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Resonant inelastic X-ray scattering (RIXS) has been observed in graphite and hexagonal boron nitride (hBN) above and below their K edges. Below the core threshold, inelastic-loss features are observed, which disperse linearly with excitation energy, but as the excitation goes above the core binding energy, nonlinear dispersive effects are observed in graphite but not in hBN. We show that these two effects, which have previously been thought of as separate processes, i.e. resonant X-ray Raman scattering (below threshold) and RIXS (above threshold), are in fact described by the same physics of coherent fluorescence. Very good agreement between experiment and simulated RIXS is achieved using a simple one-electron framework. The role core-excitons play in the RIXS process is examined in finer detail, by using narrow-band excitation. Our results indicate that core-hole effects play a minor role in the RIXS observed from graphite but are more pronounced in hBN.

Resonant inelastic X-ray scattering (RIXS) has been observed in many systems above and below their core threshold. Below threshold, inelastic-loss features are observed, which disperse linearly with excitation energy, but as the excitation increases above the core binding energy, nonlinear dispersive effects are observed. These two effects are described by the same physics of coherent fluorescence. Very good agreement between experiment and simulated RIXS is achieved using a simple one-electron framework. However, significant questions have arisen concerning core-hole effects and their influence on RIXS. In this work, the role core-excitons play in the RIXS process is examined in finer detail in graphite, by using narrow-band excitation, and through comparison between experiment and simulated spectra which include the core-hole effects explicitly in the modeling. Based on these findings, we conclude that core-hole effects play a minor but detectable role in the RIXS observed from graphite.

Inelastic x-ray scattering has been observed in the hexagonal forms of carbon (graphite) and boron nitride (hBN), both above and below their K edges. For excitation energies below the core threshold, inelastic-loss features are observed, which disperse linearly with excitation energy (Raman-like behavior). However, above the threshold, emission features that move in a nonlinear fashion are observed. We show that these two scattering regimes, which have previously been thought of as separate processes, viz., resonant x-ray Raman scattering (below threshold) and resonant inelastic x-ray scattering (above threshold), are described by the same theory for resonant fluorescence. Simulated spectra, with and without excitonic effects, are presented and compared with experiment. We conclude, based on this comparison, that excitonic effects influence these spectra in two ways. Primarily, the total fluorescence yields are enhanced or reduced simply because of changes in the absorption cross sections. Second, excitonic effects on emission features can be pronounced near some excitonic resonances, and these changes are more significant for excitation further above the K edges, which we attribute to the multiplicity of core-excited states being probed. Based on these findings, we conclude that core-hole effects do not necessarily undermine an interpretation of the scattering in terms of a one-electron (noninteracting) picture, and that resonant fluorescence spectroscopy may be successfully used to probe the band structure of solids.

Detailed experimental results and model caclulations for the evolution of the x-ray fluorescence in the vicinity of the {ital L} absorption edge of titanium in metallic Ti and TiO and insulating TiO{sub 2} and BaTiO{sub 3} are presented. Behavior characteristic of x-ray Raman scattering was found in all samples. We develop a model that describes the valence band emission below the L{sub 3} absorption threshold in terms of the calculated partial density of states and the Kramers-Heisenberg x-ray scattering formalism. Linear muffin-tin orbital band structure calculations were used for the projected density of states in the model. In the region below the L{sub 3} absorption edge very good agreement between experiment and the model was found for all compounds. We propose a mechanism for the energy shift of the emission spectra for TiO{sub 2} and BaTiO{sub 3}. An explanation of the Raman energy loss in terms of the excitation of a valence electron into localized states of {ital d} symmetry in the conduction band is also provided. {copyright} {ital 1999} {ital The American Physical Society}

