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Abstract

Resonant soft X-ray Raman scattering measurements on NiO have been made at photon energies across the Ni 2p absorption edges. The details of the spectral features are identified as Raman scattering due to d-d and charge-transfer excitations. The spectra are interpreted within the single impurity Anderson model, including multiplets, crystal-field and charge-transfer effects. At threshold excitation, the spectral features consists of triplet-triplet and triplet-singlet transitions of the 3d8 configuration. For excitation energies corresponding to the charge-transfer region in the Ni 2p X-ray absorption spectrum of NiO, the emission spectra are instead dominated by charge-transfer transitions to the 3d9L-1 final state. Comparisons of the final states with other spectroscopical techniques are also made.

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... The Mott insulators CoO, NiO, and SrCuO 2 were chosen as model materials for this study, and provide examples of resonance from strongly correlated 3d 7 , 3d 8 , and 3d 9 systems, respectively. The principle electronic excitations observed in Xray absorption spectroscopy (XAS) and RIXS with p-symmetry core holes on Co [7,16], Ni [6,8,14,15,17,18,36], and Cu [8,9] are closely identified with excitation matrix elements obtained from AM based calculations for a high-spin 2+ valence atomic state perturbed by the crystal field of surrounding oxygen atoms. The compounds CoO and NiO have a rock salt crystal structure, with an octahedral crystal field on the transition metal site, and strong antiferromagnetic superexchange interactions through σbonded oxygen atoms. ...
... To evaluate the degree to which charge transfer shake-up excitations are reduced in EUV RIXS, we examine M-edge scattering data on the model Mott insulators NiO and CoO. Previous experimental studies performed with soft X-rays at the transition metal L-edge (700-900 eV) have noted that charge transfer features from these materials have significant intensity at 4-7 eV energy loss [14,[16][17][18]. M-edge studies have noted that charge transfer excitations are weak, but stopped short of presenting data that covered the Mott gap feature [15,16]. ...
... Second, we focus only on the region within 4 eV of the band gap, as this is where most charge transfer intensity is found at the L-edge, with the exception of the high energy emission line. The emission line is far weaker at the M-edge than at the L-edge, and only low energy parts of the emission line are associated with simple metal-ligand charge transfer in recent models [17,18]. Finally, we assume that the peak intensity of charge transfer excitations is 50% greater than the average intensity of the 4 eV region and, based on the data quality in Figure 3, that a feature with peak intensity greater than 0.01 in several consecutive RIXS curves would be recognizable in the data. ...
Article
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In resonant inelastic X-ray scattering (RIXS), core hole resonance modes are used to enhance coupling between photons and low energy electronic degrees of freedom. Resonating with shallow core holes accessed in the extreme ultraviolet (EUV) can provide greatly improved energy resolution at standard resolving power, but has been found to often yield qualitatively different spectra than similar measurements performed with higher energy X-rays. This paper uses experimental data and multiplet-based numerical simulations for the M-edges of Co-, Ni- and Cu-based Mott insulators to review the properties that distinguish EUV RIXS from more commonly performed higher energy measurements. Key factors such as the origin of the strong EUV elastic line and advantages of EUV spectral functions over soft X-ray RIXS for identifying intrinsic excitation line shapes are discussed.
... Rather than constant final energy fluorescence, the spectra now feature Raman-like constant energy loss excitations that resonate differently as a function of incident energy. In this regard, the Ni electronic state looks much more similar to correlated insulators with a Ni d 8 configuration such as NiO or NiCl 2 [54][55][56]. Indeed, LTNAO has been observed to be far more electrically insulating than LaNiO 3 [17]. ...
... The atomic model used here assigns only d states to the valence band; hence, the extracted energies correspond to those of effective hybridized orbitals in the electronic structure. We find excellent agreement between experiment and theory for the peak energies, as good, or better, than multiplet calculations for much simpler bulk compounds [54][55][56]. A slightly poorer agreement is found regarding the intensity of the excitations, which is likely due to small intensity renormalizations driven by the presence of ligand holes [55]. ...
