Surface Review and Letters

We measured Si L_2,3 soft-X-ray emission spectra from Mo/Si multilayers annealed at 400°C. We showed that MoSi₂ was formed at the interface, and the thickness of the MoSi₂ interlayer increased with the annealing time. By an analysis of the soft-X-ray emission spectra and the patterns of small-angle X-ray diffraction, we estimated the thickness of the MoSi₂ interlayer and a diffusion coefficient between Mo and Si at 400°C as 1.1 (± 0.2) × 10^-18cm²/s. We showed that soft-X-ray emission spectroscopy is a useful tool for studying the thermal change of the buried interface nondestructively.

An Fe(3.0 nm)/Si(1.3 nm) multilayer had been found to exhibit a fairly strong antiferromagnetic coupling by Endo et al. However, the medium responsible for the interlayer coupling had not been assigned. We measured the Si L_2,3 emission spectra from Fe(3.0 nm)/Si(t) multilayers of t = 0.5, 1.0, 1.5, 2.0, and 4.0 nm thick. From the emission spectra the interface of the multilayers was found to consist of dominantly Fe₃Si alloy. From a rough spectral analysis, there seems to remain a very thin a-Si and/or high-resistive Fe–Si alloy layer sandwiched between the Fe₃Si layers, which is responsible for the strong interlayer coupling.

Synchrotron radiation high resolution core level spectroscopy has been used to investigate the 1/3 monolayer (ML) solid solution at well-defined values of the mutual surface concentration of the Sn and Si adatoms (x = 0.24 and x = 0.40) as a function of temperature (100–300 K). For 24% Si adatoms at the surface there is low energy electron diffraction (LEED) evidence of a nascent 3 × 3 lowering of the surface symmetry at 100 K. Despite this evidence core level spectroscopy data do not show any signature of the nascent phase transition. Our evidence is in agreement with the analogous observation of no significant spectral changes at reduced temperature of the core level spectra of the isoelectronic and isostructural 1/3 ML Sn/Ge(111) surface.

We have studied the electronic structure of Ag thin films of monoatomic height grown on a Ge(001) surface at 95 K by using photoelectron spectroscopy. All the surface states on the clean Ge surface vanished and Ag 5s derived state appeared in the bulk band gap just below the Fermi level. These suggest a strong interaction with the substrate. On the other hand, on three-dimensional (3D) islands formed on a room temperature substrate, small changes were found in Ge 3d spectrum from the clean surface. Each of the Ag 3D islands already had bulklike feature in the valence band structure. The results are consistent with recent observation by scanning tunneling microscopy.

A study of effects induced in the Be 1s core level spectrum and in the surface band structure after Si adsorption on Be(0001) is reported. The changes in the Be 1s spectrum are quite dramatic. The number of resolvable surface components and the magnitude of the shifts do decrease and the relative intensities of the shifted components are drastically different compared to the clean surface. The surface band structure is also strongly affected after Si adsorption and annealing. At the surface state is found to move down from 2.8 to 4.1 eV. The band also splits at around 0.5 Å^-1 along both the and directions. At and beyond only one surface state is observed in the band gap instead of the two for the clean surface. Our findings indicate that a fairly small amount of Si in the outer atomic layers strongly modifies the electronic properties of these layers.

Thin epitaxial Fe films were grown on singular and vicinal GaAs(001) substrates, and their magnetic and electronic structures were investigated by synchrotron-based spin-resolved and spin-integrated photoelectron spectroscopy with different Fe thickness. There were two types of substrates: one was a Si-doped n-type GaAs(001) surface with doping concentration of 2 × 10¹⁸cm^-3 (singular substrate), and the other was orientated by 3° toward the (111)A direction (vicinal substrate). Spin polarization of the secondary electron peak at different growth stages of Fe coverage for the singular substrate sample and the vicinal one were measured. In the case of singular substrates, there was a dependence of their initial surface reconstruction, which is associated with complex domain structure, while no such the dependence was observed in the case of vicinal substrates. The result from the vicinal sample suggests the geometrical influence of the initial surface stoichiometry of the substrate.

The emission properties of ZnO films deposited on Si substrates by reactive dc sputtering have been studied using synchrotron radiation (SR). The integrated luminescence spectra as well as the spectra at a fast time window from about 4 ns to 8 ns and a slow time window from about 20 ns to 160 ns were measured simultaneously, at a temperature range from several K to 300 K. Apart from the bound-exciton radiation recombination (peaked at 369.5 nm) and ultraviolet emission band (peaked at 380 nm), a new emission band peaked at 290 nm was found for the first time under SR vacuum ultraviolet excitation. The mechanisms of these photoluminescence emissions are discussed.

Sub-micrometer resolution has been achieved in photoemission microscopy thanks to the high flux and brightness of the soft X-rays provided by third generation synchrotron sources and the progress in microfabrication of focusing elements. The use of multichannel detectors in recently constructed scanning microscopes adds speed and flexibility in data acquisition. Here we present preliminary results on a study of the interactions between sub-monolayer Pd and polycrystalline Ni obtained with the scanning photoemission microscope at Elettra to illustrate the importance of multichannel data acquisition in photoemission microscopy.

The Fe LMM Auger and Auger vacancy satellite spectra of ultrathin Fe films grown on Cu(100) have been measured using photon excitation energies above and below the L₂ ionisation edge, and in coincidence with the Fe 2p_3/2 and Fe 2p_1/2 photoelectrons. A comparison of the relative intensities of the satellites and main Auger transitions indicates that the M_4,5 holes of L₃M_4,5 double hole states are localised for longer than the L₃ lifetime for the 0.3 and 10 ML coverages but have a lifetime comparable to that of L₃ holes for the 1 ML coverage. The results for the 1 ML coverage are supported by the results of the coincidence experiments. The reasons for the variation in the lifetime of M_4,5 holes are discussed in terms of changes in the number of Fe–Fe neighbours and the density of states at the Fermi energy in these systems.

Ammonia adsorption on the nonpolar surface has been studied using X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. Ammonia adsorbs on the surface molecularly at room temperature. Adsorption induces the shift of the O 2p dangling bond state to the higher binding energy side. This implies the formation of the hydrogen bond between the surface O atoms and the H atoms of NH₃. Annealing the ammonia-covered surface induces the partially decomposed NH_x (x=1,2) species. All adsorbates are removed at the surface temperature of 625–650 K. When the surface is precovered with K, no adsorbed ammonia is observed. Not only a site blocking effect but also a nonlocal effect for poisoning is proposed.

Copper adsorption on Ru(0001) has been studied by synchrotron radiation. The clean Ru 3d_5/2 spectra were found to consist of two components with a binding energy shift of 400 meV. The component with the lower binding energy represents the first layer of ruthenium atoms. Adsorption of copper gives rise to core level shifts of the Ru 3d_5/2 components, which were studied as a function of Cu coverage. Experiments were carried out with copper coverages varying from the submonolayer range up to two monolayers of copper. The binding energy of the Cu 2p_3/2 level was measured by X-ray photoemission spectroscopy.

The linewidth of two-dimensional states appearing in the valence band spectra of 6–12-monolayer Ag(111) films deposited onto H-passivated Si(111)-(1 × 1) substrates has been analyzed in order to obtain a lower limit of the photohole lifetime. An inverse quadratic energy dependence is expected according to the Fermi liquid theory. However, in contrast with the observed in the literature for Ag single crystals, an inverse linear energy dependence has been found for the photohole lifetime. This discrepancy could be attributed to the quantization in the Γ-L direction of the Ag-sp states available for the hole decay, which does not fulfill the energy-independent density of states hypothesis of the Fermi liquid theory.

Angle-scanned photoelectron diffraction patterns of the Si 2p signal of oxidized Si(111) and Si(001) surfaces have been recorded and compared with simulations. The chemically shifted components of the Si 2p signal were deconvoluted by least squares fitting. The different oxidation states exhibit individual diffraction patterns for both surface orientations, indicating a different environment for each suboxide. For the Si(111) surface the results agree with a previously proposed sharp interface, whereas a graded interface is indicated for the Si(001) surface.

We exploit the capabilities of photoelectron diffraction (PED) to provide quantitative information on the local structure of the first layers of clean and adsorbate-covered surface systems. Selected studies of low-energy PED are presented, highlighting the advantages of the angle-scanning approach. In the first experiment, we evaluate the clean Rh(110) surface layer relaxation by employing the PED of the Rh 3d_5/2 surface component. The resulting relaxation is in good accord with previous low energy electron diffraction data. In the second experiment, a system lacking long-range order is examined, namely the saturation layer formed by nitrogen monoxide on Rh(100) at 123 K. Preliminary results confirm the bridge adsorption geometry model. The last example is a chemical shift PED study of the c(4 × 2) phase of carbon monoxide on Pt(111). In this system, CO molecules are coadsorbed at two different adsorption sites, the energy separation of the respective C1s components being 0.7 eV. Structural determination has been achieved by an independent analysis of the diffraction yield originated by the two chemically shifted C1s components. The structure of Pt(111) + c(4 × 2)-2CO has been refined with an automated search of the best parameters using a modified version of MSCD (multiple scattering calculation of diffraction) code.