The advent of third-generation synchrotron radiation sources has stimulated interest in the study of X-ray emission using photon excitation. Soft X-ray fluorescence (SXF) provides a means of measuring the element and angular momentum selective valence band density of states in complex materials. Tenability of the excitation source allows a detailed investigation of the dynamical behavior of the relaxation channels and the coupling between the excitation and emission process that occurs near a resonance or an absorption edge. As an example of resonant inelastic scattering studies in solid systems by SXF spectroscopy at the insertion device beam line BL 8.0 at ALS, results from CaF 2 will be presented. When the energy of exciting photons is tuned to the Ca 2p–3d threshold absorption peak in CaF 2 , the SXF produced by recombination of the excited electrons with core holes exhibits remarkable resonances for both the 2p 3/2 and 2p 1/2 levels. When the resonant 2p–3d SXF peaks are excited, additional broad peaks appear in the spectra below the normal valence band spectrum, which are displaced from the resonant peaks by about 17 eV and track the changes in excitation energy. We identify this as evidence of a coherent, inelastic scattering process, analogous to resonant Raman scattering, in which incident photons excite a localized electronic transition and are re-radiated at an energy reduced by the energy of the excitation. Evidence is also presented which indicates that the 2p–3d absorption transitions lie about 1 eV above a weak Ca 4s absorption threshold.

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The sulfur L{sub 2,3} soft-x-ray emission spectra of wurtzite CdS and cubic ZnS and the zinc M{sub 2,3} spectra of ZnS have been measured using both photon and electron-beam excitation. The photon-excited sulfur L spectra are found to be much sharper than electron-beam-excited spectra. Using photon excitation between the L{sub 2} and L{sub 3} thresholds, a pure L{sub 3} spectrum is obtained for both materials. Broad features of the S L spectra, consisting of a lower valence band (LVB) derived from the sulfur 3s orbital and an upper valence band (UVB) derived from S 3p and Cd(Zn) s and p orbitals, are typical of materials with mixed ionic-covalent character. A narrow band between the UVB and LVB is associated with overlap of Zn 3d and Cd 4d orbitals onto the S site. The s+d local partial density of states derived from these spectra provide a rigorous test of band-structure calculations; they are in good agreement for the position, width, and structure of the LVB and UVB, but place the d band deeper in the subband gap than theory predicts. The Zn M{sub 2,3} spectrum has large lifetime broadening and thus provides limited information. However, near-threshold excitation of this spectrum showsmore » dramatic effects resulting from an electronic resonance Raman process. {copyright} {ital 1997} {ital The American Physical Society}« less

Photon-excited yttrium ${M}_{\mathrm{I}\mathrm{V},\mathrm{V}}$, and electron-excited oxygen $K$ x-ray emission spectra for yttrium oxide are presented. It is shown that, as in the case of yttrium metal, the decay of ${M}_{\mathrm{IV}}$ vacancies does not contribute substantially to the oxide ${M}_{\mathrm{I}\mathrm{V},\mathrm{V}}$ emission. The valence emission is interpreted in a one-electron picture as a measure of the local $p$-type partial density of states. The yttrium and oxygen valence emission bands are very similar and strongly resemble published photoelectron spectra. Using local-density approximation electronic structure calculations, we show that the broadening of the Y-$4p$ signal in yttrium oxide relative to Y metal are due to two inequivalent yttrium sites in ${\mathrm{Y}}_{2}$${\mathrm{O}}_{3}$. Features present in the oxide, but not the metal spectrum, are the result of overlap (hybridization) between the Y-$4p$ wave function and states in the oxygen $2s$ subband.

Photon-excited B K fluorescence spectra were measured for hexagonal boron nitride using tunable synchrotron radiation below and above the B K edge. We report Raman-like resonant inelastic scattering of soft x rays involving excitation of delocalized valence-band electrons. The inelastic scattering features track with the excitation energy below threshold, go through a resonance as the excitation is tuned to the B(1s) core-exciton energy, and evolve into incoherent fluorescence as the excitation is raised further. The energy loss is identified as equivalent to an electronic transition of valence $\ensuremath{\sigma}$ electrons to ${\ensuremath{\pi}}^{*}$ conduction-band states.

