Shallow Core Levels Research Articles

Soft X-Ray Emission and Resonant Inelastic X-Ray Scattering Studies of Transition Metal Oxides.

ABSTRACTThis paper discusses the application of soft x-ray emission and resonant inelastic x-ray scattering as probes of the electronic structure of transition metal oxides. The results of studies on the narrow gap insulator CdO, and the Mott-Hubbard oxide Cr-doped V2O3, are reported. The O 2p valence band partial density of states for CdO has been measured, and emission due to the hybridization of the O 2p valence band states with Cd 4d shallow core level has been observed. For Cr-doped V2O3, the temperature induced metal-insulator transition in samples with 1.5% Cr was observed using soft x-ray emission, and both charge transfer and dipole forbidden d-d excitations were observed using resonant inelastic x-ray scattering.

Photoemission spectroscopy study of the charge accumulation at the K3Sb:Cs surface

The alkali antimonide compounds are widely applied in light detection devices. The strong optical absorption and the low electron affinity make the photon–electron conversion very efficient in these materials. In this paper, we present the results of a photoemission spectroscopy study of K3Sb films of 100Å with and without an extra layer of 0.1ml Cs. The shallow Sb 4d core levels show a chemical shift for K3Sb, which is only very slightly affected by the extra Cs layer. The results are explained as a consequence of charge accumulation at the K3Sb surface, which is induced by the Cs surface states.

Separation dynamics of luminescence from Raman scattering in characteristic X-ray radiation processes of Y compounds

Abstract We theoretically study the separation dynamics of a luminescence from Raman scatterings in the spectrum of the X-ray radiation occurring by an electronic transition from a shallow core level to a deep one in insulating solids of Y compounds. We consider a many-body problem in a four-band model composed of dispersionless deep and shallow core bands, and conduction and valence bands. The Coulomb interactions among the conduction electrons and the valence holes are taken into account by the lowest-order perturbation theory. With this model, it is clearly shown that the radiation spectra have three components; a luminescence part and two Raman scattering parts. This result agrees with the recent experiments of YF 3 and YCl 3 . We also show the origin of the three components cannot be explained by a one-body picture.

Dynamics of electronic relaxation in characteristic x-ray radiation processes in wide-gap insulators: Separation of luminescence from Raman scattering

Electronic relaxation dynamics on the separation of luminescence from Raman scatterings are theoretically studied in the x-ray radiation process from a shallow core level to a deep one in a wide-gap insulator. A four-band model composed of dispersionless deep and shallow core bands, and conduction and valence ones, is taken as one of the simplest examples. The Coulomb interactions among the conduction electrons and the valence holes are taken into account by the lowest-order perturbation theory. By these interactions, a conduction electron is scattered together with creation of a valence-hole--conduction-electron pair. Using this model, we calculate a second-order optical process composed of an excitation of an electron from the deep core level to the conduction band by an incident x-ray, and a subsequent transition from a shallow core level to a deep one by radiating another x ray. When the incident x-ray energy is below the absorption edge, the resultant radiation spectra have only one peak due to a Raman scattering. On the other hand, when the incident energy is above the edge, the radiation spectra separate into two peaks: a luminescence peak, and a Raman scattering peaks on the higher-energy side. The luminescence is considered to occur after the following electronic relaxation. The photoexcited electron enters the conduction band by creating a valence-hole--conduction-electron pair through the Coulomb interactions. Afterwards, this pair goes away from the original site, and it results in a dissipation. From this scenario, the calculated x-ray radiation spectra agree with experimental results. We have concluded that the conditions to obtain luminescence is that the conduction- and valence-band-widths are finite, and the lifetime of the deep core hole is long enough. With this scenario, we also qualitatively explain the characteristic x-ray radiation spectra in metals, semiconductors, and atoms. Furthermore, we discuss the spectral shapes in the Auger decay process competitive with the radiation one.

