Energy band structure of cadmium fluoride

The electronic band structure of cadmium fluoride is calculated by a combined tight binding and pseudopotential method. The band structure is found to be rather similar in shape with that of CaF₂. In particular it is shown that the occupied cationic d⁺ bands do not perturb significantly the upper valence band predominantly composed of the F^-p-orbital. This explains the great similarity of the low energy part of the reflection spectra of CaF₂ and CdF₂.     

Photoemission studies of single crystals of titanium carbide

The electron distribution in the valence band from single crystals of titanium carbide has been studied by photoelectron spectroscopy with photon energies h?ω = 16.8, 21.2, 40.8 and 1486.6 eV. The most conspicious feature of the electron distribution curves for TiC is a hybridization between the titanium 3d and carbon 2p states at ca. 3–4-eV binding energy, and a single carbon 2s band at ca. 10 eV. By taking into account the strong symmetry and energy dependence of the photoionization crosssections, as well as the surface sensitivity, we have identified strong emission from a carbon 2p band at ? 2.9-eV energy. Our results are compared with several recent energy band structure calculations and other experimental data. Results from pure titanium, which have been used for reference purposes, are also presented.The valence band from single crystals of titanium carbide have been studied by means of photoelectron spectroscopy, with photon energies ranging from 16.8 to 1486.6 eV.By taking into account effects such as the symmetry and energy dependence of the photoionization cross-sections and surface sensitivity, we have found the valence band of titanium carbide to consist of two peaks. The upper part of the valence band at 3–4 eV below the Fermi level consists of a hybridization between Ti 3d and C 2p states. The C 2p states observed in our spectra were mainly excited from a band about 2.9 eV below the Fermi level. The APW^5–9, MAPW¹⁰ and EPM¹¹ band structure calculations predict a flat band of p-character between the symmetry points X′₄ and K₃, most likely responsible for the majority of C 2p excitations observed. The C 2s states, on the other hand, form a single band centered around ?10.4 eV.The results obtained are consistent with several recent energy band structure calculations^{5–11, 13} that predict a combined bonding of covalent, ionic and metallic nature.     

Electron energy loss spectra of the silicon 2p, 2s, carbon 1s and valence shells of tetramethylsilane

Inner shell excitation spectra of tetramethylsilane, (CH₃)₄Si, have been measured in the silicon 2s, 2p (L_I,II,III-shell) and carbon is (K-shell) regions using electron energy-loss spectroscopy at an impact energy of 2.5 keV and a scattering angle of ~1°. The high-resolution valence shell spectrum has also been observed at an impact energy of 3 keV and a zero degree scattering angle. The silicon 2p spectra are compared and contrasted with published photoabsorption spectra of SiF₄, SiH₄, and other related Si-containing molecules with varying ligands.     

Electronic structure of solid pyridine and pyridine adsorbed on polycrystalline silver

The electronic structure of the valence bands of polycrystalline films of pyridine (C₅H₅N), including all valence levels extending from initial energies of 4 down to 30 eV (E_VAC = 0), has been determined from photoelectron energy distribution measurements for photon energies 20eV ? hν ? 170 eV, using synchrotron radiation. The valence bands showing a one-to-one correspondence to the gas phase, a rigid relaxation shift of δe_r = 0.3 eV for the vertical binding energies, and a considerable solid state broadening (? 0.5 eV) are assigned in comparison to recent MO calculations. By tuning the photon energy and thereby achieving high surface sensitivity for hν around 45 eV, we have also studied pyridine adsorbed at 120 K (6 Langmuir) on in situ prepared polycrystalline Ag-substrates. Thus, we were able to study in detail the surface electronic structure in the range of the Ag 4d bands. Due to a strong mixing of substrate 4d and pyridine 2b₁ (π), 1a₂ (π) and 7a₁ (n) energy levels, the surface electronic structure is strongly modified in the upper part of the 4d bands, and a strong and sharp (0.4 eV FWHM) surface resonance at an energy of 3.7 e V below E^AG_F is observed, which we attribute to a 4d-7a₁ (n) bonding of the nitrogen lone-pair orbital.     

