Electronic structure of PbI2 from photoelectron spectra and the exciton problem

The valence band density of states for PbI₂ is determined from X-ray and u.v. induced photoelectron spectra. It is shown that the band derived from Pb 6s states is at 8 eV binding energy and not at the top of the valence bands as suggested by band structure and charge density calculations. A rigid shift in the predominantly iodine 5p derived bands to lower binding energy brings the band structure calculations into essential agreement with experiment. Pb 5d core level binding energies determined here are used to derive core level exciton energies of 0.7 eV from published reflectivity spectra.     

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.     

Effect of alloying on the stabilities and catalytic properties of Pt–Au bimetallic subnanoclusters: a theoretical investigation

Density functional theory (DFT) has been applied to study the geometrical and electronic structures and the catalytic properties for NO oxidation of pure Pt and PtAu clusters. The calculated results suggest that Pt₁₀ clusters shows the most stable structure among the pure Pt _n (n = 2–13) clusters with the local maximum Δ₂ E value. The doping of Au atoms reduces the stability of the clusters, and Pt₆Au₄ cluster has the most stable structure among Pt_10?n Au _n (n = 1–7) clusters, due to the closest band centers between Pt and Au atoms (0.83 eV) and the obvious s–p resonance peaks near the Fermi level. Pt₆Au₄ cluster displays the strongest activation of O₂ molecules among Pt_10?n Au _n (n = 0–7) clusters, owing to the clear overlap between O 2p and Pt 6 s and Au 6 s near the Fermi level, and the more positive d band center than the others. The interaction between NO and metals changes slightly in NO/Pt_10-nAu_n (n = 2–7) systems, which is weaker than that in NO/Pt₉Au system, as a result of the decreasing resonance peaks of sp hybridization near the Fermi level. Compared to pure Pt₁₀ cluster, the lower energy barriers and larger reaction energies on Pt₆Au₄ cluster suggest a higher catalytic activity of PtAu cluster for the O₂ dissociation and NO oxidation reactions. Our study provides atomic-scale insights into the nature of the interfacial effect that determines NO oxidation on PtAu cluster catalysts.     

Valence band structure of single crystal titanium carbide,studied by soft X-ray and ultraviolet photoelectron spectroscopy

Valence states of single crystal titatium carbide (TiC_x, X?0.88) have been studied with photon energies ranging from far ultraviolet (u.v.) to soft X-ray. The valence band consists of two peaks located at 3 and 10 eV below the Fermi level. This is in good agreement with recent APW band structure calculations that predict a strong hybridization of the Ti 3d and C 2p bands and a C 2s band at lower energy.     

Experimental valence-band study of Ti(NiCu) alloys with different compositions and crystal structures

The density of valence-band electronic states of Ti(NiCu) alloys with different crystal structures and elemental compositions has been studied by X-ray photoelectron spectroscopy. It has been established that the change in the crystal state initiated by a martensitic transformation or a transition from the amorphous state to the crystal state does not affect the valence-band electronic state density distribution of the Ti₅₀Ni₅₀ and Ti₅₀Ni₂₅Cu₂₅ alloys. It has been shown that a change in the elemental composition leads to a noticeable redistribution of the electronic density in alloys of the Ti₅₀Ni_{50 ? x} Cu _x system (x = 0, 10, 15, 25, 30, 38, 50 at. %). As the copper concentration in the Ti(NiCu) alloys increases, the contribution of the Ni d states in the vicinity of the Fermi level decreases, with the d band of nickel shifting toward higher binding energies, and that of copper, toward lower binding energies.     

Density functional study of small cobalt-platinum nanoalloy clusters

The structural, energetic, electronic and magnetic properties of small bimetallic Co_nPt_m (n+m≤5) nanoalloy clusters are investigated by density functional theory within the generalized gradient approximation. A plausible candidate for the ground state isomer and the other possible local minima, binding energies, relative stabilities, magnetic moments, the highest occupied and the lowest unoccupied molecular orbital energy gaps have been calculated. It is found as a general trend that average binding energies of Co-Pt bimetallic clusters increase with Pt doping. Planar structures of pure Co clusters become 3D with the addition of Pt atoms. CoPt₂, Co₂Pt₂, Co₃Pt₂, and CoPt₄ nanoalloys are identified as the most stable species since they have higher second finite difference in energy than the others. Pt doping decreases the total spin magnetic moment gradually. A rule for the prediction of the total spin moments of small Co-Pt bimetallic clusters is derived.     