Photoelectron emission and scanning photoelectron microscopies have been used to study the mass transport of potassium on the surface of a Rh(110) catalyst with Pt microstructures, induced by propagating chemical waves during the H₂ + O₂ reaction or by coadsorption of NO. Imaging the dynamic changes in the adsorbed layer, an energetically driven redistribution of K and formation of condensed coadsorbed patterns on the Rh(110) surface has been evidenced. The local composition of the different patterns on this spatially heterogeneous interface has been determined by microspot photoelectron spectroscopy, and the mechanism of the observed self-organization processes has been verified.

For selected examples of diatomic molecules chemisorbed on transition and noble metals [N₂ on Ru(001) and Ni(111), CO on Ru(001) and Cu(111)], we survey core-induced bond breaking by N1s and O1s excitation. We demonstrate that supplementary information on the primarily excited electronic states and on the mechanism of bond dissociation is supplied by ions, neutral molecules and atoms, and decay electrons emitted upon electronic evolution. By comparing yields of neutrals and ions, and their translational energies as a function of excitation energy for the different systems, we discriminate between molecular and surface-induced aspects of bond dissociation. In particular we analyze the potential of the substrate–adsorbate interaction to enhance atom selectivity in core-excitation-induced bond breaking.

We have studied the electronic structures in a wide temperature range for the Si(111) surface using photoemission spectroscopy combined with the laser annealing method. The temperature dependence of the Si 2p surface-sensitive core level photoemission spectra shows some gradual changes along with the thermal broadening above ~1063 K. In addition, the spectral change in the valence band photoemission spectra was also observed across the 7 × 7–1 × 1 transition temperature. These results indicate that the surface band structure is changed along with structural change at the 7 × 7–1 × 1 transition temperature. With increase of the temperature, the shift of the Si 2p core-level photoemission spectra to the lower binding energy side was observed. We discuss the temperature-induced effects such as the thermal broadening and the observed shift.

We followed in real time the thermal reaction of fullerene molecules with the Si(111) surface by means of fast photoemission spectroscopy. The formation of SiC via C₆₀ fragmentation on Si(111) is used as a key example of the capability of fast photoemission, associated with a fine temperature control, in determining the nature of thermally induced chemical reactions. By monitoring every 13 s the evolution of the C1s core level photoemission spectrum, as a function of temperature and as a function of time at fixed temperature, we were able to identify several steps in the interaction of C₆₀ with Si(111). A model describing the thermal evolution of this interaction, in agreement with these and other experimental observations, considers the initial chemisorption of C₆₀ in mainly metastable configurations, the evolution toward more stable configurations, allowed by molecular rotations and breaking of Si–Si bonds, the cage deformation to further increase the number of C–Si bonds, the final cage fragmentation and SiC formation only above 1050 ± 10 K.

For the first time, the free exciton (FE) VUV-luminescence decay curves were systematically investigated in solid Kr using photoexcitation in the energy region above band gap energy. Delayed electron–hole recombination and "prompt" (in terms of the time resolution of the experimental setup) creation of secondary excitons were separated using the time-resolved experimental technique. A detailed model for the dynamics of electron–hole recombination into the FE state was developed. The delayed component of the free exciton decay curve was reproduced with model calculations, including thermalization of the carriers via scattering on acoustic phonons, and the recombination cross-section, which depends on the actual carrier temperatures. A satisfactory agreement between experiment and theory was found.

We present a temperature-programmed X-ray photoelectron spectroscopy study of the thermal chemistry of acetylene, ethylene and propene on Ni(100) using synchrotron radiation. The use of a third generation light source makes it possible to measure high resolution photoemission spectra within a few seconds approaching the ideal of real-time analysis. The evolution of the C 1s core level spectra has been monitored in situ as a function of sample temperature from 90 to 530 K. Analysis of binding energies and changes in intensities and correlation with previous data from the literature have allowed us to distinguish the surface intermediates formed during the thermal reactions. The resultant temperature-dependent intensity curves clarify the dehydrogenation mechanisms of acetylene and ethylene and provide new insight into the decomposition of propene.

In this contribution we introduce a new apparatus combining high-resolution photoelectron spectroscopy and supersonic molecular beam techniques designed to follow simple surface reactions like adsorption, desorption or oxidation in situ in a time- and temperature-resolved manner. Using high brightness synchrotron radiation high resolution core-level spectra can be obtained in less than 3 s. The molecular beam allows for local pressures of up to 1 · 10^-5 mbar and translational energies of the molecules of up to 2 eV in the case of CO. Preliminary examples of CO adsorption on Pt(111) are shown, where kinetic parameters can be derived from the coverage dependence of the different adsorption sites.

We have used synchrotron radiation photoemission (SR-PES) and infrared absorption spectroscopy (IRAS) to investigate in situ the adsorption and decomposition of SiH₂Cl₂ on Si(100)(2 × 1). Si 2p core level photoemission spectra and IRAS spectra in the Si–H stretching vibration region of the surface exposed to SiH₂Cl₂ at room temperature have been measured to elucidate how SiH₂Cl₂ adsorbs on the surface. PES data show that monochloride (SiCl) and hydride species (SiH_x) are generated upon dichlorosilane adsorption. IRAS data demonstrate that at initial stages of SiH₂Cl₂ adsorption, the monohydride, the dihydride (SiH₂) and the Cl-substituted hydride (–SiHCl) are populated on the surface. Comparison of PES and IRAS data indicates that Si–Cl bonds of the SiH₂Cl₂ molecule are readily ruptured upon adsorption of dichlorosilane on the Si(100)(2 × 1).

A short overview is given of recent developments in the use of resonant scattering of polarized soft X-rays to investigate magnetic materials. A few studies are chosen to exemplify possible applications in the field of thin films and ordered multilayer structures.

We review our recent studies on the occurrence of atomic charge localization in magnetite and half-doped manganites. X-ray resonant scattering experiments at the iron K-edge of magnetite and X-ray absorption spectroscopy at the Mn K-edge of Ca-doped manganites show the lack of charge localization on the transition metal atom, either temporal or spatial. The evolution of the intensity of (002) and (006) forbidden reflections in magnetite across the Verwey transition, as a function of the incoming X-ray beam energy, the azimuthal angle and the polarization of the scattering process, indicates that the atomic anomalous scattering factors for the octahedral iron ions are identical to each other, i.e. all of them show an identical electronic state. High resolution XANES spectra of RE_1-xCa_xMnO₃ series cannot be reproduced by a linear combination of REMnO₃ and CaMnO₃ spectra. Accordingly, the electronic state of the Mn atom in these intermediate compounds cannot be described as a bimodal distribution of Mn³⁺ and Mn⁴⁺ pure ionic states. The claimed experimental evidence of charge ordering in Mn³⁺/Mn⁴⁺=1 manganites proposed from X-ray resonant scattering experiments has also been critically re-examined. We show that although these experiments prove the existence of two kinds of Mn atoms, they cannot be identified as Mn³⁺ and Mn⁴⁺ ions. Moreover, the so-called charge-ordering (CO) and orbital-ordering (OO) resonant scattering reflections are naturally explained due to the presence of local distortions without including any ordering of the Mn d-orbitals.

The inverse values of MCD signal intensity on PrFe₄P₁₂ and CePd₃ are plotted against temperatures, to be compared with the bulk susceptibility data. It was found that the MCD data are very different from the bulk susceptibility data. The "local" Kondo temperature observed through the present results is much lower than the "coherent" Kondo temperature.

We have measured the magnetic circular dichroism (MCD) of X-ray emission spectra (XES) for gadolinium and terbium in the 4d–4f resonant excitation region. Clear MCD signals in the XES were observed and they dramatically changed as the excitation energy changed. Especially, an extremely large MCD signal appeared with the ⁸D resonant excitation for gadolinium. Comparison between the MCD for XES and total photoelectron yield (TEY) shows that the XES are very insensitive to the surface state. As a result, the measurement of XES was proved to be a superior technique for estimating the genuine magnetic moment in the bulk.

Magnetic circular dichroism (MCD) spectra of spinel ferrites MFe₂O₄ (M = Co, Mn, Fe, Co and Ni) have been measured by using a photon-helicity-switching method. The method achieves high energy resolution and a low noise-to-signal ratio. The Fe L_2,3 absorption spectra of MFe₂O₄ show a similar spectral profile; on the other hand, the MCD spectra show the characteristic fine structures. These differences are attributed to the variety of 3d electron configuration of Fe ions (Fe²⁺ or Fe³⁺) and the local symmetry (tetrahedral or octahedral). The origin of the positive peak of MFe₂O₄ (M = Fe, Co and Ni) MCD spectra at the L₃ edge is attributed to the tetrahedral Fe³⁺ (3d⁵) state.

Magnetic circular dichroism (MCD) spectra of CrFe₂O₄ have been measured by using the helicity switching method. The method achieves a high signal-to-noise ratio and resolved the fine structure in these spectra. The theoretical spectra, which were calculated by the charge transfer model, reproduced the experimental spectra. The origin of three negative peaks around the L₃ edge in Cr 2p-MCD is attributed to the electron configuration in the initial state.

