A study of core and valence levels in β-PbO2 by hard X-ray photoemission

The core and valence levels of β-PbO₂ have been studied using hard X-ray photoemission spectroscopy (hν = 6000 eV and 7700 eV). The Pb 4f core levels display an asymmetric lineshape which may be fitted with components associated with screened and unscreened final states. It is found that intrinsic final state screening is suppressed in the near-surface region. A shift in the O 1s binding energy due to recoil effects is observed under excitation at 7700 eV. It is shown that conduction band states have substantial 6s character and are selectively enhanced in hard X-ray photoemission spectra. However, the maximum amplitude in the Pb 6s partial density of states is found at the bottom of the valence band and the associated photoemission peak shows the most pronounced enhancement in intensity at high photon energy.     

Valence band photoemission study of the copper chalcogenide compounds, Cu2S, Cu2Se and Cu2Te

The electronic structures of the copper chalcogenide compounds, Cu₂S, Cu₂Se and Cu₂Te have been investigated by taking photoemission data with synchrotron photon sources. The band calculations are done using the full-potential linear-muffin-tin-orbital method. Since the crystal structures are not clarified well, several simplified structure models are used. The calculated densities of states are compared with the observed spectra. The analysis shows that a sharp peak at −3.5 eV is due to the Cu 3d states, and that the tails at the high and low energy sides of the Cu 3d peak are due to the chalcogen p states.     

Breakup of quasiparticles in thin-film quantum wells

Quantum well states in thin films are commonly described in terms of a quasiparticle confined in a quantum box, but this single-particle picture can fail dramatically near a substrate band edge, as shown by this angle-resolved photoemission study. Atomically uniform Ag films are prepared on Ge(111) to facilitate accurate line shape and dispersion relation measurements. A quantum well peak is observed to split into two peaks near the Ge valence band edge. The unusual line shapes are shown to be due to many-body interactions and are quantitatively explained by a Green's function calculation.     

Quantization of electronic states on metal surfaces

An electron in front of a metal surface experiences an attractive force due to the induced image charge. Band gaps in the band structure can prevent a penetration into the metal along certain directions. The Coulomb-like potential supports bound states in front of the surface which correspond to a hydrogen atom in one dimension. These image states can be measured with high resolution by two-photon photoemission. The adsorption of metals modifies the states. If the electrons can penetrate into the metal, quantum-well states can develop corresponding to standing waves in the overlayer. Image states on small islands show the quantization effects due to the lateral localization. The spectroscopy of image states by two-photon photoemission permits the investigation of growth and morphology of deposited metal layers, a well as the illustration of fundamental quantum-mechanical effects.     

The influence of oxygen deficiency and Sb doping on inverse photoemission spectra of SnO2

The changes in the empty electronic states in SnO₂ produced by ion-beam induced oxygen deficiency and by Sb doping have been studied by inverse photoemission spectroscopy. Inverse photoemission in SnO₂ itself is dominated by peaks 4 and 12 eV above the Fermi level, the former associated with empty states of dominant Sn 5p atomic character. Sb doping populates states in the Sn 5s conduction band, shifting the empty state structure closer to the Fermi energy. By contrast oxygen deficiency introduces new states above the main Sn 5p peak. These are tentatively described as 5s-5p hybrids pushed up in energy from the 5p band by mixing between atomic orbitais of different parity in the non-centrosymmetric cation environment of oxygen deficient SnO₂.     

Theory of core photoemission spectra in CeO2

By using the Anderson model with a filled valence band, we calculate the core photoemission spectra of CeO₂, and compare them with experimental results. It is shown that in the ground state of CeO₂ the 4?⁰ and 4?¹ configurations are mixed strongly due to the large hybridization between the 4? states and the valence band. In the final state of the photoemission, the 4?¹ and 4?² configurations are also mixed strongly due to the final state interaction coming from a core hole potential and the large hybridization. The fractional intensity of the 4?⁰ photoemission peak is considerably different from the weight of the 4?⁰ configuration in the ground state because of the strong final state interaction.     

The electronic structure of surfaces with the matching green function method: II. fcc and bcc transition metal surfaces

Using the matching Green function method, the densities of states at (001) surfaces of Ni (fcc) and Mo (bcc) are calculated. Fixing the wavevector parallel to the surface, the surface density of states at Ni(001) is similar to the bulk, with band edge singularities rounded off and reflected in big surface resonances. Changes are much greater in the case of Mo(001), and the surface density of states has a central peak in the minimum of the bulk density of states. This peak arises from the 4d level in the surface atoms interacting weakly with the substrate and with neighbouring surface atoms. Our results are compared with angular resolved photoemission theory and experiment — in the case of Ni all the experimental peaks correspond to bulk k-conserving transitions, though some surface resonances in the Mo surface density of states show up in photoemission.     