Line shape analysis and relaxation energies in the MVV Auger spectra of zinc and zinc oxide

The oxidation of polycrystalline zinc has been studied by recording the evolution of the line shape of the corresponding Auger spectra. The fine structure of the surface-sensitive low energy M₁ VV and M₂₃VV (V = 4s ? 3d valence band) lines in pure zinc has been analyzed and a new feature involving the 4s band tentatively identified. In the course of oxidation the main peaks broaden and shift to lower energies. This behaviour is explained in terms of increase of the 3d band width and decrease in the extraatomic relaxation energies. The extraatomic relaxation is found to decrease in the oxide by ~3.8 eV. A derivation of Auger intra and Extraatomic energies involving basically experimental data is presented. These values are compared to theoretical evaluations and their connection with photoemission dynamical relaxation data is discussed.     

Ultraviolet photoemission from polymeric sulfur nitride polycrystalline films and single crystals

The (SN)_x valence band structure, for polycrystalline films as well as for single crystal samples, has been studied using He I and He II resonance radiation. In angle-resolved photoemission energy distributions from single crystals, structure in the spectra is selectively enhanced offering a possibility of assigning the photoemission as originating from particular regions of the Brillouin Zone. The observed onset of photoemission 0.2 eV below the Fermi edge is discussed.     

Nanostructured high valence silver oxide produced by pulsed laser deposition

Among silver oxides, Ag₄O₄, i.e. high valence Ag(I)Ag(III) oxide, is interesting for applications in high energy batteries and for the development of antimicrobial coatings. We here show that ns UV pulsed laser deposition (PLD) in an oxygen containing atmosphere allows the synthesis of pure Ag₄O₄ nanocrystalline thin films, permitting at the same time to control the morphology of the material at the sub-micrometer scale. Ag₄O₄ films with a crystalline domain size of the order of tens of nm can be deposited provided the deposition pressure is above a threshold (roughly 4 Pa pure O₂ or 20 Pa synthetic air). The formation of this particular high valence silver oxide is explained in terms of the reactions occurring during the expansion of the ablated species in the reactive atmosphere. In particular, expansion of the PLD plasma plume is accompanied by formation of low stability Ag-O dimers and atomic oxygen, providing reactive species at the substrate where the film grows. Evidence of reactive collisions in the expanding ablation plume is obtained by analysis of the plume visible shape in inert and reactive atmospheres. In addition, we show how the dimensionless deposition parameter L, relating the target-to-substrate distance to the ablation plume maximum expansion length, can be used to classify different growth regimes. It is thus possible to vary the stoichiometry and the morphology of the films, from compact and columnar to foam-like, by controlling both the gas pressure and the target-to-substrate distance.     

Resonant photoemission spectroscopy of Cu(InGa)Se2 materials for solar cells

The electron structure of CuIn_{1 ? x} Ga _x Se₂ single crystals is determined via resonant photoemis-sion and the main regularities of its transformation upon varying concentration x from 0 to 1 are established. The dependence of the shape of valence band spectra on the photon energy is studied. Integral photoemission intensities are shown to be determined by atomic photoionization cross sections. Processes of the direct and two-step creation of photoelectrons accompanying photoemission and the participation of internal states in the spectra of electrons from valence bands are studied. Two-hole final states in photoemission are obtained upon threshold excitation of the Cu 2p level. The strong interaction of holes leads to the multiplet splitting of these states. Partial densities of the components’ states are determined using the energy dependence of atomic photoionization cross sections.     

Electronic structure of copper halides CuI and CuCl: A comparative X-Ray photoelectron and absorption spectroscopy study

The energy distributions of the occupied and unoccupied electronic states for copper halides CuCl and CuI have been investigated using X-ray photoemission and absorption spectroscopy with a highenergy resolution on the equipment of the Russian-German beamline for outlet and monochromatization of synchrotron radiation from the electron storage ring BESSY II. A quasi-molecular analysis of the obtained experimental spectra has revealed that there is a fundamental similarity of the energy structures of the valence band and the conduction band of copper halides CuX (X = Cl, I) due to the identical atomic structure of the studied compounds. The differences in the positions of individual energy subbands in the valence band and the conduction band of CuX and in their intensities in the spectra are associated with different degrees of hybridization of the Cu 3d, 4s and X(n + 1)s, np valence states, as well as with different sizes of structural units (CuCl₄ and CuI₄ quasi-molecules) of the studied crystals.     