Magnetic circular dichroism (MCD) spectra of La_1-xSr_xCoO₃ have been measured at Co 1s and 2p XAS, La 3d and 2p XAS. The contribution of the orbital moment to the total magnetic moment of Co ions became larger with the Sr composition of the La_1-xSr_xCoO₃ system increasing. There was no polarization of the 4f orbital in La but the 5d orbital in La was polarized by the hybridization between the Co 3d orbital and the La 5d orbital.

In angle- and spin-resolved photoemission experiments, we have observed that the ferromagnetic order of the Cr(001) surface is strongly influenced by oxygen concentration. Oxygen atoms of a few atomic percent of the total volume have a substantial influence on the ferromagnetic order of the Cr(001) surface.

We have investigated electronic and magnetic properties of Co thin films epitaxially grown on Au(111) and Pd(111) substrates by spin- and angle-resolved photoelectron spectroscopy. In the Co/Au(111) system, the magnetization direction of Co changes from perpendicular to parallel to the surface at about 6 ML. The origin of the reorientation is qualitatively explained by the increasing contribution of Co 3d orbitals perpendicular to the surface. In the Co/Pd(111) system, the reorientation of the magnetization direction occurs at about 4 ML, the origin of which is explained as being due to the contribution of the upper Λ₃ band of Co with increase of film thickness as in the case of the Co/Au(111) system. The stronger hybridization between Co 3d and Pd 4d states in the Co/Pd(111) system causes larger binding energy shifts of the Λ₃ states than in the Co/Au(111) system.

Co 3d → 2p X-ray emission spectra of LaCoO₃ in the Co 2p photoabsorption region are measured at 40 and 300 K. The spectra mainly reveal features nearly independent of excitation photon energy. This indicates the highly covalent or strongly hybridized nature of the Co 3d state. The emission component showing resonant behavior is also observed. The temperature dependence of the resonant emission confirms that the wave function of Co 3d electrons is totally symmetric in the low-spin ground state and that for the spin state excited thermally through the magnetic transition around 100 K is not.

The benefits of combining soft X-ray magnetic circular dichroism and photoelectron microscopy are demonstrated by applying this combination (XMCD–PEEM) not only to magnetic domain imaging but also to quantitative evaluation of the distribution of spin and orbital magnetic moments. The latter takes full advantage of the spectroscopic aspect of XMCD–PEEM.

The angular dependence of the circular dichroism in photoemission from Pt(111) was investigated for excitation with VUV and soft X-ray radiation. VUV excitation was used to probe band structure and the circular dichroism for valence band emission. The measurements are compared to full relativistic single step photoemission calculations. XPS was used to investigate the circular dichroism in emission from the 4f core level. In this case, the dichroism is induced by photoelectron diffraction. First results from single step core level calculations are compared to the experimental observations.

The energy dependence of the circular dichroism in the angular distribution of photoelectrons (CDAD) was investigated from the shallow Cs 5p core levels. Cesium was prepared in a hexagonal ordered monolayer on W(110). The results are mainly influenced by scattering of the photoelectrons within the adlayer. At low energies, the behavior observed is described by means of a model that treats only the back-scattering of photoelectrons at the potential step between the adlayer and the substrate. It leads to a simple method determining the position of the monolayer with respect to the substrate. In this approach, the back-scattering model is matched with the measured energy dependence of the CDAD between hν = 21 eV and hν = 33 eV. This procedure yields reliable results already for a single observation angle of the photoelectron. The distance between cesium and tungsten was determined to be (2.2 ± 0.1) Å, which corresponds to the effective distance between the center of the adsorbed layer and the potential step between the layer and the substrate. Finally, the fitted result was proven to meet the complete angular dependence of the CDAD at different photon energies.

The observation of magnetic circular dichroism in the angular distribution of photoelectrons (MCDAD) from alkali and rare-gas films adsorbed on a thin remanently magnetized Co layer is reported. The observation of an antisymmetric MCDAD near normal emission for Co(0001)–Cs 5p semi core level is shown. The reversed magnetization leads to a mirror-image-like distribution. The second system under investigation, physisorbed Co(0001)–Xe 5p, does not give such an but shows significant differences in the MCDAD signal for both directions of magnetization, in particular in the |m_j| substates of the p_3/2 level. A magnetic coupling between Xe and Co is evident when we take results of spin-resolved photoemission into account.

We demonstrate the application of the advanced spin-polarized photoelectron diffraction (SPPD) method, the photoelectron diffraction measurement of specific core levels in different spins, to the investigation of the spin arrangement of magnetic overlayers on magnetic thin films. Comparing the experimental SPPD result of the Mn 3p core level with that of calculation, we have determined the possible spin arrangements of the Mn overlayer (~1 ML) on the Fe(001) thin film which was epitaxially grown on Ag(001) substrate. Consequently, one of the most plausible spin arrangements of this system is a c(2 × 2) ordering of Mn atoms on the ferromagnetic Fe substrate. The other possible spin arrangement is a p(2 × 2) ordering of Mn atoms ferromagnetically coupled with the ferromagnetic Fe substrate. In both cases, the c(2 × 2) surface reconstruction is necessary to reproduce the experimental results properly.

The valence electronic states of Fe₃O₄(111) films on Pt(111) were studied using temperature-dependent spin- and angle-resolved photoemission spectroscopy with synchrotron radiation. The majority- and minority-spin partial density of states (DOS) has been obtained at 300 K and 100 K, above and below the Verwey transition temperature (T_V ~ 120 K). The spectra still show substantial k-dependence at 100 K in the 3d region. This provides the most direct experimental evidence that the 3d electrons are itinerant even at T < T_V.

The transverse magneto-optical Kerr effect (T-MOKE) in the soft X-ray regime is used to investigate the magnetic properties of ultrathin epitaxial Fe(110) films and thermally created epitaxial Fe islands on W(110). The measurements have been carried out at the Fe 2p core levels with tunable, linearly polarized synchrotron radiation. Besides the chemical selectivity T-MOKE at core levels is characterized by a much larger sensitivity compared to the visible regime. We have analyzed the transition from thin films to a three-dimensional island system at a thickness of 4 ML and the magnetic behavior when increasing the iron coverage to 8 ML. The Fe island system is characterized by a rotation of the easy magnetization axis in this coverage regime.

By simultaneous measurements of sample resistance and of X-ray resonant scattering, we tried to establish a direct correlation between magnetoresistance (MR) and antiferromagnetic (AF) order in a Co/Cu metallic multilayer. Field-dependent scattered intensity has been measured in a geometrical configuration corresponding to the Bragg peak coming from the AF coupling, at a photon energy close to the Co L₃ absorption edge (hν = 776.5 eV). The comparison between measurements of the scattered intensity and of the resistance clearly shows a direct correlation between AF order and MR.

We present angle-, energy- and polarization-dependent magneto-optical properties of Gd/Fe and Gd/Co multilayers with periods of the order of 1 nm for the photon energy range of 700–1230 eV. In the vicinity of the 2p absorption edge of transition metals (TM) and the 3d edge of the rare earth elements (RE), the X ray magnetic circular dichroism (XMCD) was measured in transmission and the X ray resonant magnetic scattering (XRMS) was investigated in L-MOKE geometry. The magnetic state of Gd could be realized at room temperature. Based on these data a calibrated detector of the polarization state of synchrotron radiation has been developed.

We have investigated the effects of oxygen exposure on Co films grown on Pd(111) single crystal using magneto-optical Kerr effect and X-ray photoemission spectroscopy. Upon 300-langmuir oxygen exposure, the cobalt films are oxidized as CoO and a small amount of Co₃O₄, the latter being reduced to CoO on short heating at temperatures higher than 700 K. The ferromagnetic properties disappear by oxygen exposure. However, part of CoO decomposes into cobalt and oxygen on further annealing, and the ferromagnetic property reappears.

CoO layers have been grown by exposing to oxygen the (001) body-centered-tetragonal (bct) surface of a Co ultrathin film epitaxially grown on Fe(001). Different oxide thicknesses in the 2–15 ML range have been investigated by means of synchrotron-radiation-based techniques. X-ray photoelectron spectroscopy has been used to check the formation of the oxide films; X ray photoelectron diffraction has given information concerning the symmetry of their unit cell; grazing incidence X-ray diffraction has allowed to evaluate precisely their in-plane lattice constant. The films show a CoO(001) rocksalt structure, rotated by 45° with respect to the bct Co substrate, with the [100] direction parallel to the substrate [110] direction. Their in-plane lattice constant increases as a function of thickness, to release the in-plane strain due to the 3% mismatch between the bulk CoO phase and the underlying substrate.

Faraday and longitudinal magnetic Kerr rotation measurements were performed on Co and Ni single layer films around their M_2,3 edges, at room temperature under an applied magnetic field of 0.82 T generated by a magnetic circuit composed of permanent magnets. The longitudinal Kerr rotation angle spectra are in good agreement with those calculated from the Faraday rotation spectra in Ni but different in Co. The main reason for the difference in Co was explained with the difference of the magnetization between a longitudinal Kerr (saturated) and a Faraday (unsaturated) configuration under the magnetic field of 0.82 T. This was supported by the calculated result from magnetic circular dichroism (1.2 T, 140 K) measurement on Co.

