ray scattering is fairly weak in these compounds and overlaps with the ordinary fluorescence spectrum. Large spectral changes of V Lα-fluorescence in the metal-insulator transition of V2O3 have been observed.
Article
The inelastic scattering of X-rays is becoming a powerful alternative to better established techniques, based on neutrons or low energy photons, for the study of low- and medium-energy excitations in solids. When performed in the soft range the resonant inelastic X-ray scattering (RIXS) is ideal for strongly correlated electron systems based on 3d transition metals. The remarkable evolution of Cu L3 RIXS has been boosted by the steady improvement of experimental energy resolution, and by the fortunate fact that cuprates give intense and richly featured spectra. Over the last 8 years several key results were obtained using the AXES (ESRF) and the SAXES (SLS) spectrometers. This initial success is now supporting several new projects for soft X-ray RIXS worldwide. We briefly present here the case of spin excitation dispersion in insulating and superconducting cuprates and the first RIXS spectra with partial polarization analysis of the scattered photons.
Article
Resonant soft X-ray emission spectra of NiAs-type MnTe have been measured near the Mn 2p threshold using synchrotron radiation. The spectra consist of two structures due to elastic and inelastic scatterings and their relative intensity depends strongly on the excitation photon energy. The inelastic scattering peak around 3–4 eV below the elastic scattering peak can be attributed to d-d* excitation. In order to interpret the resonant behaviour, we have calculated the soft X-ray emission spectra of NiAs-type MnTe on the basis of a cluster model including electron-electron interactions and p-d hybridization. The theoretical results reproduce the basic structures of the experimental spectra, through we notice a difference in their relative intensities.
Article
Full-text available
We discuss methods for ab initio calculations of the parameters in the Anderson model. First, we present a very simple method for calculating the appropriate combination of hopping matrix elements needed in the impurity Anderson model. For a substitutional impurity, we show that to a good approximation it is sufficient to know the potential of the impurity atom and the local density of states of the unperturbed host. Calculations are performed for Mn substituting Cd in CdTe. As expected, the Mn 3d orbitals have a strong coupling to the Te 5p-derived valence band, but there is also a strong coupling to the conduction band. The dependence of the hopping matrix elements on the Mn configuration is studied. While there is a strong dependence on the Mn net charge, we find that the creation of, e.g., a core hole has a fairly small effect on the matrix elements, provided that the 3d occupancy is allowed to relax. Second, the Coulomb integrals between two Mn 3d orbitals and between a 3d orbital and a core orbital are calculated. The renormalization of these quantities due to the radial relaxation of the Mn 3d, 4s, and 4p orbitals, and due to charge-transfer effects, are analyzed in detail. Because of the nonmetallic character of CdTe, a change in the number of Mn 3d electrons is only partly screened by a charge transfer to the Mn 4s and 4p orbitals. Because of the moderate size of the band gap, this screening is, nevertheless, important. The radial relaxation of the Mn 3d, 4s, and 4p wave functions is also important. The relaxation of the neighboring atoms plays a rather small role. Results for the photoemission spectra are calculated including multiplet effects. The results are found to be in rather good agreement with experiment.
Article
Full-text available
Oxygen K and manganese L2,3 edges electron-energy-loss spectra have been measured for a series of manganese oxides with different nominal oxidation states for the cation (MnO, Mn3O4, Mn2O3, MnO2, and BaMnO4). Spectra have been processed for quantitative elemental analysis and for the evaluation of manganese L2,3 white-line intensity ratios and normalized total intensities. Prepeaks on the oxygen K edge are sensitive to the nature of local chemical bonds and their detailed substructures are interpreted with the support of a molecular-orbital picture. As for the cation point of view, the analysis of the white-line intensities provides a satisfactory measurement of the effective 3d orbital occupancy as well as a hint of the local magnetic properties. The present study also confirms that the hybridization between oxygen 2p and manganese 3d orbitals plays an important role when considering the electronic structures of these oxides.
