Bremsstrahlung Isochromat Spectra Research Articles

Theory of temperature-dependent photoemission in 3d-band ferromagnets: Application to Ni(110) and Ni(111).

The effects of electron correlations and phonons on the temperature dependence of ultraviolet photoemission and bremsstrahlung isochromat spectra of ferromagnetic Ni are investigated. The spin- and temperature-dependent photocurrent is calculated in the framework of a generalized one-step model of photoemission. Phonons are then into account via temperature-dependent phase shifts. The hole state of photoemission is described by an energy-, spin-, and temperature-dependent self-energy, which rests on a combination of density-functional theory and a Hubbard-type multiband model

Comment on "Observation of Kondo resonance in YbAl3"

We show that single-impurity Anderson-model parameter values obtained by Oh et al. [Phys. Rev. B 37, 2861 (1988)] from an analysis of their bremsstrahlung-isochromat spectra of ${\mathrm{YbAl}}_{3}$ are incompatible with their photoemission spectra also presented in the same work.

BIS and XPS Study of YbP, YbAs and Yb4As3

X-ray bremsstrahlung isochromat spectra (BIS) as well as photoemission spectra (XPS) are measured for YbP, YbAs and Yb 4 As 3 . The BIS spectra have revealed clear differences in the Yb 4f and 5d states between the Yb-monopnictides and Yb 4 As 3 . Yb 4p and 5p core XPS spectra have shown the mixed valence of Yb 4 As 3 compared with the predominance of Yb 3+ in Yb-monopnictides.

X-ray emission and absorption studies of silicides in relation to their electronic structure

The valence bands and conduction bands of about 30 transition metal silicides (of which we concentrate on 4 here) have been investigated by measurements of Si X-ray emission bandsspectra, X-ray absorption spectra near the Si K (1s) edge, photoemission spectra, and Bremsstrahlung Isochromat spectra. The densities of states have also been calculated for the materials in their real crystal structures. The influence of the core hole on some spectra has been investigated using supercell calculations, a (Greens function) generalized Clogston-Wolff model, and Auger spectroscopy. A selection of results is presented to illustrate the utility of site and selective methods in investigations of the electronic structure of silicides and the nature of the "quasi-gap" of the partial density of Si p states in the region of the transition metal d bands.

A simple parallel isochromat detector for inverse photoemission

A simple parallel detection system which allows several bremsstrahlung isochromat (inverse photoemission) spectra to be recorded simultaneously is described. It is based on the dispersive properties of a LiF lens in the vacuum ultraviolet previously described by Childs et al. [13] and operates in the photon energy range of approximately 8.0-11.5eV. In the instrument described three isochromats can be recorded at photon energy separations of 1.5eV. Its utility is demonstrated with measurements on Cu(111), the different photon energies permitting easy confirmation of the contributions of surface-localised states.

Calculated photoemission and many body effects in 3d-ferromagnets

The influence of electron correlations on the temperature dependence of ultraviolet photoemission and bremsstrahlung isochromat spectra of ferromagnetic Ni is investigated. The calculations are performed within the framework of the one-step model of photoemission using an expression for the energy-, spin- and temperature-dependent self-energy of the hole state, which rests on a combination of the results of density functional theory and those of a generalized Hubbard model. The model contains only two parameters, the Hubbard U and the interband exchange J. It is approximately solved by use of a selfconsistent moment method and has so far resulted in an excellent description of macroscopic magnetic properties, like the magnetic moment at T = 0, the Curie temperature, a Brillouin-like magnetization curve and a strict Curie-Weiss behaviour of the paramagnetic susceptibility. In this contribution we demonstrate the virtues of this theory for quasiparticle properties in Ni by calculating spin-polarized and temperature-dependent photoemission spectra and comparing with experimental results available in literature. The theoretical photoemission results for the occupied d-bands are shown to be in excellent agreement with the experiment. Since the ferromagnetism and the many body effects in Ni are mainly caused by the uppermost d-band, which is only partly occupied, we also calculate the spin-polarized inverse photoemission spectra for this subband and compare with the most recent experimental results obtained in bresstrahlung isochromat spectroscopy.

Real-space, multiple-scattering-theory method for the calculation of x-ray bremsstrahlung isochromat spectra.