This article is a summary of four different aspects of soft x‐ray spectroscopy that are being used or developed by our group to probe the interface. The first is to study the change in valence band fluorescence emission by mimicking a high density of interface atoms through the use of a multilayer. The second method uses an intense localized excited state to characterize the buried interface. The third uses Raman scattering to probe changes in band structure produced by stoichiometric changes in the valence band density of states for atoms at the interface. Finally, a method to study interface atoms by using valence emission from interface atoms that are excited by x‐ray standing waves is being developed.

In a number of solid state systems with localized conduction band states and excitonic states, sharp features in absorption spectra were observed. We have measured the x-ray absorption spectra from borides, nitrides, and some calcium and titanium compounds using BL 6.3.2, an entrance slitless bend magnet beamline operating in the 50–1000 eV region, equipped with a Hettrick-Underwood type varied line space (VLS) grating monochromator. As an example, absorption measurement of selected compounds in B-K (∼190 eV), CaL (∼350 eV), and TiL (∼460 eV) x-ray region will be presented along with their accurate energy positions.

An insertion device beamline, BL 8.0 at the ALS, utilizes the radiation from a 5.0 cm period undulator to cover the 100 – 1500 eV photon energy range. In a number of solid systems with localized conduction band states and excitonic states by tuning the energy of the monochromatic x-rays near core level absorption edges of cation atoms, a resonant inelastic scattering feature is observed. We have carried out careful studies in various systems. As an example, we present resonant X-ray inelastic scattering spectra from CaF2, CaSi and CaSi2 with distinctive localized empty 3d states, to give a general overview of resonant inelastics scattering observations in these systems. When a corelectrtron is excited into a localized state, it stays there long enough eventuall to recombine with the core hole, resulting in resonant elastic peaks. For the inelastic process, the energy loss observed indicates an electronic transition of the valence electron to the localized states in the conduction band accompanying the excitation.

Soft-x-ray emission spectra for the La {ital N}{sub 4,5}{ital O}{sub 2,3} transitions in lanthanum aluminate LaAlO{sub 3} include contributions from several relatively intense multiplet lines near 80 eV. The appearance of this spectrum contrasts sharply with the three diagram lines observed for the {ital N}{sub 4,5}{ital O}{sub 2,3} emission spectrum of Ba in BaF{sub 2}. An atomic model of the La {ital N}{sub 4,5}{ital O}{sub 2,3} transition, assuming single occupancy of the 4{ital f} shell by a spectator electron, provides a good approximation of the measured spectrum. Emission features due to the low-energy {sup 3}{ital P}{sup {ital o}} and {sup 3}{ital D}{sup {ital o}} terms of the 4{ital d}{sup 9}4{ital f} configuration are also apparent in the LaAlO{sub 3} emission spectra, but not in emission spectra from BaF{sub 2}. Reflection measurements indicate that the 4{ital f} level is not occupied in the ground state in LaAlO{sub 3}. The data suggest that charge transfer from ligand orbitals following creation of a 4{ital d} vacancy prior to the emission process is the predominant mechanism giving rise to the required 4{ital f} occupancy.

Monochromatic photon beams were used to excite barium {ital N}{sub IV,V} soft x-ray emission spectra from YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}, PrBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}, BaF{sub 2}, and BaTiO{sub 3}. Near threshold excitation was used to demonstrate that small contributions to the barium {ital N}{sub V} and {ital N}{sub IV} emission spectra in the energy region above the 5{ital p}{r_arrow}4{ital d} core-core transitions do not arise as satellite emission from transitions in multiply excited atoms but rather occur as a result of transitions from the valence states. The emission spectrum of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} and PrBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} reveals a contribution to the electronic density of states at the barium site in the region near the Fermi level. The YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} compound is a superconductor and PrBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} is an insulator. It has been proposed that the difference between them is due to mixing of praseodymium and barium among the sites occupied by yttrium and barium, with an accompanying change in electronic structure. However, our measurements indicate that the barium partial density of states for the two compounds are essentially identical.