Article
Full-text available
A propagating excitation with a velocity of sound of 3200±100 m/s is measured in H2O at 294 K between 4 and 14 nm-1, using inelastic x-ray scattering with 5 meV energy resolution. The existence of fast sound is therefore demonstrated in an energy-momentum region much wider than that of previous neutron measurements on D2O. The equivalence of the fast sound velocity in H2O and in D2O rules out models where this mode propagates only on the hydrogen network. These results show the ability of inelastic x-ray scattering to study the collective dynamics of liquids.
Article
The Ce M4,5 X-ray absorption and resonant Ce Mα,β (M4,5→N6,7) X-ray emission was measured for CeF3 and CeO2 using monochromatic synchrotron radiation. For CeF3, the recombination peak is dominant in the emission spectra recorded at different excitation energies near the Ce 3d threshold, thus indicating the highly ionic character of this compound and strong 4f localization. The CeO2 emission spectra indicate a partial but significant degree of localization for states in the main peaks of the Ce 3d absorption edge and a delocalized nature for states in the satellites which can be associated with the 4f admixture to the continuum. Our analysis suggests that CeO2 is a covalent compound but not one with a mixed Ce valency.
Article
We report the observation of strong optically forbidden 3d → 3d transitions in the electron-energy-loss spectra of vacuum-cleaved MnO(100). A striking sensitivity of the relative amplitudes of the various 3d → 3d multiplet transitions to the interference from other excitation channels is observed for the first time in the transition-metal oxides. A model reflecting the electron exchange nature of the 3d → 3d transition in MnO, similar to that used to describe the 4f → 4f transitions in rare-earth metals, is proposed to explain the large amplitudes of the transitions. However, unlike the case of the rare-earth metals, the energy dependence of the amplitude of the transitions cannot be explained simply in terms of a resonant electron emission process.
Article
We report strong magnetic linear dichroism at the Fe [ital L][sub 2,3] edge of the antiferromagnet Fe[sub 2]O[sub 3] (hematite). The relative difference in absorption for light polarized parallel and perpendicular to the magnetic moment is as high as 40% at the Fe [ital L][sub 2] edge. The spectra are in excellent agreement with calculations of magnetic dichroism for Fe[sup 3+] (3[ital d][sup 5]). The magnetic origin of this dichroism is demonstrated by the Morin transition at [approx][minus]10 [degree]C, where the moments in Fe[sub 2]O[sub 3] rotate by 90[degree]. Magnetic linear dichroism may be applied to measure the spin orientation in thin films and multilayers, and to image magnetic domains.
Article
Inconsistencies in accepted values (in x units) of x-ray reference lines have recently been demonstrated, although all are supposedly based on "good" calcite crystals. Factors supporting the selection of the W Kα1 line as the X-Ray Wavelength Standard are critically discussed. A review is given of the experimental measurements which are used to establish the wavelength of this line on an absolute angstrom basis. Its value is λ W Kα1=(0.2090100±5 ppm) Å. This may be used to define a new unit, denoted by Å*, such that the W Kα1 wavelength is exactly 0.2090100 Å*; hence 1Å*=1ű5 ppm. The wavelengths of the Ag Kα1, Mo Kα1, Cu Kα1, and the Cr Kα2 have been established as secondary standards with probable error of approximately one part per million. Sixty-one additional x-ray lines have been used as reference values in a comprehensive review and reevaluation of more than 2700 emission and absorption wavelengths. The recommended wavelength values are listed in Å* units together with probable errors; corresponding energies are given in keV. A second table lists the wavelengths in numerical order, and likewise includes their energies in keV.