Resonant spectroscopies offer a new opportunity to get more insight into excited electronic states by studying line shapes and intensities of decay processes. For photon excitations in the pre-K-edge region, the Ti KL_2,3L_2,3 Auger spectra in TiO₂ show additional peaks when an electron is promoted in localized d-like states via a quadrupolar transition. This resonant process is used to unravel the respective contributions of quadrupolar and dipolar transitions to the absorption edge prepeaks.

Magnetic circular dichroism (MCD) of core-level absorption spectra (XAS) in the soft X-ray region has been measured for the ferromagnetic Heusler alloys Co₂TiSn, Co₂ZrSn, and Co₂NbSn. The Co 2p → 3d XAS–MCD spectra show an obvious contribution of the orbital angular momentum component to the magnetic moment in these alloys. The ratio between the orbital and the spin magnetic moments derived from the MCD spectra varies by more than a factor of 2 within these alloys. The mechanism of this variation is discussed in comparison with the band structure calculation.

In order to directly investigate the energy band dispersion and the origin of the magnetic moment in intercalation compounds Fe_xTiS₂, we have measured the angle-resolved photoemission (ARPES) spectrum and the magnetic circular dichroism (MCD) of the soft X-ray core absorption (XAS) spectra using a synchrotron light source. The energy band dispersion is clearly observed in the Ti 2p_3/2 (e_g) on-resonance ARPES spectra. In the Ti 2p XAS spectra the MCD is clearly seen, suggesting that a magnetic moment is induced on the Ti atom through the hybridization with the Fe 3d orbitals. We quantitatively estimate the orbital and spin magnetic moments of the Fe and Ti atoms by using the magneto-optical sum rules.

X-ray Raman scattering from core electrons of low Z materials provides an alternative to soft X-ray absorption spectroscopy in cases where (i) exotic final states will be probed, (ii) the penetrating power of hard X rays is needed to study bulk properties, and (iii) when systems under high pressure are studied. The theoretical background and experimental requirements are discussed. The present capabilities of the technique are illustrated by two experiments, performed on the inelastic X ray scattering beamlines at the European Synchrotron Radiation Facility.

The impact of resonant scattering of polarized X-rays is illustrated taking the gadolinium 4d edge energy range as an example. It is shown that: (i) the study of the reflection of linearly polarized light by a magnetized Gd sample gives information on the electronic states of the system, similar to that given by magnetic circular dichroism in absorption; (ii) the energy-loss peaks corresponding to spin-flip transitions in the 4d → 4f excitation exhibit a strong circular dichroism.

The electronic structures of Ce compounds have been investigated by means of resonant soft X-ray emission spectroscopy (RXES) excited at resonant energy range of Ce 3d → 4f absorption. Polarization dependence of the RXES shows information on concerning electronic states. In CeO₂, the Ce 4f → 3d RXES spectra are interpreted as electronic structures hybridized between 4f⁰ and states. Peaks appearing in the spectra are attributed to bonding, nonbonding, and antibonding states between those states, while the spectra of CeRh₃ cannot be explained by only using the hybridization between 4f⁰ and states. The spectra have large broad Raman peak, especially when the excitation photon energy is set at satellite of Ce 3d → 4f absorption. We attribute the origin of the broad Raman peak to hybridization states involving electron–hole pairs.

Resonant soft X-ray scattering spectra of a La_0.5Ca_0.5MnO₃ thin film were measured at varying energies of the incident photon beam across Mn 2p thresholds. The data are analyzed within the framework of the Anderson impurity model and full multiplet theory. The scattering profiles of the film are reasonably well reproduced by model calculations for the Mn³⁺ system, in contrast to the absorption spectrum at the Mn L_2,3 edge for which the contribution of the Mn⁴⁺ sublattice is required to be taken into account. The obtained agreement between calculations and experiment indicates that in this case resonant inelastic scattering mainly probes the local electronic structure at the Mn³⁺ sites in the La_0.5Ca_0.5MnO₃ film. In turn, Mn⁴⁺ sites mainly contribute to ordinary L_α,β X-ray emission due to relaxations in the intermediate state of the spectroscopic process. Such a situation can occur when the system favors a phase separation rather than checkerboard charge ordering of Mn³⁺ and Mn⁴⁺ sites.

Recent progress in the energy and momentum resolutions in photoemission spectroscopy (PES) is remarkable, enabling a new challenge to long-standing and/or current fundamental problems in the strongly correlated electron systems. We review the history, current status, and future of high-resolution PES by showing representative experimental results on high-temperature superconductors, low-dimensional materials and heavy fermion f electron compounds.

Spin-resolved photoemission and absorption studies of Fe₃O₄ and CrO₂ epitaxial thin films have been reviewed to address the relationship between the electron correlation effects and the half-metallic properties of these two materials. Spin-resolved photoemission results suggest that Fe₃O₄ should be considered as a strongly correlated system, and that Fe₃O₄ is not a half-metal. Spin-resolved O 1s X-ray absorption measurements on ferromagnetic CrO₂ reveal that the spin polarization of the unoccupied states closest to the Fermi level approaches 100%, confirming the half-metallic ferromagnetic nature of the material. The spin polarization of the main line of the unoccupied states, on the other hand, is found to be only 50%, indicating a very atomic-like behavior of the Cr 3d² ions.

Very recently, the angle-resolved photoemission spectrum (ARPES) of [Ni(chxn)₂Br]Br₂ (chxn=1R, 2R-cyclohexanediamine), which is a typical one-dimensional Mott insulator, has been experimentally observed. The experiment has strongly suggested that a one-body gap, which is defined as double the ARPES gap, is much smaller than an optical gap given by the light absorption spectrum (LAS). The apparent difference between these two gaps indicates the breakdown of a mean field description of this strongly correlated electron system. In this paper, we show, by using a quantum Monte Carlo method, that the ARPES and LAS of [Ni(chxn)₂Br]Br₂ are consistently explained within the framework of the one-dimensional extended Hubbard model. It is suggested that the large difference between the one-body and optical gaps is caused by the dynamical Zeeman field induced by the quantum fluctuations.

Multi-Ru-site effects on the photoemission from the Ru3d core level in Sr₂RuO₄ are discussed on the basis of numerical diagonalization calculations for large-size cluster models. It is shown that the electrons on the Ru4d ions neighboring the core-hole site participate actively in the core-hole screening, which indicates the importance of so-called "nonlocal screening" in Sr₂RuO₄. However, since it occurs via the Ru4d(xy)–O2pπ bonding, the photoemission intensity which corresponds to the nonlocal screening is not large, compared with the Cu2p photoemission in high-T_c-related cuprates. From the spectral analysis, the Coulomb interaction strength between Ru4d electrons is estimated to be comparable with the width of Rudε bands. In other words, Sr₂RuO₄ belongs to a category of strongly correlated electron systems.

The physics of the Peierls compound NbSe3 is elucidated by mapping of the electron bands obtained from angle-resolved photoemission. We present experimental data on the Fermi level crossing responsible for the charge density wave along the one-dimensional axis. The data are consistent with a density functional calculation of the Fermi surfaces. The instability with periodicity q = 0.44 Å^-1 induces a backfolding of the electron bands in the nominally metallic state high above the critical temperature of 145 K.

Ce 4f electronic states in an antiferromagnetic Kondo compound CePd₂Si₂ and a strong intermediate-valence compound CeNi₂Si₂ have been investigated by means of high-resolution Ce 4d–4f resonant photoemission spectroscopy at 12 K. The relative intensity of the f_5/2¹ to f_7/2¹ final state in the Ce 4f spectrum for CeNi₂Si₂ is stronger than the relative intensity for CePd₂Si₂, indicating that the intensity of Kondo resonance scales the Kondo temperature since the Ce 4f spectrum near the Fermi level dominantly reflects the bulk component. Both Ce 4f spectra have been well fitted by the spectral calculations based on a single impurity Anderson model with consideration of the bulk and surface components. Our present calculation indicates that the model is applicable to the analyses of the Ce 4f spectra of the weakly and strongly hybridized systems.

We have performed high-resolution angle-resolved photoemission spectroscopy (ARPES) on a quasi two-dimensional (2D) conductor η-Mo₄O₁₁. The band dispersions near E_F and the Fermi-surface topology determined by ARPES show a qualitatively good agreement with the band-structure calculation, although the observed width of Mo 4d bands forming the Fermi surface is two-to-three times larger in the calculation. We have clearly identified two nesting vectors in the ARPES-derived Fermi surface, which show a good agreement with the CDW wave vectors obtained from X-ray diffraction experiments. The present ARPES results indicate that the CDW instability in η-Mo₄O₁₁ is driven by "the hidden 1D Fermi-surface nesting" due to the quasi 1D chains in the crystal.