Article
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Resonant inelastic x-ray scattering is used to investigate the electronic origin of orbital polarization in nickelate heterostructures taking LaTiO3-LaNiO3-3x(LaAlO3), a system with exceptionally large polarization, as a model system. We find that heterostructuring generates only minor changes in the Ni 3d orbital energy levels, contradicting the often-invoked picture in which changes in orbital energy levels generate orbital polarization. Instead, O K-edge x-ray absorption spectroscopy demonstrates that orbital polarization is caused by an anisotropic reconstruction of the oxygen ligand hole states. This provides an explanation for the limited success of theoretical predictions based on tuning orbital energy levels and implies that future theories should focus on anisotropic hybridization as the most effective means to drive large changes in electronic structure and realize novel emergent phenomena.
... To properly understand and describe the CT excitations, it was recognized that taking into account only the ligand band structure leads to the wide band of CT excitations seen in the experiment (Fig. 41). Such an extension of the cluster approach leads to the SIAM with full multiplet effects, which is widely used in modern semiempirical simulations of RIXS (Magnuson, Butorin, Agui, and Nordgren, 2002;Magnuson, Butorin, Guo, and Nordgren, 2002;Ghiringhelli et al., 2005Ghiringhelli et al., , 2006Chiuzbăian et al., 2008) of the RIXS. The combination of the local density approximation to the density-functional theory and the dynamical mean-field theory (Hariki, Winder et al., 2020) eliminates most of the empirical parameters of the traditional cluster method. ...
Article
An overview of both experimental and theoretical results in the field of resonant scattering of tunable soft and hard x-ray radiation is presented, with a main focus on the closely related processes of resonant inelastic x-ray scattering (RIXS) and resonant Auger scattering (RAS). The review starts with an overview of fundamental dynamical aspects of RIXS illustrated for different systems. A detailed analysis of case studies with increasing complexity, considering both gas-phase and condensed matter (liquids and solids) applications, is given. In the review, the most important achievements in investigations of coupled electron-nuclear dynamics and structural aspects in studies of liquids and solids over the last two decades are outlined. To give a perspective on the insights from RIXS and RAS, the x-ray results are discussed against the background of complementary experimental techniques like vibrational infrared absorption and Raman spectroscopy, as well as small-angle x-ray and neutron scattering. Finally, recent achievements in time-resolved studies based on x-ray free-electron lasers are described.
... Based on previous Ni L-edge XAS studies of nickel oxides, the feature at ∼855 eV likely results from changes in the local ligand field parameters and may also have the partial ligand to metal charge-transfer contributions. 52,58,60,61 Hence, this feature will be sensitive to changes in the Ni coordination environment. We hypothesize that upon wetting in 0.1 M KOH solution, a partial transformation of NiO to β-Ni(OH) 2 occurs. ...
Article
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Herein, we report the synthesis and electrochemical oxygen evolution experiments for a graphene supported Ni3MnO4 catalyst. The changes that occur at the Ni active sites during the electrocatalyic oxygen evolution reaction (OER) were elucidated by a combination of operando Ni L-edge X-ray absorption spectroscopy (XAS) and Ni 2p3d resonant inelastic X-ray scattering (RIXS). These data are compared to reference measurements on NiO, β-Ni(OH)2, β-NiOOH, and γ-NiOOH. Through this comparative analysis, we are able to show that under alkaline conditions (0.1 M KOH) the oxides of the Ni3MnO4 catalyst are converted to hydroxides. At the onset of catalysis (1.47 V) the β-Ni(OH)2-like phase is oxidized and converted to a dominantly γ-NiOOH phase. The present study thus challenges the notion that the β-NiOOH phase is the active phase in OER and provides further evidence that the γ-NiOOH phase is catalytically active. The ability to use Ni L-edge XAS and 2p3d RIXS to provide a rational basis for structure-activity correlations is highlighted.
... One part of these spectra is related to the d-d excitations (up to about -4 eV), which can be described within the crystal field model of NiO 32 . The second, dispersing part at higher energy transfers is related to the charge transfer excitations and can be described by the Anderson impurity model 31,33,34 . The main peak at -1 eV (which is present in all measured spectra) can be interpreted as corresponding to the crystal field splitting parameter 10Dq, which is given as the energy splitting between the singly occupied e g (x 2 − y 2 , z 2 ) and t 2g (xy, yz, zx) orbitals in the crystal field model. ...