Heavy-fermion-like state in a transition metal oxide LiV2O4 single crystal: indication of Kondo resonance in the photoemission spectrum

We have performed a vacuum ultraviolet laser excited photoemission spectroscopy on a d-electron heavy-fermion-like material LiV2O4 single crystal. We observed a sharp peak structure in the density of states at approximately 4 meV above the Fermi level (E(F)). The evolution of the peak height corresponds well with the crossover behavior to the heavy-fermion-like state as observed in the thermal and transport properties. The position, shape, and temperature (T) dependence of the peak structure is quite similar to the Kondo resonance observed in conventional f-electron heavy Fermion compounds.     

Origin of the peak-dip-hump line shape in the superconducting-state (pi,0) photoemission spectra of Bi2Sr2CaCu2O8

From detailed high-resolution measurements of the photon energy dependence of the (pi,0) superconducting-state photoemission spectrum of the bilayer Bi high-temperature superconductors, we show that the famous peak-dip-hump line shape is dominated by a superposition of spectral features originating from different electronic states which reside at different binding energies, but are each describable by essentially identical single-particle spectral functions. The previously identified bilayer-split CuO2 bands are the culprit: with the "superconducting" peak being due to the antibonding band, while the hump is mainly formed by its bonding bilayer-split counterpart.     

Chemisorption of CO on Ir(100) studied by photoemission

Chemisorption of cabon monoxide on a reconstructed Ir(100) surface has been studied by means of LEED and photoemission. In the photoemission spectrum, apart from a low lying peak due to the 4σ-orbital of CO, there is also a broad bump divided by a clear dip. The upper part of this double peak is assigned to the 5σ orbital and the lower part to the 1 π orbital of CO.     

Photon- and electron-induced surface voltage in electron spectroscopies on ZnSe(0 0 1)

The surface band bending in ZnSe(0 0 1), as a function of the temperature, is investigated both in the valence band (by photoemission) and in the conduction band (by inverse photoemission and absorbed current spectroscopies). Two different mechanisms are invoked for interpreting the experimental data: the band bending due to surface states, and the surface voltage induced by the incident beam. While the latter is well known in photoemission (surface photovoltage), we demonstrate the existence of a similar effect in inverse photoemission and absorbed current spectroscopies, induced by the incident electrons instead of photons. These results point to the importance of considering the surface voltage effect even in electron-in techniques for a correct evaluation of the band bending.     

Origin of localized states in graphite: Indirect photoemission processes or impurities?

The electronic band structure of different types of graphite samples have been investigated in order to identify the origin of non-dispersive density of states recently reported in the literature. A systematic series of synchrotron radiation angle resolved photoemission spectroscopy (ARPES) measurements on graphite single crystal, highly oriented graphite (HOPG) and epitaxial grown graphite single crystal on 6H-SiC(0 0 0 1) samples, have been carried out as well as compared with theoretical tight binding calculations. Our results indicate that these localized states are present in all the graphite-investigated samples showing the same non-dispersive character and at the same binding energies. The photoemission data taken at several photon energies demonstrate that these states are not surface states nor due to indirect photoemission processes. It seems that they are closely related to the level of impurities present in the studied samples.     

Non-conservation of k in photoemission into electrolytes

Recent measurements of the photoemission yield from copper single crystals into electrolyte revealed unexplained polarization and crystallographic anisotropies. It is shown that these anisotropies indicate a breakdown of parallel momentum conservation at the interface due to scattering by water molecules. The similarity between the experimental results above and below the interband transition threshold suggests the involvement of evanescent states in photoemission below threshold.     

Intrinsic accuracy in 3-dimensional photoemission band mapping

Fundamental principles of mapping 3-dimensional quasiparticle dispersions in the valence band using angle-resolved photoemission spectroscopy are discussed. Such mapping is intrinsically limited in accuracy owing to damping of the final states, resulting in equivalent broadening in the surface-perpendicular wavevector. Mechanisms of the intrinsic accuracy are discussed in depth based on a physically transparent picture involving interplay of the final- and initial-state spectral functions, and illustrated by photoemission simulations and experimental examples. Other interesting effects of 3-dimensional dispersions include ‘ghost’ photoemission peaks outside the Fermi surface and finite peak width at the Fermi level. Finally, optimization of the experiment on the intrinsic accuracy is discussed.     