It has been shown that the intensive and broad "red" photoluminescence band in porous silicon is a nonelementary one and could be decomposed on at least three elementary bands. Photoluminescence, ultrasoft X-ray emission spectroscopy, infrared absorption and atomic force microscopy methods were used to study the reasons for both luminescence band appearance in porous silicon photoluminescence spectra, prepared in different technological conditions. The mechanisms of radiative transition for both elementary bands have been discussed as well.

Electronic structures of hole-doped and electron hole-doped manganites, La_0.7Ca_0.3MnO₃ and La_0.7Ce_0.3MnO₃, have been elucidated by X-ray absorption near-edge structure (XANES) spectra of O K- and Mn L_3,2-edges. In the electron-doped system, La_0.7Ce_0.3MnO₃, the spectral features of O K-edge spectra are shifted by ~ 1 eV towards the higher energy side and reduction of Mn valency from 4⁺ and 3⁺ to 3⁺ and 2⁺ is expected when compared to hole-doped manganite. The XANES of Ce M_5,4-edges indicates a tetravalent state for Ce, similar to that in CeO₂. Ce⁴⁺ is incorporated into the lattice of LaMnO₃, resulting in the modification of MnO₆ octahedra and also indicating strong hybridization of Ce 5d and/or Ce 4f states with O 2p states.

Resonant L_α emission spectra of Ce compounds, CeF₃, CeO₂ and CeB₆, were measured around the L_III absorption threshold. As the energy of the incident photon is tuned at the pre-edge region and at the absorption peaks of the Ce L_III absorption spectrum, the Raman peaks are resonantly enhanced. Obtained spectra are decomposed into Raman and normal L_α emission peaks by line shape analysis. The results show that the Raman spectra provide more detailed information than the absorption spectra about the 4f configuration of Ce compounds.

We report results of the angle-resolved photoemission experiments on a quasi-one-dimensional halogen-bridged complex [Ni(chxn)₂Br]Br₂ (chxn = 1R, 2R-cyclohexanediamine), which is a one-dimensional Heisenberg spin chain with S = 1/2 and J ~ 3600 K. The expected two dispersions, originated with spin–charge separations as have been observed in other quasi-one-dimensional electron systems like Sr₂CuO₃, are not clearly observed in this compound. Instead, only one 'band" having about 0.5 eV energy dispersion is found in the half of the Brillouin zone.

Several series of oxides with Mn in a mixed valence state were studied by means of soft X-ray absorption spectroscopy in order to understand better the nature of the electronic structure of these 3d transition metal oxides. XANES spectra of REMnO₃, RE_1-xCa_xMnO₃ (RE: La, Pr and Tb) and RE_0.5A_0.5MnO₃ (RE–A: La–Ca, Pr–Ca, Tb–Ca and Pr–Sr) series were recorded at both Mn L_2,3 and O K edges as a function of temperature. The Mn 2p spectra shift towards higher energies with increase of the formal valence of the Mn atom. Moreover, the spectral shape shows minor changes both with the doping ratio (x) and the variation of the RE and/or A cations. No significant changes are also found with temperature across the different magnetic and electrical phase transitions. On the other hand, the O 1s spectra are much more sensitive to the variation of RE (A) cations. The spectra show a prepeak structure at the Fermi energy corresponding to empty states in the O-2p band. This prepeak feature shifts to lower energies and increases its intensity as the formal valence of the Mn atom increases. Finally, neither O K-edge nor Mn L-edge XANES spectra of RE_1-xCa_xMnO₃ compounds can be reproduced by a weighted linear combination of REMnO₃ and CaMnO₃ spectra, indicating that these intermediate composition oxides cannot be described as fluctuating Mn³⁺–Mn⁴⁺ mixed valence systems.

Valence-band and Yb 4d photoemission spectra of YbXCu₄ (X=In, Cd, Mg) have been measured with high energy resolution from 10 to 300 K. In the valence-band photoemission spectra of YbInCu₄ and YbCdCu₄, the structure due to the Yb²⁺4f_7/2 states is clearly observed near the Fermi level (E_F) at 10 K. The intensity of the Yb²⁺4f_7/2 structure decreases with increasing temperature and the structure almost disappears at 300 K. The amount of the enhancement from 50 to 107 K is much stronger for YbInCu₄ than for YbCdCu₄. On the other hand, the Yb²⁺4f_7/2 structure of YbMgCu₄ is observed as a broad peak near E_F and the spectra show little temperature dependence. In the Yb 4d photoemission spectra of YbInCu₄ and YbCdCu₄, the structures due to the Yb²⁺ and Yb³⁺ states are recognized. The intensity ratio Yb²⁺/Yb³⁺ increases with decreasing temperature. In the Yb 4d spectra of YbMgCu₄, on the other hand, almost only structures due to Yb²⁺ states are observed.

We have investigated the electronic structures of metal-to-insulator transitions (MIT) due to charge-density-wave phase transition in 1T-TaS_xSe_2-x by angle-resolved photoemission spectroscopy. In the insulating phase of the sample with x = 1.5, we observe a gap formation at the Fermi energy (E_F), while in the metallic phase of the sample with x = 1.2, we do not observe a drastic change in the spectra between at room temperature and at low temperature. We observe a peak originating from the lower Hubbard band in the insulating phase of the sample with x = 1.5. In the metallic phase of the sample with x = 1.2, the peak intensity is smaller and the peak energy shifts near E_F, but the lower Hubbard band peak still remains. These results suggest that MIT in 1T-TaS_xSe_2-x is due to the Mott localization.

High resolution angle-resolved photoemission spectroscopy of highly overdoped Bi₂Sr₂CaCu₂O_8+δ with T_c = 51 K indicates that the basic transport processes in this material are fundamentally different from both the lesser-doped cuprates and model metallic compounds. The overdoped sample has sharp ARPES peaks at the Fermi energy throughout the Brillouin zone even in the normal state, unlike the lesser-doped compounds. In particular, the spectra near the (π, 0) point show the presence of a sharp peak well above T_c. The ARPES line shapes, and thus the self-energy, at a given energy are almost independent of k. Further, the quasiparticle scattering rate at the Fermi energy seems to be closely tied to direct resistivity measurements. This leads us to the conclusion that overdoped Bi₂Sr₂CaCu₂O_8+δ is best described as a quasiparticle liquid. However, the energy dependence of the scattering rates is quite similar to that found in the lesser-doped compounds and quite different from that seen in a typical metal.

Measurements of X-ray emission and absorption of the constituents of MgB₂ are presented. The results obtained are in good agreement with calculated X-ray spectra, with dipole matrix elements taken into account. The comparison of X-ray emission spectra of graphite, AlB₂, and MgB₂ in the binding energy scale supports the idea of charge transfer from σ to π bands, which creates holes at the top of the bonding σ bands and drives the high-T_c superconductivity in MgB₂.

We have performed high-resolution inelastic X-ray emission scattering experiments at the Cu 3p-, Cu 3s-, and O 1s-resonances of the insulating cuprates CuGeO₃, CuO, La₂CuO₄, and SrCuO₂. We introduce the novel low-energy s-edge Cu-RIXS which reveals a dd-excitation peak, which was previously unobserved due to insufficient resolution and intensity in high-energy (Cu 1s RIXS). Also, O 1s-RIXS of all cuprate sample is investigated. Surprisingly, there is a large spread in the energy loss values of the RIXS features for different compounds and we explain this by assigning the larger energy features to the occurrence of a Zhang–Rice singlet while the lower energy feature (only observed for CuGeO₃) is assigned to a dd-excitation.

Ferromagnetism was found to coexist with superconductivity in Dy-doped BiPbSrCaCuO-2212 single crystals up to the superconducting critical temperature T_c ~ 80 K. Several experimental tests indicated that the phenomenon is intrinsic to the entire specimen rather than due to separate phases or to isolated impurities.

Gd/W(110) represents an important system for understanding magnetic coupling phenomena and the magnetic structure of rare earths. In connection with our study of quantum-well states in this system, there is a remarkable behavior of photoemission spectra at lowest coverages (0–3 monolayers). After the disappearance of W surface-derived features, a set of intense, sharp features appears in the energy region of the Gd bulk band gap. Their origin is tested using resonant photoemission at the Gd 4d–4f excitation threshold.

We performed angle-resolved photoelectron spectroscopy (ARPES) and resistivity measurements on the layered metal 1T-TiTe₂. We determined the quasiparticle dispersion and lifetime. In particular, we could characterize and separately evaluate the effects of the electron–electron and electron–phonon interactions on the experimental spectra. The good correlation between transport and spectroscopic scattering rates indicates that this material behaves as a model Fermi liquid system.