Influence of the core polarization on the dielectric properties of polyvalent metals

We have measured the dispersion of plasmon excitations in polycrystalline Sn, Pb, Cd and Zn using electron energy-loss spectroscopy (EELS) in transmission. These metals show a remarkable deviation of their plasmon behaviour from standard theoretical expectations which can be attributed to the presence of shallow core levels in the energetic neighborhood of the plasmon energy. The position of the plasmon resonance in the long wavelength limit is in line with predictions from model assumptions taking into account the core level polarization effects. We show that not only the plasmon energy in the optical limit but also the plasmon dispersion is altered by the interaction of the charge carrier plasmon and the core level excitations.

ELECTRONIC STRUCTURE AND CHEMICAL BONDING IN ALKALINE EARTH METAL SUBNITRIDES: PHOTOEMISSION STUDIES AND BAND STRUCTURE CALCULATIONS

The electronic structure of various alkaline earth metal subnitrides has been studied by X-ray and ultraviolet photoelectron spectroscopy and by LMTO band structure calculations. Improved methods for the synthesis of several of the materials are described. The electronic structure of compounds with discrete clusters (Na 22Ba 14SrN 6) or Ba 3N chains (Ba 3N, NaBa 3N, Na 5Ba 3N) in the crystal structure is characterized by a narrow atomic-like N-2p band occurring between 2 and 2.5 eV in the photoemission spectra. The spectra and the results from the band structure calculations support the idea of ionic bonding between N 3− ions and surrounding partly oxidized alkaline earth metal atoms. The N-2p regions in valence band spectra and density-of-states curves of Ca 2N, Sr 2N and Ba 2N, which have layered crystal structures, extend over a broader energy range, suggesting a more pronounced dispersion of the N-2p band in these compounds. Several feature seen in NeI, HeI, NeII and HeII spectra of the series M 2N are assigned to Auger transitions involving a primary hole in a shallow core level (N-2s or M- np) and either electrons from the metal valence band or from N-2p levels.

Separation dynamics of characteristic X-ray radiation into luminescence and Raman scattering in wide gap insulators

Abstract We theoretically study the separation dynamics of luminescence from Raman scattering in the spectral shape of X-ray radiation from a shallow core level to a deep one in wide gap insulators. We employ a four-band model composed of dispersionless deep and shallow core bands, and conduction and valence bands. The effects of the conduction electron-valence hole pair, created by the original electron excited from the deep core level to the conduction band, are taken into account by the lowest order perturbation theory. When the excitation energy is above the absorption edge, the resultant radiation spectra separate into two parts: a luminescence part, and a Raman scattering part peaked in its higher energy side. The condition for luminescence is that the conduction and valence band widths are finite, and the lifetime of the deep core hole is long enough.

Sb/Cu(111): GROWTH, DISSOLUTION AND SEGREGATION STUDIED BY AES, LEED and PES

The room temperature deposition of Sb on to Cu (111) gives rise to two different Sb states as seen from the Sb 4d shallow core level: an adstate similar to the [Formula: see text] structure formed after annealing at 400°C, and an overlayer of unreacted Sb . The unreacted Sb overlayer is rapidly dissolved into a subsurface position in the crystal during annealing, where it forms a quasistable structure which is dissolved very slowly.

Spin-polarized photoemission from shallow core levels in localized materials

We present spin-resolved photoemission measurements from the 5p core levels of Gd and Tb metal. In contrast to the 3d transition-metal 3p spectra, the rare-earth 5p spin-resolved spectra show a detailed fine structure that cannot be explained within a one-electron model. The spin-orbit, Coulomb, and exchange interactions have to be treated on an equal footing. For the case of Gd, a remarkable agreement is obtained between experiment and theory when the spin polarization of the 5p emission is analyzed on the basis of atomic-multiplet calculations in intermediate coupling. For Tb, however, small discrepancies remain that cannot be analyzed in the absence of a more detailed magnetic characterization. The results emphasize strongly the potential of spin analysis in core-level studies. {copyright} {ital 1997} {ital The American Physical Society}

Electronic structure of ultrathin Fe films on TiO2(110) studied with soft-x-ray photoelectron spectroscopy and resonant photoemission.