Preprint
The performance of the recently commissioned spectrometer PEAXIS for resonant inelastic soft X-ray scattering (RIXS) and X-ray photoelectron spectroscopy (XPS) and its hosting beamline U41-PEAXIS at the BESSY II synchrotron are characterized. The beamline provides linearly polarized light from 180 eV - 1600 eV allowing for RIXS measurements in the range of 200 eV - 1200 eV. The monochromator optics can be operated in different configurations for the benefit of either high flux, providing up to $10^{12}$ photons/s within the focal spot at the sample, or high energy resolution with a full width at half maximum of <40meV at an incident photon energy of ~400 eV. This measured total energy resolution of the RIXS spectrometer is in very good agreement with the theoretically predicted values by ray-tracing simulations. PEAXIS features a 5 m long RIXS spectrometer arm that can be continuously rotated about the sample position by 106{\deg} within the horizontal photon scattering plane, thus enabling the study of momentum-transfer-dependent excitations. To demonstrate the instrument capabilities, d-d excitations and magnetic excitations have been measured on single-crystalline NiO. Measurements employing a fluid cell demonstrate the vibrational Progression in liquid acetone. Planned upgrades of the beamline and the RIXS spectrometer that will further increase the energy resolution by 20 - 30% to ~100meV at 1000 eV incident photon energy are discussed.
... Second, several other peaks have been observed in both absorption and emission from ∼1 to 6 eV above the edge 4-12 , but not satisfactorily assigned. Third, and perhaps most crucially, despite their observation and unambiguous assignment from early optical absorption 4 , later electron energy loss (EEL) 28 and other spectroscopies 10,11,29 and most recently inelastic X-ray spectroscopy 30 , with the notable exception of Haverkort et al. 31 , there are sparse reports of calculated d → d excitation energies of NiO within the framework of those calculations that, apparently, predict correctly the value of the band gap and the accepted p 6 d 8 → p 5 d 9 description of the gap states. While the d → d transitions are on-site transitions, nevertheless, they are transitions across the fundamental energy gap, and, as such, should be calculable from these all-electron calculations. ...
Article
This paper reports calculated energies and electronic structures of nineteen excited states in NiO based on a Δ-SCF approach reported previously for the [Formula: see text] transitions in NiO and Sr2CuO2Cl2. They are the spin-flip: [Formula: see text]; eight [Formula: see text]: (one electron) 3A2g → 3 T 2g , (two electron) 3A2g → 3 T 1g , 3A2g → a 1 E g , 3A2g → 1 T 2g , 3A2g → 1 T 1g , 3A2g → b1 E g , 3A2g → b1 T 2g ; two O(2p) → Ni(3d); seven Ni(3d) → O(2p) (including two spin-flip) and the O(2s) → Ni(3d) charge transfer excitations, which span an energy range from 0.25 eV to 17.53 eV, and include the fundamental band gap associated with an excitonic O(2p) → Ni(3d) transition at 4.23 eV. These are compared to absorption and emission spectra, and previous calculations. In the case of the O(2p) → Ni(3d) excitations, comparisons are given for gap and Γ-point energies derived from HF, PBE0, HSE06, B3LYP, B1WC, GGA and LDA one-electron approximations. Finally, the directly calculated Stokes shifts and associated luminescence energies for the two O(2p) → Ni(3d) transitions are reported, where the excitonic value is found to be in good agreement with the recently reported experimental value.
... 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. ...
Chapter
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XAS of light elements, such as lithium (Li), boron (B) and carbon (C), occurs in the soft X-ray energy range at about 60, 180, and 280 eV, respectively. In general, XAS can be measured in transmission, electron, or ion yield or fluorescence yield mode. Due to the path lengths of soft X-rays, transmission X-ray absorption measurements in this soft X-ray energy range of 50–300 eV are very difficult. The electron yield mode of XAS is an alternative surface-sensitive measure for the standard transmission XAS mode. Concerning in situ XAS studies, it can as yet only be performed at the mbar to bar pressure range, for example, as shown in Refs. [7, 9]. Fluorescence yield XAS probes deeper into the sample, but this probe has very low yield for soft X-ray energies and may suffer from saturation effects in concentrated systems. At the same time, the incoming X-ray probe of 60–280 eV does not penetrate deep enough and still mostly surface is probed with fluorescence yield XAS. With XRS, one is able to measure more bulk-like properties of light elements [11]. There are a few dedicated XRS setups in the world, where I would like to mention a setup at the European Synchrotron Radiation Facility (ESRF) [17, 18] and another setup at the Stanford Synchrotron Radiation Lightsource (SSRL) [19, 20].