We introduce a new method for the calculation of the x-ray bremsstrahlung isochromat spectra. The method is worked out within a real-space multiple-scattering theory and disposes of the conceptual and practical difficulties inherent in the band-structure calculations. Our concept makes possible to calculate spectra in both near and extended energy regions even for systems lacking translational symmetry. The method is illustrated through a calculation of four isochromat spectra of Cu and Pd. For the first time the extended x-ray-isochromat fine structure is calculated.

Phase and amplitude analysis of EXBIFS as compared to EXAFS

X-ray bremsstrahlung isochromat spectra of Cu, Ni, Pd, and Ag measured at a photon energy of 5415 eV within an extended energy range (EXBIFS) were analysed by using standard EXAFS methods and quantitatively compared with the K-shell EXAFS's.

Relationship between Auger and autoionization processes.

The energy difference between Auger and autoionization processes is shown to be readily calculable and to correlate with electronegativity. Furthermore, a comparison of bremsstrahlung isochromat spectra of unfilled states just above the Fermi level with high-resolution autoionization spectra for the transition-metal and lanthanide series has demonstrated a correlation between them.

The energy dependence of the Bremsstrahlung transition in solid materials

The energy dependence of the Bremsstrahlung transition in the X-ray region is investigated within the framework of inverse photoemission. Theoretical Bremsstrahlung isochromat spectra (BIS) are calculated for solid Ni and Pd and compared with experimental BIS data reported for isochromat energies between 0.5 and 5 keV. It is shown that this formulation provides a detailed description of the BIS measurements for all the isochromat energies considered. In particular it is demonstrated that the drastic redistribution of spectral weight observed for the 3d and 4d transition metals is related to a strong dependence of the transition-matrix elements for d symmetry. This affects the analysis of BIS data not only for the unoccupied states near the Fermi level but also over a wide energy range.

Selection rules and extended x-ray bremsstrahlung isochromat spectra of Cu and Pd

We have calculated matrix elements for bremsstrahlung isochromat spectroscopy for the extended energy 50-380 eV above the Fermi level. We have found that the transition rate depends on the atomic number, photon energy of the isochromat, and the angular momentum quantum number of the final-state wave function. The ratios of $s$, $p$, and $d$ components of transition probability for the photon energies 1487 and 5415 eV are presented for Cu and Pd. We have demonstrated that the decelerated final electron originates in the core region of an atom, and we have explained why the main information to be gained from studying fine structure in the extended region of x-ray bremsstrahlung isochromat spectra is actually structural.

Interpretation of photoemission and bremsstrahlung isochromat spectra of uranium compounds.

Bremsstrahlung isochromat (BIS) and U 4f core-level photoemission (PE) spectra of UPt and ${\mathrm{UPt}}_{5}$ are presented and compared with corresponding data---including U 5f valence-band PE spectra---of ${\mathrm{UBe}}_{13}$, as well as with the results of band-structure calculations. The shapes of the BIS, U 5f, and U 4f PE spectra are consistently explained in terms of two differently screened final states, namely an f-screened main peak reflecting the delocalized 5f part of the electronic ground state, and a poorly screened satellite increasing in intensity with decreasing 5f hybridization. The considerably higher relative intensity of the f-screened main peak in 5f PE as compared to BIS spectra is readily explained on the basis of the degeneracies of the relevant 5f screening orbitals in U compounds. The positions of the poorly screened satellites relative to the f-screened main peaks in BIS spectra are found to be related to those of the Pr 4${f}^{3}$ BIS multiplets in homologous Pr compounds, i.e., they reflect chemical shifts. It is also realized that the relative intensities of these BIS satellites follow those of the satellite peaks in U 4f core-level PE spectra.

The empty electron-states in MoS 2: An inverse photoemission spectroscopy investigation

k-integrated Bremsstrahlung isochromat spectra, collected at different UV-photon energies (hv=13.2−24 eV), from 2H-MoS 2 are presented. Two structures (energy separation=1.2 eV) of d-character are resolved close to the Fermi level. A third broad spectral feature appears at higher energy values and is assigned to hybridised metal-cation sp derived states. The results are compared to different electron-states calculations and are found to give support to the theoretical predictions derived within a LCAO scheme.

Bremsstrahlung-isochromat study of the layered compounds InSe, TiSe2, SnSe2, SnS2, and Bi2Te3.