Magnetic properties and electronic structure of GdNi4Si compound

Electronic structure of the ternary GdNi₄Si compound, crystallizing in hexagonal CaCu₅ structure (P6/mmm space group) was studied by magnetic measurements, X-ray photoelectron spectroscopy (XPS) and ab initio calculations. Core levels and valence band were investigated. The valence band of the XPS spectra is determined mainly by the Ni(3d) and Gd(4f) bands. The peaks’ positions are in good agreement with binding energies of a metallic gadolinium and nickel. The experimental valence band spectrum as well as the calculated density of states exhibit the domination of the Ni(3d) states in region from −4 eV to the Fermi level.     

Energy band structure and refractive properties of LiRbSO4 crystals

The energy band structure of mechanically free and compressed LiRbSO₄ single crystals is investigated. It is established that the top of the valence band is located at the D point of the Brillouin zone [k = (0.5, 0.5, 0)], the bottom of the conduction band lies at the Γ point, and the minimum direct band gap E _g is equal to 5.20 eV. The bottom of the conduction band is predominantly formed by the Li s, Li p, Rb s, and Rb p states hybridized with the S p and O p antibonding states. The pressure coefficients corresponding to the energies of the valence and conduction band states and the band gap E _g are determined, and the pressure dependences of the refractive indices n _i are analyzed.     

A study of some potassium hexachlorometallate complexes using electron spectroscopy

X-ray photoelectron (ESCA) spectra of the core (Cl 2p K 2p and metal 4f, if present) and valence orbitals are reported for K₂ReCl₆, K₂OsCl₆, K₂IrCl₆· 3 H₂O, K₂PtCl₆, K₃MoCl₆, and K₂SnCl₆. The K 2p_{$\text{3}\text{2}$} binding energy was found to be nearly constant (292.7 eV) and that of Cl to increase very slightly with increasing atomic number for the third row transition metals. The chemical shifts of Re(IV), Os(IV), Ir(IV), and Pt(IV) relative to the metals were in qualitative agreement with atomic calculations utilizing configurations obtained from extended Hückel calculations. The valence spectra of the transition metal complexes exhibit a three-band structure. On the basis of MO results and intensity considerations the high binding energy band is assigned as a composite of the a_1g, e_g, 1t_2g MO's. The middle band represents the t_2u, 2t_1g MO's; and the low binding energy band the 2t_2g MO. Calculated nd orbital photoionization cross sections correlate reasonably well with the relative intensifies of the valence manifolds. Comparison of band separations and charge-transfer transition energies suggests that interelectronic repulsion and MO energy separation contribute about equally to the overall charge-transfer energy.     

Structure electronique des oxydes de cobalt CoO et Co3O4

The total density of occupied states in the valence band of CoO and Co₃O₄ is determined by XPS and UPS. From variations of excitation probability of the bands, the 4 e V wide O2_p band is shown to be located around 5 eV for both oxides, while structures obtained from photoionisation of the localized 3d band spread over 10 eV range below the Fermi level overlapping with O2_p band. The 3d peaks located at binding energy <3 eV correspond to the calculated energy of the d_n ?1 manifold final state in the octahedral and tetrahedral crystal field of CoO and Co₃O₄. The 3d levels at higher binding energy are shown to occur from configuration interaction in both final and initial states. These last peaks are higher in intensity for CoO relative to Co₃O₄. A superior limit for the width of the 3d initial band in a one electron energy diagram is given to be <3 eV. This value associated to the Coulomb correlation energy measured equal to ~3 eV. This value associated to the Coulomb correlation energy measured equal to ~3 eV from shake-up and Auger energy confirms the Mott insulator nature of CoO.     

Electronic structure of the valence bands of H2-, Mg- and Pt-phthalocyanine derived from soft X-ray emission and photoelectron emission spectra

The soft X-ray emission and photoelectron emission spectra of H₂-, Mg- and Pt- phthalocyanine (PC) obtained using synchrotron radiation are reported and compared. In this way, an overall view of the pattern of valence bands is obtained and the electronic structure determined in terms of the component partial densities of states. In particular, from the valence p → 1s carbon and nitrogen K-emission spectra we determine for all three compounds the C and N 2p-like valence-band density of states with strong maxima located at binding energies of 8, 11 and 13.5 eV (carbon 2p) and 8 eV (nitrogen 2p) below the vacuum level. For PtPC the partial density of d-like valence states is determined from photoelectron emission difference-spectra and compared to previous XPS results. The sharp (1.2 eV FWHM) maximum of the Pt-derived partial density of states, observed at 6.9 eV binding energy, is assigned to the ⁴F term of a 5d⁸6s final-state configuration. A second, broader maximum at around 9.5 eV binding energy contains contributions from other terms of this 5d⁸ configuration, as well as from a 5d⁷ satellite (shake-up multiplet).     