The Ca {ital L}{sub 2,3} and Si {ital L}{sub 2,3} fluorescence spectra for CaSi{sub 2} and CaSi are studied. These spectra provide {ital s} and {ital d} partial density-of-states (PDOS) information for the Ca and Si atoms. The measured PDOS`s are in good agreement with available calculations. Using monochromatized x rays to excite core electrons to localized Ca {ital d} states near threshold produces resonant recombination fluorescence peaks for the {ital L}{sub 3} and {ital L}{sub 2} edges of Ca in both compounds. In CaSi, with resonant excitation, inelastic Raman peaks are also observed. The energy position of the inelastic peaks indicates that the creation of the core hole shifts the Ca {ital d} states by several eV. The intensity of the resonance peaks indicates the degree of localization of the Ca {ital d} states, which is reduced by their participation in the bonding of the silicides. The empty Ca {ital d} states are found to be quite localized in CaSi, and less so in CaSi{sub 2}.

The IBM/TENN/TULANE/LLNL/LBL Beamline 8.0 at the advanced light source combining a 5.0 cm, 89 period undulator with a high‐throughput, high‐resolution spherical grating monochromator, provides a powerful excitation source over a spectral range of 70–1200 eV for surface physics and material science research. The beamline progress and the first experimental results obtained with a fluorescence end station on graphite and titanium oxides are presented here. The dispersive features in K emission spectra of graphite excited near threshold, and found a clear relationship between them and graphite band structure are observed. The monochromator is operated at a resolving power of roughly 2000, while the spectrometer has a resolving power of 400 for these fluorescence experiments.

The full spectrum of occupied and unoccupied σ and π orbitals is presented for solid C60, C70, and graphite, using Cls emission and absorption spectroscopy. There are significant diffcrences between C60 and C70, and even larger changes relative to their infinite analog graphite (C∝). A comparison is made with photocmission and inverse photoemission results, along with first principles quasiparticle calculations.

The Al ${\mathit{L}}_{2,3}$ soft-x-ray reflection and emission spectra of ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As have been measured for x between 0.17 and 1.0. The valence-band maximum was monitored via the emission spectra using a fit based on band-structure parameters. The dominant features in the reflection spectra are produced by the ${\mathit{L}}_{2,3}$ core excitons. Since both the reflection spectra and emission spectra were obtained using the same soft-x-ray spectrometer, the energies of the excitons above the valence-band maximum are accurately obtained. The known values of the optical band gap, as a function of x, are then used to determine the core exciton binding energy. The core excitons are found to follow and lie above the conduction-band minimum by 0.3 eV as the Al composition changes. Our results are compared to various core exciton theories and to previous experiments.

The Al {ital L}{sub 2,3} and O {ital K} thresholds for single-crystal {alpha}-Al{sub 2}O{sub 3} have been studied by photoemission. Energy-distribution curves, constant-initial-state (CIS), and constant-final-state (CFS) spectra are reported and compared to the absorption spectrum reported previously. An exciton appears as a doublet at threshold in the Al {ital L}{sub 2,3} CFS, CIS, and absorption spectra. The details of the Al {ital L}{sub 2,3} CFS spectrum and absorption spectrum are similar, while the exciton is the only feature present in the CIS spectrum. Comparisons of the various Al {ital L}{sub 2,3} spectra allow the probabilities of different exciton decay channels to be determined. The probability for nonradiative direct recombination of the exciton is found to be (8{plus minus}1)% and the probability for Auger decay of the exciton is found to be (72{plus minus}20)%. Comparisons of the O {ital K} CIS and CFS spectra suggest that no O {ital K} exciton is formed.

We have compared the Al L23 and Si L23 emission and reflection spectra of Al2O3 and SiO2 to obtain information on the nature of the excited states in the presence of the L23 holes. A 5 m Rowland spectrometer, using a 600 l/mm grating mounted in grazing incidence was used to detect both electron excited soft X-ray emission and near normal soft X-ray reflection. We report fine structure in the emission spectra above the L23 edges which coincides with structure in the reflection spectra. These features appear both in the bandgap and in the conduction band. Such features within the bandgap are typically identified as excitons, while those in the conduction band must be localized excited states. Therefore, measurements of ϵ2, for SiO2 and Al2O3 above the L23 edge should be interpreted in terms of localized excitations in the presence of a core hole, rather than interband transitions.