... A tentative explanation for this major difference between the DPE spectra is the very different energy scale for the excitation of d electrons. The minimal energy to excite a d electron at the NiO and CoO surfaces is in the range of 0.5-1.1 eV for the d-d transitions [270,271], which have been measured by optical absorption [272,273], electron energy loss spectroscopy [274][275][276][277], resonant inelastic x-ray scattering [278][279][280], and explained in detail by theoretical calculations [281,282]. This energy scale is much smaller than the threshold energy to excite a d electron from the top of the Cu and Ag d bands to the E F (2 and 4 eV [238,243]). ...
Article
The recent development of double photoemission (DPE) spectroscopy at surfaces using laser-based high-order harmonic generation in combination with time-of-flight electron spectroscopy is reviewed. Relevant experimental conditions including the solid angle for collecting photoelectron pairs, the energy and angular resolutions, as well as the repetition rate and the photon energy range of light sources are introduced. As examples, we provide an overview of laser-based DPE results on the noble metals Ag and Cu as well as transition metal oxides NiO and CoO. The DPE energy and angular distributions of photoelectron pairs are compared with emphasis on the possible indications of electron-electron interaction. Potential further developments including femtosecond time-resolved DPE experiments are outlined.
... For x = 0.05, the double-peak structure has essentially vanished in comparison to at x = 0.16 due to the superposition of the strong fcc contribution. (iv) The 6 eV feature [19,31] above the main 2p 3/2 peak is prominent in Ni metal x = 0.0 that is associated with electron correlation effects and narrow-band phenomena [32]. The intensity of the 6 eV feature is very low in the Ni 1−x C x films in comparison to Ni metal even at x = 0.05 due to more delocalized bands. ...
... For x = 0.05, the double-peak structure has essentially vanished in comparison to at x = 0.16 due to the superposition of the strong fcc contribution. (iv) The 6 eV feature [19,31] above the main 2p 3/2 peak is prominent in Ni metal x = 0.0 that is associated with electron correlation effects and narrow-band phenomena [32]. The intensity of the 6 eV feature is very low in the Ni 1−x C x films in comparison to Ni metal even at x = 0.05 due to more delocalized bands. ...
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Photoemission and bremsstrahlung-isochromat-spectroscopy data on a cleaved NiO single crystal are presented and compared to band- and cluster-theory predictions. In contrast to band-theory predictions the band gap is found to be large but not determined solely by the even larger d-d Coulomb interactions so that NiO is not a Mott-Hubbard insulator in the simplest sense. A large d-d interaction need not prevent NiS from being a metal.
Article
Recent developments in resonant X-ray emission and resonant photoemission are discussed. It will be shown that the scattered X-ray energy and intensity distribution of a resonant X-ray Raman scattering experiment contain the excitation spectrum resulting from a single local spin-flip, a well as dd excitations accompanied by local spin-flip satellites. This phenomenon can be used in experiments to determine the interatomic exchange and superexchange interactions, and more generally the local single magnon density of states. Within resonant photoemission, it will be demonstrated that a `hidden peak' is present in the Auger spectrum of NiO. Proof for the existence of this peak is found in the analysis of the 2p3s3p and 2p3p3p resonant photoemission spectra.
Article
Total and partial atomic-level widths of the K-, L-, M-, and N-levels of the elements (covering Z up to 120 for K- and L-levels) are presented in graphic form. Graphs are based on theoretical predictions; total widths are natural level widths as governed by the rates of the various deexcitation processes for a single hole in that level. The drawings make possible, throughout the periodic table, a quick survey of the magnitude and trends of the total level widths, rates and yields of radiative, Auger, and Coster-Kronig processes,and widths of x-ray, Auger, and Coster-Kronig lines.
Article
The photon energy dependences of the valence-band photoemission spectra of the Mott insulator NiO, the Pauli paramagnetic metal NiTe, the diamagnetic metal NiSb, and the semiconductor Cu2O have been measured in the vicinity of the transition-metal 3p threshold. Resonant satellites similar to those of Ni and Cu metal were found in all of these compounds. The resonance behaviors of the satellites and the main 3d emissions are also similar to those of the metals. From these data it is concluded that (1) the satellites and the resonance behaviors are in the first approximation of an atomic origin, (2) the Fano line shapes in the constant initial-state spectra of Ni and its compounds arise from the super-Coster-Kronig interaction between a discrete state 3p53dn+1 and continuum states 3p63dn-1εl following the 3p→3d photoabsorption, (3) initial-state configuration mixing contributes significantly to the Ni "6-eV" satellite, and (4) the Ni metal main 3d band resonates strongly due to a 3d84s final state. Different interpretations of the Ni "6-eV" satellite are critically assessed in the light of these findings, and it is argued that these conclusions imply an appreciable amount of polar fluctuation to the 3d84s2 configuration in the Ni metal many-body ground state. The importance of Hund's-rule coupling for the ferromagnetism of Ni is also discussed in view of different interpretations of the Ni satellite.