Conference Paper
This report contains papers on the following topics relating to x-ray and inner-shell processes: theory; instrumentation; x-ray lasers; x-ray microscopy; ion-cooler rings; photoionization; inner-shell ionization; inner-shell transitions; special spectroscopies; alignment and polarization; x-ray spectroscopy of solids; molecular studies; and biological studies. (LSP)
Article
We present a theoretical formulation of resonant inelastic x-ray scattering (RIXS) of molecules with an element of symmetry, and study the consequences of this formulation both with respect to short and long lifetimes of the intermediate core-excited scattering states and with respect to localized versus delocalized descriptions for these states. Strong polarization and excitation energy dependencies are predicted. The ‘‘core-hole localization’’ problem is explored also from the point of view of diffractional scattering of an x-ray photon showing that x-ray photons distinguish core holes in the short-wavelength limit only. By analyzing the properties of the x-ray scattering tensor, strict selection rules for the RIXS process are derived. The local decomposition model (one-center approximation) frequently employed in ordinary x-ray emission is generalized for the calculation of intensities and polarization dependence of the RIXS process, assuming either localized or delocalized core holes. It is argued that Stokes doubling should be a commonly observable feature in RIXS spectra. By means of model calculations we demonstrate the crucial role of RIXS selection rules, of channel interferences, of Stokes doubling, and of the form of the excitation frequency function for the appearance of RIXS spectra.
Article
Both the interpretation of atomic spectra and the application of atomic spectroscopy to current problems in astrophysics, laser physics, and thermonuclear plasmas require a thorough knowledge of the Slater-Condon theory of atomic structure and spectra. This book gathers together aspects of the theory that are widely scattered in the literature and augments them to produce a coherent set of closed-form equations suitable both for computer calculations on cases of arbitrary complexity and for hand calculations for very simple cases.
Article
Soft x‐ray emission spectroscopy is a common tool for the study of the electronic structure of molecules and solids. However, the interpretation of spectra is sometimes made difficult by overlaying lines due to satellite transitions or close‐lying core holes. Also, irrelevant inner core transitions may accidentally fall in the wavelength region under study. These problems, which often arise for spectra excited with electrons or broadband photon sources can be removed by using monochromatized synchrotron radiation. In addition, one achieves other advantages as well, such as the ability to study resonant behavior. Another important aspect is the softness of this excitation agent, which allows chemically fragile compounds to be investigated. In this work we demonstrate the feasibility of using monochromatized synchrotron radiation to excite soft x‐ray spectra. We also show new results which have been accomplished as a result of the selectivity of the excitation. The work has been carried out using the Flipper I wiggler beamline at HASYLAB in Hamburg using a new grazing incidence instrument designed specifically for this experiment. The photon flux at the Flipper I station (typically 5×10<sup>1</sup><sup>2</sup> photons per second on the sample with a 1% bandpass) is enough to allow soft x‐ray fluorescence spectra to be recorded at relatively high resolution and within reasonable accumulation times (typically, the spectra presented in this work were recorded in 30 min). The spectrometer is based on a new concept which allows the instrument to be quite small, still covering a large wavelength range (10–250 Å). The basic idea involves the use of several fixed mounted gratings and a large two‐dimensional detector. The grating arrangement provides simple mounting within a limited space and, in particular, large spectral range. The detector can be moved in a three‐axis coordinate system in order to cover the- different Rowland curves defined by the different gratings. The arrangement permits the use of gratings with different radii, which further facilitate the achievement of optimum performance over a large range. Two‐dimensional detection is used to allow a large solid angle, without suffering from loss of resolution due to imaging errors. The detector is based on five 2‐in. MCPs with resistive anode read out. The sensitivity of the detector, which is normally very low for soft x rays, especially at grazing angles, is enhanced by CsI coating and by using an entrance electrode.