Ultraviolet photoemission study of LiF

Photoemission from evaporated films of LiF were measured at photon energies of 10-27 eV. The photoelectron spectra exhibit features that can be identified as density-of-states structures in the valence and conduction bands of LiF. Regions of high density of states can be seen at ca. 3.3 and 7.8 eV above the vacuum level. The valence-band spectrum shows a doublet structure similar to the calculated density of states for the F^?2p band of LiF. The base width of this structure is found to be 4.6 ± 0.3 eV. The photoelectron spectra for photon energies > 15 eV indicate that the highest occupied states of the F^?2p band are located at 11.8 ± 0.3 eV below the vacuum level. The photoelectron spectra in the exciton region, however, show photo-emission from higher occupied states.     

Electronic structure of HgGa2S4

The electronic structure and chemical bonding in HgGa₂S₄ crystals grown by vapor transport method are investigated with X-ray photoemission spectroscopy. The valence band of HgGa₂S₄ is found to be formed by splitted S 3p and Hg 6s states at binding energies BE=3-7 eV and the components at BE=7-11 eV generated by the hybridization of S 3s and Ga 4s states with a strong contribution from the Hg 5d states. At higher binding energies the emission lines related to the Hg 4f, Ga 3p, S 2p, S 2s, Hg 4d, Ga LMM, Ga 3p and S LMM states are analyzed in the photoemission spectrum. The measured core level binding energies are compared with those of HgS, GaS, AgGaS₂ and SrGa₂S₄ compounds. The valence band spectrum proves to be independent on the technological conditions of crystal growth. In contrast to the valence band spectrum, the distribution of electron states in the bandgap of HgGa₂S₄ crystals is found to be strongly dependent upon the technological conditions of crystal growth as demonstrated by the photoluminescence analysis.     

Comparison of site-specific valence band densities of states determined from Auger spectra and XPS-determined valence band spectra in GeS (001) and GeSe (001)

Auger lineshapes of the Ge M₁M_4,5V and M₃M_4,5V and Se _M1M_4,5V transitions in GeS (001) and GeSe (001) are measured and compared to XPS valence band spectra. Distortions in both types of spectra due to inelastic scattering, analyzer and source broadening, and core level lifetime broadening are removed by deconvolution techniques. The valence band consists of three main peaks at ?2 eV, ?8 eV, and ?13 eV. There is excellent agreement of peak positions in AES and XPS spectra. The Auger lineshapes can be interpreted in terms of site-specific densities of states. They indicate that the states at ～?8 eV and at ～?13 eV are associated with the cation and anion sites respectively. The bonding p-like states at the top of the valence band have both cation and anion character. The Auger lineshapes indicate that the states closest to the valence band maximum are preferentially associated with Ge.     

Photoemission studies of clean and oxygen-covered Pt 6(111) × (100)

Employing the enhanced sensitivity obtained by using synchrotron radiation near the Cooper minimum for the 5d valence electrons, we have located the oxygen 2p and 2s levels for oxygen chemisorbed on a Pt 6(111) × (100) crystal. We find the oxygen 2p level located ?6 eV with a FWHM of 3 eV and the 2s at ?21.6 eV. A factor of four difference in saturation coverage is measured between temperatures of 300 and 120 K, but the position and width of the 2p level is independent of temperature. We observe also the 1b₁ orbital of weakly adsorbed H₂O molecules, which has pure O 2p parentage; from the intensity of this orbital, we are able to suggest why it is difficult to observe the oxygen 2p signal at low photon energies. In addition, we note a strong preferential attenuation in the Pt states near E_f for the adsorbed H₂O in spite of the weak nature of the bond.     