Article
We report photoemission spectra and intensity profiles of the 3d emission and the resonant valence-band satellites in NiO and Ni. Ni in Ni metal is in a 3d9 configuration, but Ni in NiO is in a 3d8 configuration. NiO gives an opportunity to study the effect of a larger number of holes in the d configuration: For NiO we find a very strong resonant satellite at 9.2 eV and a weak satellite at ∼22 eV below the main 3d peak. The intensity of the 9.2-eV satellite is about twice that of the 6.2-eV satellite in Ni, which reflects the increased number of empty 3d states of NiO as compared with Ni. The intensity profile of the main 3d emission in NiO shows a strong (30%) dip at the resonance threshold, the energy of which corresponds to the main absorption threshold as measured with the constant-final-energy spectroscopy technique and to the binding energy of the 3p core levels as determined by x-ray photoelectron spectroscopy. Our interpretation is incomplete because of the clouded theoretical picture of NiO, but the results for Ni and NiO agree qualitatively with recent calculations indicating that similar processes are involved in both cases. The strong increase of 3 eV in the satellite-main line separation energy could be due to the strong localization of the 3d electrons in NiO, which should lead to an increase in the effective Coulomb interaction.
Article
The interatomic coupling of magnetic moments in the insulating antiferromagnetic transition-metal compounds MnO, MnS, and NiO is calculated using a theory based on the itinerant-electron picture (energy-band theory and the local-spin-density treatment of exchange and correlation). Calculated values of these "Heisenberg" coupling constants agree with measured values to the extent that can be expected in light of the approximations required to execute the calculations. The calculations emphasize the importance of covalent interactions between the metal d states and the anion p states. These interactions are spin conserving and fundamentally nonmagnetic; they enter the coupling of the magnetic moments because the intra-atomic spin splitting of the metal d shell makes the covalent interactions dependent on the relative angle of the two magnetic moments.
Article
The hopping-matrix elements Vm(ɛ) between the conduction states with energy ɛ and the different crystal-field-split 4f states m are calculated for CeCu2Si2. With use of the Anderson impurity model, the effect of this m dependence on the static T=0 K susceptibility is studied. We also take into account that the one-particle hopping-matrix elements entering in the f0→f1 and the f1→f2 hopping processes are different (configuration dependence). It is concluded that for CeCu2Si2 larger hopping-matrix elements are needed for the description of thermodynamic properties than for spectroscopic properties. These results are consistent with renormalization effects, which the Coulomb interaction between the 4f and conduction electrons is expected to cause.
Article
The augmented-plane-wave (APW) method is applied to calculate the nonmagnetic band structures for the 3d transition-series monoxides CaO, TiO, VO, MnO, FeO, CoO, and NiO, all of which form with the rocksalt structure. The APW energy-band results at seven symmetry points in the fcc Brillouin zone are fitted with the Slater and Koster linear-combination-of-atomic-orbitals (LCAO) interpolation method involving nonorthogonal orbitals. A nonlinear least-squares fitting procedure is applied to determine 16 two-center energy and overlap LCAO parameters which characterize the oxygen 2s-2p and transition-metal 3d energy bands and their interactions. It is found that both the APW results and the corresponding LCAO parameters exhibit systematic variations across this series of compounds. These calculations predict that: (a) CaO is an insulator with a 10-eV band gap; (b) the metallic oxides TiO and VO have 3d bandwidths of about 0.5 Ry, which are two and a half times larger than those for the antiferromagnetic insulators MnO, FeO, CoO, and NiO; (c) the energy of the 3d band decreases systematically (relative to the bottom of the transition-metal 4s-4p band) as the nuclear charge is increased, causing the metal s-d bands to overlap for the early but not the later members of this series.