Article
The analysis of x-ray absorption spectra to determine the electronic and magnetic structure of transition metal compounds is discussed. The models to describe the ground state of transition metal compounds (single-particle, impurity, crystal field) are introduced. Some basic aspects of the interaction of x-rays with matter are recapitulated and the description of x-ray absorption is separated into single-particle models for the 1s edges and multiplet models for the 2p edges. Magnetic circular dichroism is introduced and the six Thole sum rules are discussed. The complications and experimental problems of the sum rules are outlined. The last section briefly mentions some aspects of resonance studies, for which a detailed knowledge of x-ray absorption is crucial.
Article
It is shown that fluorescence yield (FY) detection does not measure a pure X-ray absorption spectrum if multiplet effects are important. The reason is that while the Auger decay is approximately constant, the fluorescence decay shows a strong dependence on the final state. With a coherent description of the X-ray absorbtion and X-ray emission processes, the observed differences between TEY and FY detected nickel 2p edges can be reproduced accurately. In the, approximate, incoherent description the FY spectral shape identifies with the X-ray absorption spectrum multiplied with its state dependent fluorescence decay. Self-absorption effects are calculated and the implications for X-ray Magnetic Circular Dichroism (X-MCD) and generalisations to other systems are discussed.
Article
The theory of indirect exchange in poor conductors is examined from a new viewpoint in which the d (or f) shell electrons are placed in wave functions assumed to be exact solutions of the problem of a single d-electron in the presence of the full diamagnetic lattice. Inclusion of d-electron interactions leads to three spin-dependent effects which, in the usual order of their sizes, we call: superexchange per se, which is always antiferromagnetic; direct exchange, always ferromagnetic; and an indirect polarization effect analogous to nuclear indirect exchange. Superexchange itself is shown to be closely related to the poor conductivity, in agreement with experiment. By means of crystal field theory the parameters determining superexchange can be estimated, and in favorable cases (NiO, LaFeO3) the exchange integrals can be evaluated with accuracy of several tens of percent. Qualitative understanding of the whole picture of exchange in iron group oxides and fluorides follows from these ideas.
Article
The authors present the 2p (L2,3) absorption spectra of first-row transition metal ions in tetrahedral and octahedral crystal field symmetry. These have been calculated using a localized description for the 3dn to 2p53dn+1 excitation including electrostatic and spin-orbit interactions. The spectra are significantly different from those already presented where the 3d spin-orbit interaction was neglected. The spectral shape provides a valence- and symmetry-selective probe. Whereas it changes gradually with the crystal field, abrupt changes in the spectra are indicative of high-spin to low-spin transitions. These spin transitions are accompanied by a strong decrease in the 2p branching ratio. The calculated spectra provide a basis for the use of L2,3 absorption spectroscopy in materials science and biological science. The limitations of these calculations and the use of configuration interaction are discussed.
Article
Various electron-spectroscopy techniques are used to determine the electronic structure of Mn impurities in Ag and Cu. The spectral distributions of both minority- and majority-spin impurity d states are determined experimentally and compared to model calculations considering the photoemission matrix elements. The exchange and Coulomb integrals and energetic positions of the impurity d states as well as the impurity-host d-s and d-d hybridizations are determined. We find that the impurity minority-spin states are quite wide and lie close to the Fermi level, which raises questions concerning the validity of the use of a Kondo Hamiltonian to determine the low-energy scale properties. In addition, the hybridization of the majority-spin states with the host d band is larger, causing these to delocalize, whereas the magnetic moment remains localized. This large hybridization can introduce new exchange mechanisms not foreseen in conventional models.
Article
In ab initio calculations of the hopping matrix element V in the Anderson model, it is found that V2 may vary by a factor of 2-8 for Mn, Ce, and U compounds when diffferent relevant configurations are used. We give a prescription for which configuration to use in an ab initio calculation of V, and we make a corresponding modification of the hopping term in the Anderson model, which can easily be implemented in methods for solving the model. We discuss how this influences the calculated properties of Ce compounds.