The {ital L}{sub 2,3} emission spectrum of {ital c}-Si, excited by monochromatized synchrotron radiation, has been recorded with a 5-m Rowland spectrometer. Dramatic spectral changes are observed as the excitation energy is varied from the 2{ital p} binding energy up to 144 eV. It is proposed that a spectator electron, close to the bottom of the conduction band, influences the emission spectrum. The observations suggest that intraband shakeup is important in the excitation process, and that a population of low-lying levels, via initial-state shakeup, influences the high-energy-excited Si {ital L} emission spectrum.

Si L 2.3 x-ray emission stimulated with narrow-band synchrotron radiation has been studied as a function of excitation energy. In strong contrast to crystalline silicon, amorphous silicon yields emission spectra that are independent of the excitation energy, thereby demonstrating that long-range order plays a necessary role in the excitation-energy dependence of the crystalline emission

X-ray optical processes in solids—absorption, fluorescence and resonant scattering—are modeled within a band-structure-based approach to describe electron states. The theory goes beyond a simple one-electron treatment by considering self-energy corrections for electron states, the perturbation acting on electron states because of a core hole, and, to a lesser degree, lifetime effects for high-energy electron states in solids. Aspects of the work that suggest extensions to be made in the future are discussed. Theoretical results for all three types of x-ray spectra (absorption, fluorescence and resonant scattering) are presented and compared to experiment. Results are presented for diamond, graphite, NaF and Al.

This paper is a summary of some of the recent activities of our soft x-ray spectroscopy group. We are using soft x-ray emission spectroscopy to probe the electronic properties of matter, emphasizing atoms in the bulk and at interfaces. In particular we have used incoherent photon excitation to obtain a basic understanding of the electronic properties of a wide variety of materials such as yttrium oxide and titanium diboride, we have used the penetrating power of x-rays to study multilayers of silicon and iron and have shown that iron silicide is present in the silicon layer and provides evidence that in the antiferromagnetic Fe/Si multilayer system the FeSi2 layer is conducting rather than insulating. The ubiquitous presence of Raman scattering has been used to elucidate the electronic band structure of materials including graphite and hexagonal boron nitride. Such scattering can produce dramatic changes in the emission spectrum that can further the basic understanding of the electronic band structure. We...

Soft x-ray fluorescence spectroscopy has been employed to obtain information about the Si-derived valence band states of Fe/Si multilayers. The valence band spectra are quite different for films with and without antiferromagnetic interlayer exchange coupling, demonstrating that these multilayers have different silicide phases in their spacer layers. Comparison with previously published fluorescence data on bulk iron silicides shows that the Fe concentration in the silicide spacer layers is substantial. Near-edge x-ray absorption data on antiferromagnetically coupled multilayers in combination with the fluorescence data demonstrate unambiguously that the silicide spacer layer in these films is metallic. These results on the electronic structure of buried layers in a multilayer film exemplify the wide range of experiments made possible by new high-brightness synchrotron sources., 11 pages, 4 postscript figures, submitted to Phys Rev B15, Rapid Communications

Soft x-ray fluorescence (SXF) spectroscopy using synchrotron radiation offers several advantages over surface sensitive spectroscopies for probing the electronic structure of complex multi-elemental materials. Due to the long mean free path of photons in solids ({approximately}1000 {angstrom}), SXF is a bulk-sensitive probe. Also, since core levels are involved in absorption and emission, SXF is both element- and angular-momentum-selective. SXF measures the local partial density of states (DOS) projected onto each constituent element of the material. The chief limitation of SXF has been the low fluorescence yield for photon emission, particularly for light elements. However, third generation light sources, such as the Advanced Light Source (ALS), offer the high brightness that makes high-resolution SXF experiments practical. In the following the authors utilize this high brightness to demonstrate the capability of SXF to probe the band structure of a polycrystalline sample. In SXF, a valence emission spectrum results from transitions from valence band states to the core hole produced by the incident photons. In the non-resonant energy regime, the excitation energy is far above the core binding energy, and the absorption and emission events are uncoupled. The fluorescence spectrum resembles emission spectra acquired using energetic electrons, and is insensitive to the incidentmore » photon`s energy. In the resonant excitation energy regime, core electrons are excited by photons to unoccupied states just above the Fermi level (EF). The absorption and emission events are coupled, and this coupling manifests itself in several ways, depending in part on the localization of the empty electronic states in the material. Here the authors report spectral measurements from highly oriented pyrolytic graphite.« less