The optical absorption associated with charge carriers in NiO and CoO has been obtained from transmission measurements on pure and Li-doped crystals in the wavelength region 05 to 700 μm, at temperatures between 100 and 500°K. The absorption increases smoothly with frequency to a maximum near 1 ev and then shows a further increase extending to 2 ev and higher. In the region from 05 to about 4 μm the absorption increases slowly with temperature and correlates with the Li concentration. In the far infra-red the absorption σFI is independent of frequency and, for T <or= 300-400 °K, σFI > σdc. The activation energy associated with σFI is smaller than the d.c. value. The behaviour between 1 and 700 μm is very similar to that predicted by the theories of Klinger, Reik et al. and Bogomolov et al. for non-diagonal (hopping) transitions by small polarons. Similar absorption in TiO2 and other 3d oxides between 1 and 10 μm has been attributed to a hopping component in the free-carrier mobility. In NiO and CoO, however, we conclude that the absorption is primarily due to small-radius polarons hopping around Li centres with high-temperature activation energies Ea of 02-025 ev and 03-04 ev respectively. Above 400°K in NiO, σFI -> σdc and electrical studies indicate that about 20-30% of the carriers are ionized. No new features appear in the absorption but the above Ea values give an upper limit for the hopping energy of any non-diagonal component in the d.c. mobility. We infer a relaxation time of less than 10-13s for the diagonal component implying a bandwidth of at least 5 mev. Recent d.c. mobility data support a band picture, but a closer analysis indicates that an activated mobility with Ea less, similar 02 ev cannot be excluded.
Article
Wenn ein Atom äußerer monochromatischer Strahlung von der Frequenz ν ausgesetzt ist, sendet es nicht nur sekundäre monochromatische Kugelwellen von der Frequenz ν aus, die mit der einfallenden Strahlung kohärent sind, sondern das Korrespondenzprinzip verlangt, daß im allgemeinen auch noch Kugelwellen von anderen Frequenzen ausgesandt werden. Diese Frequenzen sind alle von der Form ∣ν±ν*∣, wo hν* den Energieunterschied des Atoms im betrachteten und in irgend einem anderen Zustand bezeichnet. Die nicht kohärente Streustrahlung entspricht zum Teil gewissen Prozessen, die kürzlich von Smekal ins Auge gefaßt wurden anläßlich Betrachtungen, die an die Vorstellung von Lichtquanten anknüpfen. In der Abhandlung wird gezeigt, wie sich eine wellentheoretische Analyse der streuenden Wirkung des Atoms an der Hand des Korrespondenzprinzips m ungezwungener und scheinbar eindeutiger Weise durchführen läßt. Die Ausführungen bauen durchaus weiter auf der Auffassung der Verbindung der Wellenstrahlung des Atoms mit den stationären Zuständen, die in einer neuen Arbeit von Bohr, Kramers und Slater vertreten ist, und die Folgerungen, wenn sie sich bestätigen sollten, dürften eine interessante Stütze für diese Auffassung bilden.
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
A design of a small size grazing incidence instrument is presented, which offers large spectral range and high resolution without sacrificing luminosity. The instrument is particularly suited for use at synchrotron radiation sources since it can be conveniently attached to existing experiment chambers. The basic idea of the design is the use of fixed mounted gratings of diffent radii and groove densities and a big two-dimensional position sensitive detector mounted on a x−y angle table. The design is discussed in some detail and performance is presented.
Article
The conditions necessary in metals for the presence or absence of localized moments on solute ions containing inner shell electrons are analyzed. A self-consistent Hartree-Fock treatment shows that there is a sharp transition between the magnetic state and the nonmagnetic state, depending on the density of states of free electrons, the $s$-${}d$ admixture matrix elements, and the Coulomb correlation integral in the $d$ shell; that in the magnetic state the $d$ polarization can be reduced rather severely to nonintegral values, without appreciable free electron polarization because of a compensation effect; and that in the nonmagnetic state the virtual localized $d$ level tends to lie near the Fermi surface. It is emphasized that the condition for the magnetic state depends on the Coulomb (i.e., exchange self-energy) integral, and that the usual type of exchange alone is not large enough in $d$-shell ions to allow magnetic moments to be present. We show that the susceptibility and specific heat due to the inner shell electrons show strongly contrasting behavior even in the nonmagnetic state. A calculation including degenerate $d$ orbitals and $d$-${}d$ exchange shows that the orbital angular momentum can be quenched, even when localized spin moments exist, and even on an isolated magnetic atom, by kinetic energy effects.