Article
The origin of nonstatistical branching ratios in spin-orbit-split x-ray absorption spectra is explained. Atomic calculations for transition metals show a systematic change which is due to initial-state spin-orbit splitting and electrostatic interactions between core hole and valence electrons. We have formulated the results of these atomic calculations in general rules, which are also applicable to solids. In the free atom the branching ratio reaches a maximum for the Hund’s-rule ground state and its value decreases gradually for S, L, and J levels of higher energy. The presence of a crystal field results in a lower branching ratio when it produces a low-spin ground state. The rules can be used to assess the spin state and the spin-orbit splitting from the experimental branching ratio in transition-metal and rare-earth compounds. A specific example is given for the influence of second-order spin-orbit interactions in high-spin Ni compounds.
Article
We study the renormalization of hopping integrals between localized and delocalized levels due to a Coulomb interaction between these two types of levels. We find that the renormalization enhances the effective hopping matrix elements, due to effects of the type that enhance x-ray spectra at the threshold. This effect tends to make the hopping integrals appear larger for thermodynamic properties than for spectroscopic properties.
Article
The localized character of the 3d electrons in the antiferromagnetic oxides NiO(100) and CoO(100) has been studied with electron energy-loss spectroscopy at primary energies between 20 and 1200 eV. The spectra of both charge-transfer compounds exhibit weak but sharp loss structures within the insulating gap region due to crystal-field excitations of the 3dn configuration (dn-->dn*). These parity forbidden d-d excitations are strongly enhanced at low primary energies due to exchange scattering. Their observed loss intensity depends on the momentum transfer at the scattering process as is shown by angle-dependent measurements. At NiO(100) a surface d state is found that is due to the lower symmetry of the ligand field at the surface. The measurements demonstrate the existence of spin-forbidden exchange excitations 3A2g-->(1Eg,1T1g) in NiO(100) and reveal their excitation energies. A resonant enhancement of all intra-atomic d-d transitions in NiO(100) is found at the Ni 3s excitation threshold. Temperature-dependent measurements up to the Néel point show only little influence of the antiferromagnetic ordering on the EELS spectra of NiO(100). The different shapes of the absorption edges due to interatomic d-d transitions across the insulating gaps of NiO(100) and CoO(100) are discussed.
Article
The dielectric function ε(q,omega) of Li-intercalated graphite (LiC6) with q||c axis was deduced from inelastic x-ray scattering data and compared with ε(q,omega) of pristine graphite. A displacement and broadening of the first peak of Imε(q,omega) upon intercalation was found, which we attribute to a 2-eV shift of the ``metal'' sigma state of LiC6 to lower energies relative to the sigma-type interlayer state of graphite.
Article
We present carbon K emission spectra of diamond excited with high-resolution undulator radiation. The valence-band emission spectra are shown to be strongly dependent on the excitation energy, up to 20-30 eV above the C K edge. It is proposed that the dependence is indicative of the resonant inelastic scattering description of these emission spectra, i.e., the absorption-emission process should be described as a single scattering event where the momenta of the photoelectron and the valence hole in the final state are related by momentum conservation.
Article
Over the past ten years, the resolving power from new designs of bending-magnet-based soft X-ray monochromators has increased by more than an order of magnitude. This has led to a revolution in soft X-ray spectroscopy, but the limited flux at this high resolution has allowed only relatively efficient measurements to be made, such as photo-absorption. Application of this new tool of high-resolution spectroscopy to photoemission, energy-resolved fluorescence spectroscopy and microscopy has now been made possible with the advent of undulator sources of soft X-rays. Here we have reviewed the recent development of undulator-based soft X-ray monochromators, the special features of undulators in general and the resulting benefits and problems, and describe the state of the art undulator beamline, 7.0 at the Advanced Light Source. In addition, we offer some speculation as to the possible routes to the next or ultimate generation of soft X-ray monochromator.
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On leave from the Institute of Metal Physics
  • Lund
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S-221 00 Lund, Sweden. On leave from the Institute of Metal Physics, Yekaterinburg, Russia.