Grazing incidence X-ray photoemission spectroscopy of SiO2 on Si

We have applied grazing incidence X-ray photoemission spectroscopy to the determination of the thickness of SiO₂ layers on Si, as well as surface carbon that is present. The measurements take advantage of the different optical constants of the layers. X-rays incident on the surface at grazing angle undergo total external reflection, where the fields in each layer are subject to highly non-linear changes as a function of incidence angle. X-ray photoemission excited by these fields gives information on atomic species, chemical state, and layer thickness. Simultaneous fits are made to the photoemission spectra in each layer. The method is illustrated for a thermally grown oxide layer and a native oxide on Si.     

How different is pyrimidine as a core component of DNA base from its diazine isomers: A DFT study?

Recent photoemission spectroscopic (X‐ray photoemission spectra) study revealed less dramatic chemical changes for pyrimidine (PyM, 1, 3‐diazine) with in its ionization potential. Present systematic study using density functional theory calculations shows that PyM is indeed quite different from its diazine isomers (PyD, 1, 2‐diazine and PyA, 1, 4‐diazine). It is discovered that the most stable isomer PyM is relaxed from C_2V to C₁ point symmetry with a total electronic energy deduction of ?15.86 kcal.mol^?1. Although not substantial, PyM has the smallest molecule shape (electronic spatial extent) and the largest HOMO‐LUMO energy gap of 5.65 eV; only one absorption band in the region of 200–300 nm of the UV‐Vis spectrum but three clusters of chemical shift in the carbon and hydrogen NMR spectra. The energy decomposition analyses revealed that the interaction energy (ΔE_Int) of PyM is preferred over PyA by 4.08 kcal.mol^?1 and over PyD by 22.32 kcal.mol^?1, with the preferred N? C? N bond revealed by graph theory.     

X-ray photoelectron spectroscopy of surface-treated indium-tin oxide thin films

Angle-resolved X-ray photoelectron spectroscopy and photothermal deflection spectroscopy are used to study the oxygen-plasma or aquaregia treated indium-tin oxide (ITO) anodes for organic light-emitting diodes. Detailed analysis of the O1s core-level spectra and their dependence on photoemission angle was performed. The results indicate the presence of different chemical forms of oxygen atoms (two types of O²⁻, OH⁻, organic oxygens and H₂O) which evolve with surface treatment. We find that the treatments lead to a modification of the surface chemical states and therefore of the physico-chemical properties of ITO, which in turn control the performance of organic light-emitting diodes.     

Effect of nonadiabaticity on the Fermi-edge photoemission and tunneling spectra of superconductors

Temperature dependence of the experimental photoemission spectra of the high-T_c superconductors have been interpreted and simulated by the recently formulated molecular nonadiabatic theory of the electron-vibrational interaction. The change of the spectral feature below T_c, particularly formation of the narrow, near Fermi surface (FS) edge, peak separated by the dip from a broad spectral band at higher binding energies has been shown to be the consequence of the nonadiabatic electron-phonon coupling and corresponding properties of the nonadiabatic polarons. Simulation yields very good fit to the experimental temperature-dependent spectra, with the calculated value of the gap 30.5 meV. © 1996 John Wiley & Sons, Inc.     

Oxidation state of tungsten in the NaxWO3bronzes

X-ray photoemission spectra of W4f electrons in vacuum-cleaved cubic Na_xWO₃ covering the entire composition range give no evidence for the existence of multiple W valences states in the bulk. Strong oxidation effects are observed in air exposed surfaces.     

Intramolecular band mapping of n-CH3(CH2)34CH3 over the whole Brillouin zone by angle-resolved photoemission

Angle-resolved photoemission spectra were measured for an oriented film of a model compound of polyethylene (hexatriacontane n-CH³(CH₂)₃₄CH₃) using synchrotron radiation. The photon energy (hv) dependence of the normal emission spectra was confirmed to be due to intramolecular energy-band dispersion, and the dispersion over the whole Brillouin zone was determined.     

X-ray and UV photoemission studies of valence electronic structure of NiO

The X-ray and UV photoemission valence band spectra of NiO are interpreted using the molecular orbital theory for the NiO^10?₆ cluster and the sudden approximation (monopole selection rules). They exhibit the effects of crystal field splitting, multiplet splitting, electron shake-up (O 2pe^b_g→ Ni 3de^a_g). relaxation and Ni 3dO 2p hybridization. Shake-up satellite data indicate that the NiO optical absorption edge near 4 eV is associated with an O 2p → Ni 3d transition. The NiO valence electronic structure obtained in this work is compared with band structure models of Wilson and Mattheiss.     