We report on soft-x-ray photoelectron spectroscopy (SXPS) of a ${\mathrm{TiO}}_{2}$(110) surface during deposition of Fe in the monolayer regime. At low fractional monolayer coverages, the adsorbed Fe atoms are oxidized and Ti cations at the interface become reduced due to Fe adsorption. SXPS from shallow core levels and valence bands show that Fe starts to exhibit metallic character at a coverage of approximately 0.7 equivalent monolayers. Two well separated defect states appear in the band gap of ${\mathrm{TiO}}_{2}$ at iron coverages well below one monolayer. We use resonant photoemission to obtain information on the partial density of states, and we assign these defect states as being Fe-derived and Ti-derived states, located at the Fe and Ti sites, respectively. We suggest that a position change of oxygen is involved in the bonding of Fe on the ${\mathrm{TiO}}_{2}$(110) surface.

Layer-by-layer growth of Hg on W(110)

Using photoelectron spectroscopy, the overlayer electronic structure was observed to be modulated by the Hg growth mode for Hg adsorbed on W(110) at 200 K. The Hg layer-by-layer growth was also characterized by variations in the ratio between the Hg 5d shallow core level and W 4f photoemission intensities. This layer-by-layer growth occurs in spite of surprisingly weak electronic interactions between the adatoms and the substrate.

Substitution of vanadium by chromium in thallium pentavandium octasulfide. Part II. Electronic structure. an XPS and UPS study

UPS investigations reveal that the metallic character within the substitution series TIV 5−yCr yS 8 decreases with increasing Cr content. The Cr richest sample is a poor metal or a semiconductor. The emission just below the Fermi level is mainly due to metallic states. This emission shifts to lower binding energies with increasing substitution of Cr for V. The different core levels exhibit only small chemical shifts and are indicative of low formal charges on the different atoms. The shape of the emission lines as well as the occurence and intensity of shake-up peaks are sensitive to the composition and suggest a variation of the covalency within the series. The Cr 3s core level exhibits a multiplet splitting. The intensity of the satellite increases with rising Cr content. The asymmetry of the Tl 5d shallow core level is more pronounced in the V-rich samples than in the Cr-rich compounds and can be regarded as an evidence for the stronger ionic character of Tl in the Cr-rich compounds.

Processing of conducting DCNQI radical ion salt films by solid state reactions

Self-organization of small molecular units in stack-forming radical ion salts can be used to prepare anisotropic thin films. The alkali-2,5-dimethyl- N, N′-dicyanoquinonediimine (DMe-DCNQI) radical ion salts films represent model systems to study solid state redox reactions when their components are deposited on cooled substrates. While heating the films this reaction is monitored by in situ measurements of the conductivity, the UV-Vis-IR absorption, the electron spin resonance and the photoelectron emission of the valence band and shallow alkali core levels. Initial experiments demonstrate the use of the alkali DCNQI films as wireless sensors for spin-carrying gases, thus pointing at future device applications.

Comment on the band effects in the 5p levels of Ho(0001)

The holmium 5p branching ratio is observed to change in angle-resolved photoemission as a result of changes in the photoemission geometry. The authors suggest that these changes are a result of final state interactions between the 5p hole and the valence 5d band structure rather than a consequence of the initial state band structure of the shallow 5p core levels. The 5d-5p final state interactions are postulated to be wave-vector dependent in angle-resolved photoemission.

Vibrational structure in core-level photoelectron spectra: Periodic trends.

High-resolution (\ensuremath{\sim}0.1 eV) core-level photoelectron spectra of gas-phase ${\mathrm{SiH}}_{4}$, ${\mathrm{PH}}_{3}$, ${\mathrm{GeH}}_{4}$, and ${\mathrm{AsH}}_{3}$ have been recorded using monochromatized synchrotron radiation. These spectra, multiconfiguration self-consistent-field calculations of the vibrational profile, and the core-equivalent model all show that core-level vibrational structure decreases dramatically, both down a group of congeneric molecules and across a periodic row of isoelectronic molecules. The theoretical calculations also show that the Franck-Condon factors do not change for relatively shallow core levels (e.g., Si 2p,2s or Ge 3d,3p) but increase significantly for deep core levels (e.g., Si 1s).

Thermal and surface core-electron binding-energy shifts in metals.