Photoemission from laser excited semiconductors: Dynamic response of the electronic structure in Si

Using angle-resolved photoemission in coincidence with a pulsed Nd:Yag laser beam (hν_L = 2.33 eV), we have studied the dynamic rearrangement of the electronic states in Si in the presence of a large number of electron hole pairs created by laser photon excitation. We observed considerable changes in the valence band structure, a renormalization of the bandgap, and a satellite peak in the 2p core level emission due to more effective core hole screening.     

Electronic structure of Ce-pnictides

High-resolution x-ray photoemission has been used to study the electronic structure of the Ce-pnictides (CeN, CeP, CeAs and CeSb). This series of isostructural compounds allows us to follow the evolution of the 4f level as the distance between Ce atoms varies. Core level spectra show clearly that the 4f state is localized in all the compounds. In the valence band region, the width of the 4f peak is strongly influenced by the energy overlap with the extended states originating from anion p states. The spectra of CeN reveal the existence of a mixed configuration.     

Unusual spectral renormalization in hexaborides

Employing high resolution photoemission spectroscopy, we studied the evolution of the spectral features in rare earth hexaboride single crystals as a function of temperature and 4f binding energy, where the variation of the 4f binding energy is obtained by changing the rare earth element. High energy resolution helped to reveal the distinct features corresponding to the various photoemission final states. Experimental results of CeB(6), a dense Kondo system, exhibit the growth of the features near the Fermi level with the decrease in temperature relative to the uncompensated local moment contributions. The valence band spectra of the antiferromagnetic compounds, PrB(6) and NdB(6), exhibit multiple features-the 4f ionization peaks (poorly screened features) appear at higher binding energies and the features in the vicinity of the Fermi level possessing significant 4f character are due to the well-screened photoemission final states. These results indicate finite hybridization between the 4f and B 2s2p conduction electronic states. Interestingly, the well-screened features in PrB(6) and NdB(6) exhibit unusual enhancement in intensity at low temperature.     

The temperature dependence of the interaction of oxygen with Pd(111); A study by photoemission and auger spectroscopy

The temperature dependence of the different binding states of oxygen on Pd(111) has been studied using photoemission spectroscopy and Auger spectroscopy. By examining the oxygen 2s binding energies, three distinct states are observed, which are identified as surface chemisorbed oxygen, oxygen incorporated into the first Pd layer, and subsurface oxygen. Changes in the relative oxygen Auger peak heights for 1000 K exposure temperature are explained if the oxygen is assumed to become more negatively charged as it penetrates the Pd surface. A model for the interaction of oxygen with the Pd(111) surface is proposed.     

Electronic exchanges between adsorbed Ni atoms and TiO2(1 1 0) surface evidenced by resonant photoemission

Nickel was deposited on stoichiometric TiO₂(1 1 0) surface in the 0.02–2.1 equivalent monolayer (eqML) range and analyzed by means of photoemission and resonant photoemission. In the case of very low coverage (lower than 0.1 eqML), deposited nickel reacts with the surface through an electronic transfer from nickel atoms towards titanium ions. This exchange caused the filling of unoccupied Ti3d states leading to the increase of a peak in the TiO₂ band gap. These states can be better characterized through resonant photoemission experiments at the Ti 3p → 3d absorption edge: for very low coverage, these states in the TiO₂ band gap have resonant behavior of Ti3d electrons rather than Ni3d ones, confirming the filling of Ti3d states and thus electron transfer between nickel and titanium. For coverage higher than 0.14 eqML, nickel peaks (both Ni3p core level and valence band) should be related to the presence of metallic nickel in small clusters.     

Core level shake up structures of N2 adsorbed on nickel surfaces: Cluster models

Self-consistent Hartee-Fock-LCAO calculations on a linear NiN₂ cluster are used to explain the two-peak structure observed in core level photoemission from N₂ adsorbed on the Ni (100) surface. The two observed peaks are due to different N₂ core hole final states, a screened and an unscreened final state that can be clearly identified in the cluster model. The energy separation and intensity ratio (computed using the sudden approximation) of the two final states in NiN₂ are comparable with the experimental peak separation and intensity ratio for reasonable cluster geometries. A comparison with previous model studies on NiCO is used to explain the different shape of the core level spectra of N₂ and CO adsorbed on nickel surfaces.