The full spectrum of occupied and unoccupied σ and Π orbitals is presented for solid C60, C70, and graphite, using C1s emission and absorption spectroscopy. There are significant differences between C60 and C70, and even larger changes relative to their infinite analog graphite C∞. A comparison is made with photoemission and inverse photoemission results, along with first principles quasiparticle calculations.

We are using resonant soft x-ray fluorescence at the Advanced Light Source to probe the electronic and geometric structure of novel materials. In the resonant process, a core electron is excited by a photon whose energy is near the core binding energy. In this energy regime the absorption and emission processes are coupled, and this coupling manifests itself in several ways. In boron nitride (BN), the resonant emission spectra reflect the influence of a “spectator” electron in an unoccupied excitonic state. The resonant emission can be used to distinguish between the various bulk phases of BN, and can also be used to probe the electronic structure of a monolayer of BN buried in a bulk environment, where it is inaccessible to electron spectroscopies. For highly-oriented pyrolytic graphite (HOPG) a coherent absorption-emission process takes place in the resonant regime, whereby crystalline momentum is conserved between the core excited electron and the valence hole which remains after emission

We investigate the excitation energy dependence of the boron [ital K] soft-x-ray emission of B[sub 2]O[sub 3] and hexagonal-BN using synchrotron radiation. When exciting into the core exciton state the spectra are shifted to lower energy by 1.5 and 1.8 eV, respectively. This shift is found to be a result of phonon relaxation and differences in initial-and final-state electronic screening.

Monochromatized synchrotron radiation has been employed as the excitation source for soft x-ray emission spectroscopy. In the present paper changes in the emission spectra that occur as the excitation energy is varied near the core-absorption threshold are discussed. In the case of crystalline silicon, strong variations are seen in the L2,3 emission for excitation energies up to 30 eV above threshold. These variations are shown to be dependent on the crystalline order of the material and can be interpreted in terms of restrictions on the crystal momentum that arise in an inelastic scattering description of the combined absorption and emission. On the other hand this description is less relevant to the excitation-energy dependence of ionic insulators, in which strong phonon coupling removes these restrictions on crystal momentum. In the insulators B2O3 and BN strong variations in the emission are observed at threshold, upon creation of a core exciton: the exciton affects the emission through its influence on the phonon coupling as well as on the initial and final-state screening.

Phonon-relaxation effects on the ${\mathit{L}}_{2,3}$ soft-x-ray emission of MgO, ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$, and ${\mathrm{SiO}}_{2}$ are measured by comparing soft-x-ray emission spectra to photoelectron spectra. The observed shifts of the soft-x-ray emission spectra relative to the photoelectron spectra are identified as Stokes shifts and are described in terms of partial phonon relaxation. The observed Stokes shifts are 0.5, 0.9, and 1.2 eV for MgO, ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$, and ${\mathrm{SiO}}_{2}$, respectively. Similar measurements on Mg, Al, and Si show no detectable Stokes shift within the experimental uncertainty of 0.2 eV. The larger Stokes shifts observed in the oxides are due to the longer core-hole lifetime, the faster phonon relaxation, and larger phonon relaxation energy in the oxides.

We present the first soft x-ray-emission spectra for solid Kr (4{ital p}-3{ital d}) and Xe (5{ital p}-4{ital d}) that are free of complicating satellite spectra. Monochromatic synchrotron-radiation excitation is used to suppress the satellites. The data are analyzed to determine the {ital P}{sub 3/2} bandwidth for both elements; these results are compared with both photoemission data and available theoretical calculations. Bandwidths are found to be larger than those predicted by most electronic structure calculations.