We present bremsstrahlung-isochromat spectra (BIS) for the layered compounds InSe, ${\mathrm{TeSe}}_{2}$, ${\mathrm{SnS}}_{2}$, ${\mathrm{SnS}}_{2}$, ${\mathrm{SnSe}}_{2}$, and ${\mathrm{Bi}}_{2}$${\mathrm{Te}}_{3}$ at a photon energy of 1486.6 eV. The spectra exhibit several sharp peaks in the lower conduction bands. These spectral features are compared with theoretical calculations of the density of states and with optical data. In several cases, discrepancies between the BIS spectra and the optical spectra confirm that large core excitonic shifts occur for layered compounds.

4f multiplets in bremsstrahlung-isochromat spectroscopy as a probe for local electron density in Ce compounds.

The splitting of 4${f}^{2}$ final-state multiplets in bremsstrahlung-isochromat spectra of Ce compounds is found to vary between 0.8 and 1.2 of the ${\mathrm{Ce}}^{2+}$ free-ion value. Similar, but smaller variations are observed for Pr compounds, and no variations are found for systems of Nd and Sm. These observations are caused by changes in the nuclear potential, effective for the radial distribution function of the 4f electrons, both by delocalization of Ce core electrons and by valence-electron screening.

Electronic structure of some 3D transition-metal pyrites

Bremsstrahlung Isochromat spectra of FeS2, NiS2 NiS1.2Se0.8 and NiSe2 are reported. These are the first direct experimental evidence for a sharp antibonding p-like state above the Fermi level. A comparison is made with experimental results in the literature. For FeS2 band structure calculations are also presented; these are in good agreement with experiment. The authors show that it is necessary in this calculation to include 'empty spheres' when using a muffin-tin approximation. This is because the pyrite crystal structure is far from close packed. Furthermore, they briefly discuss the electrical and magnetic properties of the compounds.

Electronic structure, chemical bonding and spectral properties of the intermetallic compounds FeTi, CoTi and NiTi

Self-consistent PAW band-structure calculations were performed for B2 FeTi, CoTi and NiTi. Using the resulting electron and difference electron densities together with partial local densities of states and band charges, the covalent bonding contribution is shown to increase distinctly in the series NiTi-CoTi-FeTi. Structure factors are extracted from the calculated electron densities and they can be compared with measured X-ray diffraction data as soon as the latter become available. X-ray emission and absorption spectra as well as angle-integrated photoelectron and bremsstrahlung isochromat spectra are calculated from the APW eigenvalues and wavefunctions and compared with measured spectra from the literature, if available. For the XPS of NiTi some discrepancies between theory and experiment can be eliminated by applying an estimated self-energy correction to the eigenvalues before the calculation of the spectrum.

Theory of Photoemission, Photoabsorption and Inverse Photoemission in CeO2 and Metallic Ce Compounds

Ce 3d core X-ray photoemission (3d-XPS), 3d core photoabsorption (3d-XAS), 3d core resonant photoemission (3d-RXPS) and bremsstrahlung isochromat spectra (BIS) in a mixed valent insulator CeO2 and typical metallic mixed valent Ce compounds such as CeRh3 are studied theoretically on the basis of the Anderson model. By assuming that the weight of 4f0 configuration in the ground state, the 4f-valence band mixing and the 4f-4f Coulomb interaction for CeO2 are considerably larger than those for metallic Ce compounds, a remarkable difference in 3d-XPS and an apparent similarity in 3d-XAS between the two systems are consistently explained. Furthermore differences in 3d-RXPS and in BIS between the two systems expected from the present analysis are predicted. A preliminary discussion about the effect of 4f-core hole and 4f-4f exchange interactions on core absorption spectra in mixed valent systems is also presented.

Recent spectroscopic studies of UO2

Recent spectroscopic results obtained on UO2 single crystals are reviewed. Special emphasis is put on the various optical spectroscopies such as reflectivity, absorption, Faraday rotation, magnetic circular dichroism and Raman scattering, but also X-ray photoemission, bremsstrahlung isochromat spectra and neutron scattering data are discussed to show the state of the art of our knowledge of the electronic structure of UO2.

Electron spectroscopy study of the heavy Fermion compound URu 2Si 2

We report x-ray photoemission, resonant photoemission and bremsstrahlung isochromat spectra of the 4f core levels, the valence band and the conduction band, respectively, of the heavy fermion compound URu 2Si 2. The resonant photoemission behavior of the uranium 5f weight resembles that of cerium 4f weight in that the portion away from the Fermi energy has a delayed resonance relative to that at the Fermi energy. We also assess current efforts to interpret uranium spectra.