High-resolution photoemission spectra from the shallow core levels of alkali metals and of In have been obtained between 78 K and room temperature. The data yield values for the alkali-metal surface-atom core-level shift and show thermal shifts of comparable size for bulk and surface. The positive surface shifts are due to the spill-out of conduction-electron charge, which is responsible for the surface dipole layer. The surface shifts are in good agreement with values obtained from a Born-Haber cycle expressed in terms of surface energies. The thermal shifts are proportional to the lattice expansion, and arise from both initial-state and final-state effects. As the lattice expands, the Fermi level decreases, decreasing the core-electron binding energy. At the same time, the expansion of the conduction-electron charge increases ${\mathit{r}}_{\mathit{s}}$, thereby decreasing the potential at the core level and increasing the binding energy. The expansion also decreases the relaxation energy, further increasing the core-electron binding energy. In the alkali metals, the combined potential- and relaxation-energy terms dominate the Fermi-level term, making the shifts positive. In divalent metals the three terms tend to cancel, while in trivalent metals it is the Fermi-level term that dominates, making the shifts negative.

Multielectron x-ray photoexcitation observations in x-ray-absorption fine-structure background.

Multielectron excitations involving deep and shallow core levels have been identified for the first time in the x-ray-absorption fine-structure (XAFS) energy range for crystalline samples, RbBr and \ensuremath{\beta}-${\mathrm{PbO}}_{2}$. The XAFS oscillations have been fitted and subtracted from the experimental data using theoretical standards. The positions of steps in the extracted background are in excellent agreement with the Z+1 approximation. The ability to determine this structure has important implications for XAFS and multiexcitation studies.

Core-level spectroscopy study of the initial formation of tin-germanium interfaces

We have studied the initial stages of the Schottky barrier formation at Sn/Ge(100) interfaces, in comparison with Sn/Ge(111) ones, with high resolution core-level spectroscopy measurements (Ge 3d and Sn 4d shallow core levels) using synchrotron radiation. A detailed stringent decomposition of the core lines reveals three different components for both Ge and Sn on the two surfaces. The three components of the Sn 4d line on the Ge(100)2 × 1-Sn surface are interpreted as due to three different adsorption sites simultaneously occupied, while the persistence of the surface components in the Ge 3d core lines suggests that the building block of the reconstruction remains an asymmetric Ge dimer.

Multielectron excitations in the K-edge x-ray-absorption near-edge spectra of V, Cr, and Mn 3d0 compounds with tetrahedral coordination.

Multielectron excitations involving metal deep 1s and shallow 3p core levels have been identified in the x-ray-absorption spectra in the range 50--60 eV above the K edge of transition-metal ions V, Cr, and Mn with tetrahedral coordination geometry. The many-body transitions have been found after subtraction of the dominant one-electron transitions to continuum states giving the x-ray-absorption near-edge structure. The presence of many-body excitations involving valence electrons has been found at \ensuremath{\sim}7 eV above threshold. The energy positions are in good agreement with calculated excitation energies for the many-body configurations, where the two excited electrons are in the unoccupied molecular ${\mathit{t}}_{2}^{\mathrm{*}}$ orbital. Experimental comparison of the intensities of corresponding many-body satellites between the Cr 2p x-ray photoelectron spectrum and Cr K-edge absorption spectrum is reported for ${\mathrm{K}}_{2}$${\mathrm{CrO}}_{4}$.

Design and performance of a VUV ellipsometer

Abstract Spectroscopic ellipsometry provides a rapid, precise and direct measurement of both real and imaginary parts of the dielectric function from bulk materials and thin films. The technique is used routinely in the NIR-VIS-UV spectral region to determine the optical constants of solids over the photon energy range 1.5–5.0 eV. By making use of synchrotron radiation with its high degree of linear polarization and triple-reflection polarizers we have built a new automated instrument which extends the spectral range of spectroscopic ellipsometry up to 35 eV. Our instrument is ideal for investigating the band structure of wide-band-gap semiconductors and insulators and can also be used to measure the excitonic structures arising from transitions from shallow core levels. In this paper the design of the instrument is described and the factors that limit the ultimate performance are analyzed.