Electronic structure of polyenes and polyacetylene

The CNDO/S2 model is applied to evaluate the molecular orbitals and ultraviolet absorption spectra of the series of polyenes C_4nH_4n+2, n = 1, 2, 3, 4; extended to encompass the infinite polyene chain, i.e., polyacetylene; and utilized to interpret the photoemission and absorption spectra of polyacetylene.     

The interaction of electrons with the molecular vibrations of benzene

The linear coupling constants describing the interactions with lattice vibrations of π-electrons in benzene are extracted from ultraviolet absorption and photoemission spectra. They are predicted adequately by molecular-orbital theory which further predicts large quadratic-coupling terms.     

Calculation of transition metal compounds using an extension of the CNDO formalism. V. many body effects in the inner valence shell photoionization spectra of free and coordinated carbon monoxide

The inner valence shell region in the photoionization spectra of NiCO, Ni(CO)₄ and Cr(CO)₆ is studied by means of a diagonal two-particle—hole Tamm—Dancoff approximation. Many body effects are found in the energy range of the CO-ligand ionizations. The effects are more pronounced in NiCO and Ni(CO)₄ than in Cr(CO)₆. Since similar results have to be expected for adsorbate systems, the interpretation of photoemission spectra of such systems on the basis of the simple one particle picture should be handled with care.     

A photoelectrochemical study of the anodic oxidation of lead in alkaline solution

The oxidation of lead in sodium tetraborate and in sulphuric acid solution has been studied. The analysis of photocurrent spectra has shown that tetragonal PbO is formed in both electrolytes. Evidence for photoemission from the metal substrate into the anodic oxide film is presented and discussed. The photocurrent behaviour is related to changes in electrode impedance which accompany the interconvertion of PbO and PbO₂.     

Direct observation of the anderson transition in hxwo3 bronzes by high-resolution x-ray photoelectron spectroscopy

We report high-resolution X-ray photoemission measurements of semiconducting and metallic hydrogen bronze H_xWO₃ as a function of the hydrogen/tungsten ratio. Complex structures in W 4f core-level spectra can be attributed to two discrete initial states in the semiconducting phase and to three states in the metallic phase. These results, and a comparison of valence and conduction band spectra with band structure calculations, are consistent with an Anderson transition at x ≈ 0.2 and conflict with percolation or Mott-type transition models.     

Computational Study of the Electron Spectra of Vapor-Phase Indole and Four Azaindoles

After geometry optimization, the electron spectra of indole and four azaindoles are calculated by density functional theory. Available experimental photoemission and excitation data for indole and 7-azaindole are used to compare with the theoretical values. The results for the other azaindoles are presented as predictions to help the interpretation of experimental spectra when they become available.     

Growth and characterization of ultrathin carbon films on electrodeposited Cu and Ni

Ultrathin carbon films were grown on different types of metallic substrates. Free‐standing foils of Cu and Ni were prepared by electroforming, and a pure Ni film was obtained by galvanic displacement on a Si wafer. Commercial foil of Ni 99.95% was used as a reference substrate. Carbon films were grown on these substrates by chemical vapour deposition in a CH₄‐H₂ atmosphere. Obtained films were characterized by Raman spectroscopy, X‐ray photoelectron spectroscopy (XPS), Auger electron spectroscopy, and ultraviolet photoemission spectroscopy. The XPS at grazing collection angle was used to determine the thickness of carbon films. Depending on the deposition parameters, the films of graphene or graphite were obtained on the different substrates. The uniformity of graphene and its distribution over the sample area were investigated from Raman data, optical images, and XPS chemical maps. The presence of graphene or graphite in the films was determined from the Raman spectra and Auger peak of C KVV. For this purpose, the D parameter, which is a fingerprint of carbon allotropes, was determined from C KVV spectra acquired by using X‐rays and electron beam. A formation of an intermediate layer of metal hydroxide was revealed in the samples with graphene overlayer.     