Rare earth nickel oxide perovskites (RNiO₃, R=rare earth) have, except for LaNiO₃, a metal–insulator (MI) phase transition as temperature decreases. The transition temperature (T_MI) increases as the R-ion becomes smaller. They present also, at low temperatures, a complex antiferromagnetic order. For lighter R-ions (e.g. Pr and Nd), the antiferromagnetic transition temperature (T_N) is close to T_MI, while for heavier R-ions (e.g. Eu, Sm), T_MI and T_N are very far apart, suggesting that the magnetic and electronic behaviors are not directly coupled. Although RNiO₃ perovskites are placed in the boundary of the Mott–Hubbard and charge transfer regimes, there are several evidences pointing to a charge transfer gap, mainly controlled by ligand-to-metal charge transfer energy, and thus strongly dependent on hybridization. Ni L-edge absorption spectroscopy (transition 2p → 3d) gives direct information on the density of Ni 3d empty states, and in particular on the multiplet splitting and hybridization between Ni3d and O2p bands. Here we present Ni L₃ and L₂ absorption spectra measured for NdNiO₃ and EuNiO₃ (T_MI = 200 and 480 K). At room temperature, dramatic differences are observed between EuNiO₃ (insulating) and NdNiO₃ (metallic). The normalized spectra give evidence for a higher density of 3d unoccupied states and a larger multiplet splitting in EuNiO₃. Both effects might be correlated to a decrease in hybridization. The same behavior is observed for NdNiO₃ as it is cooled down to the insulating phase (T < 200 K), revealing that in these compounds the opening of the gap is directly related to the degree of hybridization.

We have performed angle-resolved photoemission spectroscopy on a layered Mott system BaCo_1-xNi_xS₂ across the phase transition from the antiferromagnetic insulator (x = 0.18) to anomalous metal (x = 0.28). In the anomalous metallic phase, we found a large Fermi surface centered at the M(A) point with a dominant Co (Ni) 3d_3z²-r² character. Hubbard bands in the insulating phase have a remnant of Fermi surface in the metallic phase and gradually evolve into metallic dispersive bands upon carrier doping. We conclude that the electronic structure of BaCo_{1 - x}Ni_xS₂ is basically different from that of the high-T_c cuprates although the transport and magnetic properties share some common features between the two layered compounds.

The concept of the layer-resolved optical conductivity applied by means of a conventional band structure method is introduced. It is demonstrated that it allows a detailed discussion of the magneto-optical properties of magnetic multilayer systems. In particular it is found that the layer-projected optical conductivity of an atomic layer is influenced by only very few neighboring layers. This property can be exploited within the Baukasten principle, which aims to predict the magneto-optical properties of a complex layer system from the properties calculated for a closely related but simpler one.

A new theory for near-threshold photoionization of inner electrons of atoms confined in multicentered atomic formations, e.g. molecules or clusters, is developed. The formulas for fixed-in-space molecules have been derived. The interaction of the photoelectron in the continuum with atoms that surround the atom being ionized, is replaced by the suitable boundary conditions imposed on the photoelectron wave function at the location of nuclei of these atoms in the molecule. The general formulas derived are used to calculate photoelectron angular distributions of diatomic molecules. The calculated data are in qualitative agreement with experimental data and results of other calculations.

We calculate the U 5f → 3d_5/2 resonant X-ray emission spectra (5f → 3d_5/2 RXES) of a uranium compound, UO₃, by means of the Anderson impurity model with full multiplet effects. The calculation is based on the second-order optical formula, and the effect of the X-ray polarization is taken into account. The calculated results of 5f → 3d_5/2 RXES show the strong dependence on the X-ray polarization. Using the selection rule of the dipole transition with polarized X-rays, the calculated spectral features are explained.

Compton ionization of krypton and xenon outer and intermediate subshells is investigated. Results are presented covering the energy transferred to the atoms from their ionization thresholds up to 30 Ry and momentum transferred up to 4 a.u. Calculations are performed in the one-electron Hartree–Fock approximation and with many-electron effects taken into account in the random phase approximation with exchange. It is demonstrated that many-electron correlations play an important role not only in dipole but also in monopole and quadrupole transitions, significantly modifying the cross sections as well as leading to new maxima and minima.

Several cross sections of two-electron processes at high but nonrelativistic photon energies ω are considered, which are expressed solely via the initial state wave function of the ionized two-electron object. The new high precision and locally correct nonvariational wave functions describing the ground and several lowest excited states of H^-, He and helium-like ions are used in calculations of different cross sections in the pure dipole approximation and with account of first order corrections in ω/c², and a number of the cross sections' ratios. The dependencies of all these quantities on the nuclear charge Z and the principal quantum number n (for 1 < n < 5) of the initial state excitation are studied.

We report theoretical results for resonant X-ray scattering (RXS) and linear dichroism (LD) in X-ray absorption spectra at the Mn L_2,3 edge in LaMnO₃, with special attention to the observation of orbital ordering. Calculations were performed on a (MnO₆)^10- cluster with D_4h symmetry, including the effect of intra-atomic multiplet coupling in the Mn ion. Anisotropy effects are also included, through the anisotropic Mn 3d–O 2p hybridization and crystal field parameters for Mn 3d states. The agreement between theory and experiment is good. Moreover, we show that the intensity of the peaks at the L₃ edge are strongly dependent on the Jahn–Teller distortion, while the peaks at the L₂ edge are mostly related to the orbital ordering.

We have investigated the influence of optical properties of Cu(001) on the spin polarization in Cu3p photoelectron diffraction patterns. The refraction and absorption of light have been taken into account in the calculation of the dipole transition matrix element. Therefore, a general polarization vector of light is defined within a macroscopic theory of electromagnetic fields.

Results are shown and discussed for Cu3p photoelectrons excited by linearly or circularly polarized light, respectively. It is shown that the optical behavior of crystals may cause a symmetry breaking in the angular dependence of the photoelectron intensity. Besides, there are strong quantitative changes in the related spin polarization of Cu3p photoelectrons.

A version of the Kubo formula is presented, which allows one to calculate the resonant magneto-optical properties in the X-ray regime for arbitrary materials. This is achieved by representing the underlying electronic structure in terms of the electronic Green's function. Calculating the Green's function within the framework of spin-polarized relativistic multiple scattering theory (SPR-KKR) in particular gives access to spin–orbit-induced resonant magneto-optical properties. Results obtained for the Faraday effect and for the magnetic dichroic part of the complex reflective index n at the K-edge of Co and L_2,3-edges of Fe are found in very good agreement with recent experimental findings. The relation of n with the local magnetic moment is discussed on the basis of the calculated results for the L₃-edges of Pt in fcc-Co_xPt_1-x disordered alloy.

We present analytical and numerical results on the relativistic calculation of the two-photon bremsstrahlung process. For the approximate calculation of relativistic free–free two-photon matrix elements in the central field, we applied the method based on the accurate treatment of the diagonal singularities in the relativistic free–free single-photon matrix elements. Numerical calculations are carried out with the Furry–Sommerfeld–Maue wave functions. The obtained results are compared with the results of the relativistic Born approximation.

A formalism for the full relativistic treatment of the polarizational bremsstrahlung formed in collision of a charged particle with a many-electron target is developed. The multipole series for amplitude and cross section are evaluated. The presented results of numerical computation of polarizational bremsstrahlung cross sections and angular distributions show the importance of the relativistic and higher-multipole effects in the case of relativistic collision with a high-Z target. The logarithmic growth of relativistic cross section with increasing incident energy is demonstrated. The calculations are performed on the basis of the nonrelativistic and relativistic algorithms and compared with each other and with available experimental data.

We investigate the Ti and V K edge resonant X-ray scattering spectra in YTiO₃ and YVO₃ by using ab initio band structure calculation based on the local density approximation. The calculated spectra show good agreement with recent experiments, although the effect of the orbital ordering is neglected, while that of the lattice distortion is taken into account in the calculation. Particularly, the observed drastic change of the spectra for the (100) reflection between the low temperature and intermediate temperature phases of YVO₃ is nicely explained by the difference of the Jahn–Teller distortion between the two phases. These results cast doubt on an assertion that the resonant X-ray scattering is a direct observation of the orbital order in YTiO₃ and YVO₃.

We study the origin of the resonant X-ray scattering (RXS) signal near the Ce L_III absorption edge in the antiferroquadrupolar (AFQ) ordering phase of CeB₆. In this rare-earth metal compound, the intra-atomic Coulomb interaction plays a dominant role to give the large RXS intensities, which shows a sharp contrast with the Jahn–Teller distortion mechanism realized in the transition metal compound such as LaMnO₃. The temperature and magnetic field dependences show good agreement with the recent experiment, indicating that the RXS signal is a direct reflection of the AFQ order parameter.

Line shapes of photoionization satellites are theoretically investigated. Calculations were carried out for the satellite states 1s2s(³S)3s(²S) and 3s3p⁵(¹P)4s(²P) of Ne 1s and Ar 3p vacancies, respectively, created in photoionization. Theory excellently reproduces experimental line shape parameters of the first satellite and predicts the two-peak structure of the second.

We use multiple scattering photoelectron diffraction (MSPD) theory to calculate the angular patterns of electrons photoemitted from the K-shell of CO and N₂ gas-phase oriented molecules, as recently measured by several groups. For low (E < 50 eV) kinetic energies of the photoemitted electron, the electron scattering cannot be adequately represented by spherically symmetric potentials. We thus include nonspherical scattering potentials in our formalism through nondiagonal scattering matrices. We show that intramolecular scattering and interference are responsible for the experimentally measured patterns. This MSPD approach represents a more accurate and versatile method for dealing with such angular distributions as compared to prior calculations of these effects.