Relationship between the local electronic and local crystal structures of intermediate-valence Sm1−x Y x S

The valence state of samarium and the local environments of samarium and yttrium ions in the intermediatevalence compound Sm_1?x Y _x S (x = 0.17, 0.25, 0.33, or 0.45) are studied over a wide temperature range of 20–300 K using x-ray absorption spectroscopy. The temperature dependence of Sm-S, Sm-Sm(Y), Y-S, and Y-Sm(Y) bond lengths and the Debye-Waller factors has been found. A direct correlation of changes in the valence of samarium with changes in its local environment parameters depending on yttrium concentration and temperature has been revealed. The main characteristics of the intermediate-valence state of samarium have been determined: the energy width of the 4f level and its position with respect to the Fermi level.     

Photoionization branching ratios and asymmetry parameters for inner- and outer-valence shells of H2S in the 18–70 eV energy range

Branching ratios, asymmetry parameters and relative partial cross sections have been obtained for photoionization of the outer and the inner valence shells of H₂S. These measurements were made in the photon energy range 18–70 eV using synchrotron radiation. Our results are compared to a set of calculations using a developed extension of the self-consistent field-Xα scattered-wave method to the continuum states. This comparison shows a qualitative agreement between the experimental and calculated β curves of the outer valence shells. The largely predominant sulfur 3p contribution to the outer valence orbitais 2b₁, 5a₁, 2b₂ is revealed in the corresponding β curves by a Cooper minimum also predicted in the same energy range for the β(3p) of the atomic sulfur. This comparison also shows discrepancies in the branching ratios curves and we suggest that this theoretical framework is better adapted to predict photoionization processes in the outer valence shells than in the inner ones.     

Low temperature features of the local structure of Sm<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Y<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>S

The particular features of the local electronic and local crystal structures of the mixed-valence compound Sm_{1 ? x} Y _x S are studied by the XAFS spectroscopy methods in the temperature range 20–300 K for the yttrium concentration x = 0.17, 0.25, 0.33, and 0.45. The temperature behavior of the valence of Sm, as well as of the lengths and the Debye-Waller factors of the bonds Sm-S, Sm-Sm(Y), Y-S, and Y-Sm(Y), has been determined. The violation of the Vegard law has been observed. A model for the estimation of the energy width of the 4f level and of its position with respect to the Fermi level is proposed.     

(e, 2e) spectroscopy of N2—valence momentum distributions and configuration interaction

The noncoplanar symmetric (e, 2e) reaction has been applied to N₂ at 1200, 600 and 400 eV. Separation energy spectra are obtained in the valence region, the observed structure extending to above 60 eV. Electron momentum profiles are measured at a number of separation energies. They agree very well with the momentum distributions for valence orbitals given by SCF calculations. Considerable configuration interaction structure is observed, being primarily due to configuration interaction in the 2σ_g hole state. At 1200 eV the spectroscopic sum rule is satisfied within experimental error, confirming the validity of the analysis.     

Surface electronic structure of the organic superconductor κ-(ET)2Cu(NCS)2 studied via photoemission microscopy

The surface electronic structure of cleaved single crystals of the organic superconductor κ-(ET)₂Cu(NCS)₂ has been studied using photoemission microscopy. Two types of cleaved surfaces were observed, displaying different valence band photoemission spectra and different spectral behavior near the Fermi level, E_F. In particular, spectra from one surface type display relatively broad spectral features in the valence band and finite spectral intensity at E_F, while spectra from the other surface type show well-defined valence band emission features and zero photoemission intensity at E_F. We propose that the spectral differences are due to a very short electron mean free path in this material, and our results are used to explain the differences between previously published photoemission spectra from this superconductor. We also report the results of an investigation of the electronic structure of defects in this material.     