Inner shell excitation of SO2 by high-energy electron impact: a comparison with multichannel quantum defect theory

High-resolution electron energy loss spectra of the sulfur 2p, 2s and oxygen 1s inner shells of SO₂ have been obtained under conditions of high impact energy (1–3 keV) and zero-degree scattering angle. The results are also compared with photoabsorption spectra in the sulfur 1s and 2p regions. Multichannel quantum defect theory calculations of energies and oscillator strengths for S 2p, 2s, O 1s and S 1s are in excellent quantitative agreement with the measurements and are used to assign the spectra.     

Angle-resolved ultraviolet photoemission of pyridine adsorbed on Pd(111)

The valence levels of phridine adsorbed on Ph(111) at room temperature are investigated by angle-resolved UV photoemission using Hel and Hell resonance radiation. The spectra are interpreted in terms of a reduction of the surface molecule symmetry from C_2v to C₈. We suggest that pyridine is adsorbed with its aromatic ring plane tilted wit respect to the surface interaction with the substrate taking place through both N lone-pair and π electrons.     

X-ray photoelectron spectra of Ti4+ in TiO2. Evidence of band structure

Satellite structure has been observed at about 3.1, 5.4, 6.4 and 14.5 eV below the main peaks in the X-ray photoelectron spectra of the Ti and O levels of TiO₂. These satellites arise from transitions, accompanying primary photoemission, between predominantly O_2p states of the ligand and various excited states in the conduction band. The energies found fit the transitions calculated by Daude et al. for an electronic band structure calculated by a combined tight-binding and pseudopotential method.     

In Situ and Theoretical Studies for the Dissociation of Water on an Active Ni/CeO2 Catalyst: Importance of Strong Metal–Support Interactions for the Cleavage of O–H Bonds

Water dissociation is crucial in many catalytic reactions on oxide‐supported transition‐metal catalysts. Supported by experimental and density‐functional theory results, the effect of the support on O? H bond cleavage activity is elucidated for nickel/ceria systems. Ambient‐pressure O 1s photoemission spectra at low Ni loadings on CeO₂(111) reveal a substantially larger amount of OH groups as compared to the bare support. Computed activation energy barriers for water dissociation show an enhanced reactivity of Ni adatoms on CeO₂(111) compared with pyramidal Ni₄ particles with one Ni atom not in contact with the support, and extended Ni(111) surfaces. At the origin of this support effect is the ability of ceria to stabilize oxidized Ni²⁺ species by accommodating electrons in localized f‐states. The fast dissociation of water on Ni/CeO₂ has a dramatic effect on the activity and stability of this system as a catalyst for the water‐gas shift and ethanol steam reforming reactions.     

Core level study of alanine and threonine

Core level X-ray photoemission spectra (XPS) and near edge X-ray absorption fine structure (NEXAFS) spectra of alanine and threonine in the gas phase have been measured at the carbon, nitrogen, and oxygen K edges and interpreted in the light of theoretical calculations. For the computations, a set of approximations is made which allows sufficiently accurate calculations of several conformers to be performed in reasonable computing time. The accuracy has been checked by comparing results obtained for proline to our previous, higher level calculations. The photoemission spectra at the carbon and oxygen edges are assigned and compared. The nitrogen 1s photoemission peaks show anomalous broadening which we relate to the populations and types of conformers. The carbon K-edge NEXAFS spectra of alanine and threonine are compared with our previous data on glycine and resonances assigned accordingly. The nitrogen K-edge NEXAFS spectra of alanine and threonine do not show measurable effects due to the population of conformers, in contrast to the photoemission results. At the oxygen K edge, the spectra of these amino acids are similar with two prominent peaks assigned to transitions of O 1s electrons from the oxo and hydroxyl groups to vacant pi* and sigma* orbitals and additional intensity for threonine due to the second OH group. Conformer effects are observable in photoemission but appear to be more difficult to resolve in photoabsorption. We explain this by energetic shifts of opposite sign for the core hole states and unoccupied orbitals, which causes partial cancelation in NEXAFS but not in photoemission.     

The transport gap of organic semiconductors studied using the combination of direct and inverse photoemission

The combination of valence band photoemission and inverse photoemission spectroscopy is applied to study the densities of occupied and unoccupied states of perylene derivative and phthalocyanine organic layers on inorganic semiconductors. The ionisation energies and electron affinities are determined and it is proposed that the transport gap of the materials can be evaluated from the distance of the HOMO and LUMO edges. The resulting values for the transport gap which are somewhat smaller than other published data are in good agreement with e.g. electrical measurements. The experimental spectra are compared with simulated ones obtained by density functional theory calculations.