An extremely high quality soft X-ray beam line was constructed at BL25SU of SPring-8. A varied line spacing plane grating monochromator (VLSPGM) equipped with gratings with the central groove density of 600 and 1,000/mm can cover the energy region from 0.22 to 2 keV by the use of the fundamental radiation from the twin helical undulator. The energy resolution of VLSPGM is beyond 20,000 at 1 keV. The total resolution of practical photoelectron spectroscopy (PES) of better than 80 meV is achieved at 880 eV. The PES measurements performed above several hundred eV have shown the bulk sensitivity in many materials and provided spectra much different from the surface-sensitive spectra so far obtained below slightly above 100 eV or by He II and I light sources. As a breakthrough, bulk-sensitive angle-resolved photoemission spectroscopy (ARPES) is demonstrated to be a very powerful means of studying the bulk electronic states of correlated electron systems.

Using the technique of photoelectron spectrometry in conjunction with synchrotron radiation, we have carried out a systematic study of the partial cross sections of the main photoelectron lines arising from the ionization of the 3d and 4s subshells of atomic Fe in the vicinity of 3p → 3d autoionizing resonances. Our results confirm the presence of two broad and intense resonances which have been observed earlier. However, our high resolution results also clearly indicate the presence of numerous other weaker resonances. Through a fitting procedure we were able to accurately determine both resonance widths and positions.

The structure of the K/Si(111)-(3 × 1) surface was studied by high-resolution core-level photoelectron spectroscopy. Five surface components were observed in the Si 2p core-level spectra. Compared to the bulk component, three components are shifted to lower and two to higher binding energies. The two components with the lowest binding energies are assigned to the top-layer Si atoms bonded to the K atoms with different configurations. The component with highest binding energy has a contribution from the π-bonded Si atoms of the top layer. The two other components originate from the Si atoms of the second and third layers.

Nickel (111) and the interface of hexagonal boron nitride on Ni(111) serve as model systems for an itinerant ferromagnet that is truncated with vacuum or a single layer insulator. These systems are investigated with angle-scanned He I and He II photoemission. Upon formation of the h-BN/Ni(111) interface the work function drops by 1.8 eV and the minority Ni Λ₃d-band shifts by 0.13 eV to higher binding energy. This indicates that the Ni magnetic moment decreases in the interface. Spin-polarized band structure calculations from bulk nickel identify the observed minority and majority bands. The experimental Fermi surface maps show that h-BN distorts a minority d-band in a way which is consistent with the decreasing magnetic moment. It can be seen that h-BN affects the scattering of the electrons. Effects in the photoemission final and initial state are recognized.

Synchrotron-radiation-excited angle-resolved photoemission spectra for the (100) and (111) faces of AuAl₂ and the (111) face of PtGa₂ are presented, along with derived experimental dispersion relations. These bands are compared with those obtained from the first-principles theoretical band-structure calculations. The total densities of states at the Fermi energy for these two materials are compared with those obtained from the specific-heat measurements. The energy variations of Au and Pt 5d bands for AuAl₂ and PtGa₂, respectively, show atomic-like behavior in the photon-energy range of 80–220 eV.

We have measured the 4d–4f high-resolution resonant photoemission spectra of RFe₄P₁₂ (R = La, Ce, Pr), PrSn₃ and PrAl₂. The Ce and Pr 4f spectra show the spectral features corresponding to a strongly hybridized system. However, the and the ³H₄ peaks located just below the Fermi energy in the Ce 4f and Pr 4f spectra, respectively, are observed as a shoulder instead of the prominent peak expected for the strongly hybridized or high-Kondo temperature systems.

Photoelectron spectra of compounds having a linear carbon chain terminated by phenyl groups to stabilize the chain have been measured. The spectra consist of the contribution of phenyl groups and that of the chain part, which indicates small interaction between them. The highest occupied molecular orbital (HOMO) of these compounds derives from the carbon chain part. The longer the chain length becomes, the lower the binding energy of the HOMO. This fact implies the instability of a very long linear carbon chain. The spectra are well explained with the aid of semiempirical molecular orbital calculations.

Photoemission valence band spectra of submonolayer fullerene C₆₀F₄₈ films, evaporated in situ on d-metal foils of Au, Pt, Ta, W, Ni, were acquired under He II (40.8 eV) excitation at 300 K. Spectral structure transformations versus film thickness and substrate material were under investigation. For coverage ~ 0.5–1.0 ML C₆₀F₄₈ the main spectral features were found to be close to those measured for thick films: with a main peak at E_b ≈ 9 eV (F2p) and a broad peak at 11.4 eV (C2p or C2s). However, a new low intensity band with E_b ≈ 1.5 eV (FWHM ≈ 1.4 eV) was found to arise at a film thickness of about 0.4–0.5 ML and to decrease beyond 1 ML. It may be attributed to a strong ion-covalent bonding between d-metal and fullerene at low coverage. For coverages exceeding 0.3 ML the spectra were found to be weakly dependent on substrate material, except for Ni, where this new band was found at E_b = 2.2 eV and the main peak at E_b = 9.7 eV.

Resolving the 3D wave vector k in photoemission mapping of the band structure E(k) requires knowledge of the unoccupied final states. Both dispersions and lifetimes of these states can be achieved by very-low-energy electron diffraction (VLEED). By incorporating the non-free-electron and excited-state effects in the final states, combining VLEED with photoemission provides accurate mapping of the valence E(k) resolved in the 3D wave vector and under control of the intrinsic accuracy. We here concentrate on the most accurate combined method which uses angle-dependent VLEED and photoemission measurements. It provides access to many Brillouin-zone lines using one crystal surface, and benefits from an intensity gain and a better intrinsic accuracy near the Fermi level.

We report on self-energy effects in the electronic structure of Cu as a prototype weakly correlated system containing electron states of different localization. The unoccupied and occupied excited states were mapped fully resolved in the three-dimensional k using very-low-energy electron diffraction and photoemission, respectively. The self-energy corrections to the density-functional theory show distinct band- and k-dependence. These results are well described by quasiparticle GW calculations, especially for less localized states. We find correlation of the self-energy effects with spatial overlap of the one-electron wave function with the electron density, and elucidate the essential physics of this effect based on the electron-gas exchange correlation behavior.

We already reported the spin- and angle-resolved photoelectron measurements of Ni(110), Ni(110)-p(2 × 1)O and Ni(110)-c(2 × 2)S along the line of the Ni(110) surface Brillouin zone using He I radiation. We found that the structure at about 1.3 eV binding energy showed unusual character near the point. The spin polarization of this structure was observed to be negative. After the adsorption of oxygen and sulfur on the Ni(110) surface, the degree of negative spin polarization was found to decrease. To study the property of this state, that it is a bulk or surface-related, and to clarify the origin of the negative polarization, we performed spin- and angle-resolved photoemission measurements for Ni(110) along the LW line of its bulk Brillouin zone. The results show that this negative polarization structure can be attributed to a surface state near the point.

This work presents the results of experimental complex study of electronic excitation dynamics for single BeO crystals. The decay kinetics (1–500 ns) of the photoluminescence, reflectivity (8–35 eV), time-resolved emission (2.5–10.4 eV) and luminescence excitation (6–35 eV) spectra were measured at 10 K for the oriented BeO crystals. The diversity among the channels of the radiative relaxation of the electronic excitations as well as the various multiplicity of the excited states of self-trapped excitons, depending on the crystal orientation and exciting photon energy, are discussed.

Core-level photoelectron spectroscopy with the combination of synchrotron radiation (SR) and a laser was used for exploring the surface-photovoltage (SPV) effect and its temporal profiles in a GaAs–GaAsP superlattice (SL). It was observed that the SPV value in the SL is suppressed as compared with a bulk GaAs. However, no significant difference was found in the temporal profile between the bulk and the SL. It is suggested that the suppression of the SPV in the SL is dominantly due to the small value of band bending under thermal equilibrium.

The nature of Na 1s photoabsorption in NaF and NaCl is investigated using the resonant Auger decay of Na 1s core excitations. The appearance of new peaks in Auger spectra, when the energy of excitation coincides with the photoabsorption thresholds, shows that the core excitons are created in the threshold region of the Na 1s photoabsorption spectrum. The core excitons around 1074.2 eV originate from the dipole-forbidden 1s^-13s final state, the population of which is made possible by the symmetry disruption at the photoabsorption site. Another core exciton, excited at photon energies of 1076.8 eV in NaF and 1076.3 eV in NaCl, originates from 1s^-13p states similar to atoms of Ne and Na. Resonant Auger spectra show also that in NaCl the first sharp photoabsorption maximum at 1076.6 eV has only partially excitonic character.

Boron K-edge soft X-ray emission and absorption are used to address the fundamental question of whether divalent hexaborides are intrinsic semimetals or defect-doped band gap insulators. These bulk-sensitive measurements, complementary and consistent with surface-sensitive angle-resolved photoemission experiments, confirm the existence of a bulk band gap and the location of the chemical potential at the bottom of the conduction band.