Intraband and interband optical transitions in Zn3AS2

Absorption measurements were made on single crystals and thin films of Zn₃As₂; within the photon energy range of 0.12–1.16 eV at temperatures of 300, 80 and 5 K and reflectivity was measured in the range of 1.0–5.5 eV at 300 K. Absorption below the fundamental edge has been interpreted as a process involving three mechanisms: (i) free-carrier absorption, (ii) intraband transitions between levels in the valence band, and (iii) direct transitions from valence levels to the acceptor level/band. The fundamental absorption edge has been ascribed to direct interband transitions from three valence levels to one conduction level. An isotropic three-level Kane band model has been used to interpret the experimental data, modified by introducing the light-hole level split from the heavy-hole level due to the tetragonal crystal field. A reasonable fit of the model to the experimental results has been obtained in the region of both intraband and interband absorption for the following set of parameters: E_g = 0.985 eV, Δ_SO = 0.30 eV, Δ_CF = 0.05 eV, m^*_hh = 0.36 m₀ and P = 4 × 10^?10eVm (at 300 K). A proposed Zn₃As₂ energy-band model near the Γ point is described to interpret the observed absorption.     

X-ray photoelectron spectroscopy characterization of the layered intercalated compound K2xMn1 − xPS3

Polycrystalline powders of the layered MnPS₃ compound have been intercalated with K⁺ ions by ion-exchange to yield the K_2xMn_1 − xPS₃ intercalate. X-ray photoelectron spectroscopy has been applied to learn about the electronic structure of this compound. In particular, we have studied the XPS spectra of the Mn 2p and 3p, P and S 2p, K 2p and 3p core levels and of the valence band region. The binding energies for various core levels of the elements present in this compound and their observed chemical shifts are analyzed. The data give evidence for the lack of non-equivalent atoms of K, Mn, P and S. Shake-up satellites are present at the Mn 2p and 3p core levels. The occurrence of such lines allows us to hypothesize that K_2xMn_1 − xPS₃ is a large-gap insulating Mn compound. Confirmation that only an ion transfer accompanies the intercalation process is given from both the strong observed similarity with the corresponding XPS spectra in MnPS₃ and the observed binding energy positions of the K 2p and 3p levels. As regards the valence band XPS spectrum, the observed analogies with the corresponding XPS spectra of the pure compound and of other K compounds have allowed us to single out two regions and their probable contributors.     

Valence and core state modifications in Pt3Ti

XPS data for the valence band, the Pt 4? states, and the Ti 2p states are presented for the intermetallic Pt₃Ti. Relative to the Pt valence band, the Pt₃Ti band shows a decrease in the density of states just below the Fermi level and a shift of the centroid to higher (binding) energy. Ti 2p and Pt 4? binding energies showed relatively large shifts with respect to the pure metals. These changes in the valence band density of states and core level binding energies are interpreted as arising from hybridization of the d-orbitals in both metals to form strong intermetallic bonds.     

Electronic band structure of partially inverse spinel MgIn2S4 as calculated by the semiempirical pseudopotential method

The electronic band structure of the partially inverse spinel MgIn₂S₄ has been calculated on the symmetry lines ΓΛL, ΓΔX and ΓΣK by the semiempirical pseudopotential method. The general features of the band structure of MgIn₂S₄ are quite similar to those of the normal spinel CdIn₂S₄. The conduction band minimum is located at Γ and the valence band maximum is along the Σ line. The indirect energy gap (Γ_1cΣ_4v) is 2.50 eV. The effects of magnesium vacancy and variations in the cation distribution and in the parameter u are examined and shown to be small.     

Valence band photoemission spectroscopy of a ternary layered semiconductor: ZnIn2S4

UV photoemission spectroscopy (UPS) experiments have been carried out on the layer compound ZnIn₂S₄ employing several different photon energies in the range $\text{h}\text{?}{}_{\mathrm{\omega }}\text{= 9.5?21.2}\text{eV}$. The energy distribution curves (EDC's) exhibit four valence band density of states structures besides the Zn 3d peak. These five peaks appear 0.90 eV, 1.6 eV, 4.3 eV, 5.8 eV and 8.7 eV respectively below the top of the valence band, E_v. The atomic orbital character of the shallowest peak A appears different from that of the three deeper valence band peaks B, C and D and this is discussed in terms of the more or less pronounced ionic character of the intralayer chemical bonds. These results demonstrate that an overall understanding of the electronic states in complex structures can be achieved by an approach based on photoemission experiments and chemical bonding considerations which has been widely used in the past to study simple binary layer compounds.