Article
The absorption spectrum of single crystal NiO has been measured in the 0.1 to 6 ev range and its reflectivity spectrum from 0.025 to 10 ev. The absorption spectrum shows a series of lines from 1 to 3.5 ev arising from internal transitions of the Ni ion. A continuous background absorption occurs in the range from 0.1 to 3.5 ev whose magnitude increases with impurity concentration. The absorption coefficient rises steeply above 3.5 ev and reaches a value of 106 cm−1 at and above 4 ev. An absorption line at 0.24 ev is found to be temperature sensitive in both intensity and frequency in the range above 300°K. Its behavior suggests that it is connected with the antiferromagnetic ordering. The reststrahlen spectrum was observed with the following parameters: high- and low-frequency dielectric constants 5.4 and 12, respectively; energies of longitudinal and transverse optical mode vibrations are 0.076 and 0.044 ev, respectively.
Article
High resolution L2,3 near edge absorption spectra of Ni compounds are compared to an impurity model calculation including the d-d correlation effects. Covalent mixing in the final state gives a change in the observed structure which can be used as an analytical tool to study the ground state properties. From the analysis we obtain the d-d Coulomb interactions, the charge transfer energy and the covalency in the ground state.
Article
In this paper we present a study of the effects of superexchange on the isotropic Ni 2p x-ray-absorption spectrum of NiO. The exchange interactions are included by performing model calculations on a cluster with two nickel atoms. It is shown that the spectra are sensitive to the nearest-neighbor exchange interactions and that the effect is larger than that obtained from a molecular-field approximation. We will also show that the first moment of the spectrum is proportional to 〈${\mathbf{S}}_{\mathit{i}}$$\cdot${}${\mathbf{S}}_{\mathit{j}}$〉.
Article
Resonant photoelectron spectroscopy studies have been performed on the NiO valence band at photon energies corresponding to the Ni 2p, 3p, and O 1s absorption thresholds. Strong resonances are seen in the vicinity of the Ni 2p threshold, which confirm earlier conclusions from the weaker resonances seen at the Ni 3p threshold. No valence-band resonance is observed at the O 1s threshold. The analysis of this data confirms the picture of NiO as a strongly correlated charge-transfer insulator by identifying the highest-lying states as being of mainly 3d8L_ final-state character. The existence of localized excited Ni states, as well as the delocalized nature of the O states, are confirmed. Comparisons with the configuration-interaction model and quasiparticle calculations are also made.
Article
The excitation spectra of the antiferromagnetic transition-metal oxides MnO, FeO, CoO, and NiO are studied by developing a local three-body scattering theory within a multiorbital tight-binding model. The self-energy corrections to the Hartree-Fock solutions are carried out by taking account of local three-body correlations. Thereby the multiplet structures of three-particle states are fully taken into account. It is shown that the self-energy correction improves drastically the excitation spectra for FeO, CoO, and NiO, leading to the correct value of the band gap and the satellite structure, in agreement with photoemission experiments. Quasiparticle dispersions are obtained in good agreement with the angle-resolved photoemission data of CoO and NiO. The self-energy correction for MnO is found to be quite small.
Article
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.}
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
A method to determine the absorption coefficient near the onset of core-electron transitions for concentrated samples using fluorescence-yield (FY) detection is presented. Measuring the FY signal for different experimental geometries, we are able to calculate the true absorption coefficient. Thus we are able to correct fully for saturation effects present in FY spectra of concentrated samples. The technique is demonstrated for Co and a buried layer of CoSi2.
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
We present a method for calculating the self-energy in the GW approximation that can be applied to systems containing 3d and 4f electrons. The method is applied to NiO and a gap of ~5.5 eV is obtained, which is in reasonable agreement with the experimental value of 4.0 eV. The local density O p band is also improved. The high binding energy satellite at 8 eV, however, is not obtained and there is no substantial increase of O p character at the top of he valence band compared to the local density result. Based on our results, we discuss to which extent the GW approximation is capable of describing highly correlated systems such as NiO.
Article
Electron-exchange processes in dipole-forbidden d-d excitations in NiO(100) have been examined by ``complete'' spin-polarized electron energy-loss spectroscopy over a wide primary energy range and for different scattering geometries. We observe unexpected strong exchange scattering at energies two orders of magnitude higher than the excitation energy. The angular dependence of exchange scattering was found to be correlated to the possibility of resonant excitations. At primary energies corresponding to resonant excitations, it differs dramatically from that observed off resonance.
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