The valence-band and conduction-band electronic structure of pyrite-type CoSe₂ has been investigated by means of ultraviolet-photoemission, synchrotron-radiation-photoemission and inverse-photoemission spectroscopies. The fully occupied Co 3d states with t_2g symmetry are observed as a main peak at -1.0 eV relative to the Fermi level (E_F), and the partially filled Co 3d states with e_g symmetry are observed as a shoulder at the E_F side of the main peak. On the other hand, the structure due to the unoccupied Co e_g states appears at 0.6 eV in the inverse-photoemission spectrum. The electronic structure of CoSe₂ is similar to that of CoS₂. The electron correlation is more important for CoSe₂ than for CoS₂.

Angle-resolved photoemission in the soft X-ray region has so far been used to study crystal (surface) structure by using photoelectron diffraction phenomena. In this energy region, the observation of the band structure was considered to be difficult, because the momentum of the incident photon cannot be negligible, the emitted electron is strongly influenced by the electron diffraction effects, and the cross section of the valence band is very small. We have performed the angle-resolved photoemission from the valence bands of highly oriented pyrolytic graphite by using the soft X-ray (hν=980 eV) and successfully observed the band structure. The appearance of the structures of the band density of state is also discussed. Since the escape depth for the electrons with high kinetic energies is longer than ARUPS, this experimental method is better than the usual ARUPS in order to study the bulk band structures in solids.

We have performed Co 2p X-ray absorption spectroscopy (XAS) on a mixed valence hexagonal layered system NaCo₂O₄, which shows large thermopower and metallic conductivity. We also measured XAS for SrCoO₃ and LaCoO₃ as examples of typical Co ionic states. The Co ions of the last two materials are octahedrally surrounded by oxygen atoms. A Co atom in NaCo₂O₄ also has octahedral coordination with oxygen atoms but it is distorted slightly. The Co ions in SrCoO₃ and LaCoO₃ are in the d⁵ and d⁶ configuration respectively. The spectrum of NaCo₂O₄ resembles that of LaCoO₃ rather than that of SrCoO₃; the Co ions in NaCo₂O₄ can be regarded as the d⁶ configuration. Also, we have tried to calculate the XAS spectra by using the charge-transfer multiplet model (CT-M). Although we could not find out the best-fitting parameters, the results suggested the itinerancy of the Co 3d states.

Valence-exciton luminescence under inner-shell excitation of the rare gas solids Xe, Kr, and Ar has been measured using time-resolved photoluminescence. Two different processes for exciton creation can be distinguished: creation of "prompt" excitons immediately after excitation (within the experimental time resolution), and creation of "delayed" excitons through electron–hole recombination. The decay structure of the exciton emission in the range of inner-shell excitation is characterized by the coexistence of the two processes. Time-resolved excitation spectra near the 2p edge in Ar, the 3d edge in Kr, and the 4d edge in Xe are discussed. The process of prompt exciton creation is strongly enhanced above an excitation threshold at the energy position of the ionization limit of the core state plus the energy of the valence free exciton.

We have investigated the reduction of initial C₆₀ electronic structure under the influence of a growing number of attached fluorines in C₆₀F₄₈, C₆₀F₃₆, C₆₀F₂₄, and C₆₀F₁₈. The reduction of the low-energy π-electron subsystem reveals itself in lowering of the optical absorption in the visible region and in gradual blueshift of the luminescence. The absorption of C₆₀F₂₄ and C₆₀F₁₈ films strongly resembles that of C₆₀, thus showing the similarity of electronic structures of C₆₀ and these compounds. This resemblance disappears in the absorption spectra of C₆₀F₄₈ and C₆₀F₃₆. The absorption of these compounds looks like that of C₆H₆, because the π-bonds are isolated.

The luminescence of C₆₀F₁₈ film has an unexpected two-component structure, with the first component close to the C₆₀ luminescence spectra, and the second one lying pproximately in the same region with C₆₀F₃₆. We attribute this splitting of luminescence spectra to the two different channels for relaxation of the electronic excitation. The first one is correlated with the fluorinated part of the C₆₀F₁₈ molecule, the second one with the part free of fluorine atoms.

Results of X-ray fluorescence measurements of MA₂Ca_n-1Cu_nO_{2n + 3} compounds with [M-12(n-1)n] type structures, (C_0.8N_0.2Sr₂CuO_5.3, [Cu_0.25(C,N)_0.75]Sr₂CaCu₂O_y, Cu_0.5V_0.5Sr₂CaCu₂O_7.5, Cu_0.5V_0.5Sr₂Ca₂Cu₃O_9.4, Cu_0.5V_0.5Sr₂(Ca_3.6Sr_0.4)Cu₅O_13.5 and Cu_0.5V_0.5Sr₂(Ca_4.7Sr_0.3)Cu₆O_15.3, are presented. The experiments show that C-atoms form a distorted CO₃ oxyacid group with a partial substitution of copper and nitrogen atoms in the M-sites. We conclude that the CO₃^2- group in (C,N) Sr₂Ca_n-1Cu_nO_y and (Cu,C,N) Sr₂Ca_n-1Cu_nO_y is substituted by the NO₂^- which induces the creation of the hole carriers in these compounds. We have found that V-atoms in the given compounds form (VO₄)^3- tetrahedrons and have a pentavalent state.

Absorption spectra of solid HCl, HBr and HI films deposited on LiF single crystals cooled at 103–104 K and those annealed have been obtained in the 4–11.5 eV region. The first peaks found around the absorption edges correspond to the first bands (dissociative) in isolated molecules, so that they are regarded as Frenkel exciton peaks. The spectral feature of as-deposited films changed after annealing irreversibly, which suggests the structural transition from an amorphous phase to a crystalline phase in HCl and HBr. In HI, it was revealed that the first band in the gaseous phase consists of three components. The broad structures found above the first peaks do not resemble the structures of the isolated molecules in the same energy region which consist of many lines. The band structures of solid hydrogen halides seem to resemble those of sodium halide crystals and solid rare gases.

The valence-band and conduction-band electronic structure of CoSb₃ crystals with a skutterudite type has been investigated by means of ultraviolet photoemission (UPS) with a He discharge lump (hν = 21.2 and 40.8 eV), synchrotron-radiation photoemission (SRPES) and inverse-photoemission spectroscopies (IPES). In the valence bands, the structures at -1.1, -2.5 and -5.8 eV relative to the Fermi level (E_F) are observed in both UPS spectra and SRPES spectra. Taking into account the photoionization cross sections of Co 3d and Sb 5p states and the crystal field splitting of Co 3d states due to the Sb-square ring around Co-ion sites, the main peak at -1.1 eV is attributed mainly to the Co t_2g states, while the Sb 5p states are expected to contribute to the structures at -2.5 and -5.8 eV with a small admixture of Co 3d states. On The other hand, in the conduction bands, a broad feature is observed from E_F to ~ 4 eV and a dip is slightly found at around 1 eV. The structures near E_F and at around 3.5 eV are considered to be due to the Co e_g and hybridization bands between Sb 5p and Co 3d states, respectively.

The creation of primary and secondary cation excitations was investigated for MgO and BaF₂ crystals using the energy-and time-resolved luminescence method. It was shown that the primary cation excitations in MgO created by photons of 50–65 eV decay with the formation of anion free excitons and electron–hole pairs. The formation of secondary cation holes by hot photoelectrons in the Ba²⁺ 5p shell causes the increase of cross-luminescence intensity whereas the creation of neither primary nor secondary cation excitons causes the appearance of cross-luminescence in BaF₂.

Resonant soft X-ray emission (RSXE) spectra of V₂O₃, VO₂ and NaV₂O₅ were recorded for a series of excitation energies at resonances of the V L- and O K-absorption band. The V L- and O K-emission in these vanadium oxide bands possess considerable overlap. By resonant excitation we can tune the energy to the absorption thresholds, thereby eliminating this overlap. Hereby we obtain the V 3d and O 2p projected density-of-states of the valence band. Resonant inelastic X-ray scattering (RIXS) is found to be weak in V₂O₃, which we explain as being due to its metallic character at room temperature. Vanadium dioxide (VO₂), semiconducting at room temperature, shows considerable RIXS features only at the O K-emission band. Distinct RIXS structures are visible in the RSXE spectra of the insulator NaV₂O₅. In the emission spectra excited at the V L-thresholds of this ternary vanadium oxide, dd-excitations of the V d_xy subband at an energy loss of -1.7 eV are observed. Our observation, that RIXS is stronger for insulators than for metals, should be taken advantage of for studying insulator-to-metal transitions in vanadium compounds in the future.

Using angle-resolved photoemission, we observe quantum-well-state formation as pronounced peaks up to large thicknesses in epitaxial Ag and Au films on W(110). These systems serve as a test case for the assignment of sp and d orbital character to quantum-well states since overlap of sp with d states is strong for both systems but differs in detail. Starting out from peak assignments on the basis of the extended phase accumulation model, the angle dispersion of several features is discussed in detail and related to the orbital character.