Energy loss mechanisms in low energy electron scattering from W(100) and their use as a surface sensitive spectroscopy

The energy loss spectrum of low energy (0 < E_p < 200 eV) electrons scattered from W(100) has been experimentally investigated, and mechanisms giving rise to the fine structure analyzed using a dielectric response formalism. The dielectric medium is characterized by available optical data and energy band calculations for tungsten. All of the structure for loss energies, w, less than 18 eV is attributed to intra- and interband transitions involving the bulk valence and conduction bands. The surface and bulk plasmon excitations are observed at w = 21 eV and w = 25.5 eV respectively which is in reasonable agreement with the optical data. A very narrow peak in the density of conduction d-band states apparently functions strongly in well defined excitations involving the $\text{5p}\text{3}\text{2}$ and 4f tungsten orbitais and the 2s and 2p orbitais of adsorbed oxygen. These conduction band states form a “window” with which to measure the electronic orbital structure of both the substrate and adsorbate during adsorption and reaction. We demonstrate this for the room temperature adsorption of oxygen on W(100) in which we observe the sequential filling of two electronically inequivalent binding states. The stability of the “d-band window” during thermally activated reaction, and the likelihood of its existence in other transition metals makes this an attractive surface sensitive spectroscopy.     

Interpretation of ESCA spectra for non-stoichiometric titanium carbides on the basis of MO—LCAO calculations

The results of electron structure calculations for the single clusters TiC₆^20?, TiC₅□^16?, TiC₄□₂^12?, TiC3□3^8?, TiC₅□^16.64?, TiC₄□₂^13.32?; and double clusters Ti₂C₁₀^32?, Ti₂C₈□₂^24?, Ti₂C₁₀□^32.72? and Ti₂C₈□₂^24.80? by the semi-empirical Mulliken—Wolfsberg—Helmholtz method with self-consistency of charges and configurations are given. The data obtained are compared with measurements of the Ti 2p-level binding energies and X-ray photoelectron valence band spectra for TiC_x(x = 0.86–0.97). The variation of the metal atom charge in the limits of the homogeneity region for TiC and the appearance of this effect as a chemical shift of the Ti 2p-level are discussed. A special set of cluster calculations was performed to check the possibility of Ti valency variation in the homogeneity region of TiC_x.     

The effect of gold on platinum oxidation in homogeneous Au-Pt electrocatalysts

Oxidation of Au-Pt thin films was carried out in ambient air at room temperature and characterized by X-ray photoelectron spectroscopy. The homogeneous films were prepared by RF co-sputtering with concentrations varying from Au₉Pt₉₁ to Au₈₉Pt₁₁ and compared to pure Pt and Au thin films. Spectral deconvolution of the Au 4f and Pt 4f core levels revealed linear peak shifts for both the Au-Au and Pt-Pt bonding components as a function of alloy mixture and metallic component peak asymmetry that remained constant for all alloy stoichiometries. The predominant oxidation products were PtO and PtO₂ and were characterized by stable core level binding energies for all films. A gradual decline in the Pt-O_x products and corresponding levels of elemental oxygen was observed with increasing Au content but was similar in proportion to the metallic Pt components. Based on these results, variations in Pt oxide phases and/or concentration do not appear to contribute to enhanced electrocatalytic activity for oxygen reduction observed for the intermediate alloy stoichiometries.     

Effective 4ƒ energy level in virtual excitation and in photoemission processes in Ce-pnictides

Effects of relaxation of occupied band electrons to the ?-hole state through the hybridization between ? and band states are studied based on a detailed model for Ce-monopnictides. The effective 4? level is shifted about 1–2 eV to shallow energy side from the unrenormalized bare level in processes in which the 4? electron is only virtually excited, such as in excitation to the vacant p band states through the p-? mixing. Photoemission spectra show two peaks, one near the Fermi energy and the other about 3 eV below it. The latter is shifted to deep energy side about 0.5–1 eV from the bare level when it lies near the bottom of the valence band. The discrepancy between the 4? level estimated from the low energy phenomena and that from photoemission is resolved.     

Co chemisorption on PtCu surfaces III. CO on PtCu(111)

Selected thermal desorption and valence band photoemission data on the chemisorption of CO on PtCu(111) surfaces are presented. The main objective is to make a comparison with CO chemisorption on an annealed (1 × 3) reconstructed Pt_0.98Cu_0.02(110) surface. The (111) alloy surfaces are unreconstructed (1 × 1) surfaces, with average near-surface Cu concentrations ranging from ? 7.5% to ? 20% as indicated by the Cu 920 eV Auger signal. It is observed that the effect of alloying Pt(111) with Cu is to progressively lower the desorption peak temperature and hence the free energy of CO desorption from Pt sites. A second observation is that the energy distribution of the Cu 3d-derived states is little affected by CO adsorption on Cu sites at 155 K. Both these results offer a contrast to the results for CO/Pt_0.98Cu_0.02(110) reported earlier.