Showing papers on "Photoemission spectroscopy published in 1979"

High-Resolution X-Ray Photoelectron Spectroscopy as a Probe of Local Atomic Structure: Application to Amorphous Si O 2 and the Si-Si O 2 Interface

Abstract: The chemical structure of thin Si${\mathrm{O}}_{2}$ films and Si${\mathrm{O}}_{2}$-Si interfaces has been investigated using high-resolution x-ray photoelectron spectroscopy. The data are consistent with a continuous random network of four-, six-, seven-, and eight-member rings of Si${\mathrm{O}}_{4}$ tetrahedra joined together by bridging oxygens. This distribution changes substantially within 30 \AA{} of the Si${\mathrm{O}}_{2}$-Si interface. The near-interface region is comprised of ${\mathrm{Si}}_{2}$${\mathrm{O}}_{3}$, SiO, and ${\mathrm{Si}}_{2}$O. This structure is interpreted by means of a structure-induced-charge-transfer model.

Interface chemistry of metal‐GaAs Schottky‐barrier contacts

Abstract: A survey of the metal‐semiconductor interface chemistry for GaAs and seven metals, Ag, Al, Au, Cr, Fe, Sn, and Ti, by using x‐ray photoemission spectroscopy (XPS) is reported Sn and Ag each form an abrupt inert interface with GaAs Au, Al, Fe, Cr, and Ti each form a chemically reacted nonabrupt interface with a trend for increasing dissociation of GaAs in the order listed Also reported is the first observation of epitaxial Fe growth on GaAs

Ultraviolet photoemission and low-energy-electron diffraction studies of Ti O 2 (rutile) (001) and (110) surfaces

Abstract: We have examined the (001) and (110) faces of Ti${\mathrm{O}}_{2}$ (rutile) using ultraviolet photoelectron spectroscopy (UPS), low-energy-electron diffraction (LEED), and Auger electron spectroscopy (AES). We found that the (001) surface is unstable and reconstructs or facets when annealed while the (110) surface is stable and does not reconstruct. Other indications of the relative instability of the (001) surface include the presence of a surface phase change for $Tg950\ifmmode^\circ\else\textdegree\fi{}$C, significant oxygen-adsorption effects, and thermal instability in the concentration of surface ${\mathrm{Ti}}^{3+}$-O-vacancy defects.

Determination of the Ni{001}c(2×2)-CO Structure by Low-Energy Electron Diffraction

Abstract: We have resolved the inconsistency between low-energy electron diffraction (LEED) and ultraviolet photoemission spectroscopy (UPS) analyses concerning the orientation of molecular CO when chemisorbed in a $c(2\ifmmode\times\else\texttimes\fi{}2)$ structure on the Ni(001) surface. Dynamical analysis of new LEED data indicates that CO stands perpendicular to the Ni surface with the C atom on top of a Ni atom; preliminary results for the Ni-C and C-O separations are 1.72 and 1.15 \AA{}, respectively.

Energy position of 4f levels in rare-earth metals

Abstract: The energy position of the occupied and unoccupied 4f levels relative to the Fermi energy is studied for the rare-earth metals. This is done by treating the excited state as an impurity in an otherwise perfect crystal. This picture is first considered in the complete screening approximation. In this approximation thermochemical data can be used directly to give energy values for the unoccupied f levels which are accurate to +- 0.05 eV; for the occupied f levels the uncertainty is somewhat larger. The obtained values are then compared with recent x-ray photoemission spectroscopy (XPS) and Bremsstrahlung isochromal spectroscopy data. The deviations found are discussed in terms of impurity effects left out in the complete screening approximation. When these effects are included a general good agreement with experiment is obtained. The consistency found in the XPS results is utilized for an improved estimation of the fourth ionization energy of the rare earths.

d -like surface-state bands on Cu(100) and Cu(111) observed in angle-resolved photoemission spectroscopy

Abstract: Surface states at the top of the bulk $d$ bands of Cu(100) and Cu(111) single crystals have been observed using angle-resolved photoemission spectroscopy. The measured dispersion of the surface-state bands falls in absolute energy gaps of the projected bulk band structure near the symmetry point $\overline{M}$ of the two-dimensional Brillouin zone, for both faces. Since the parent energy gaps are not caused by the hybridization of crossed energy bands, the observed states are of the Tamm rather than the Shockley type; surface states of the latter type have been observed before in $\mathrm{sp}$ gaps of several noble metals. Tamm states require a sufficiently strong surface "perturbation." Only the self-consistent calculations of Gay, Smith, and Arlinghaus recently reported for Cu(100) contain surface states in the energy-wave-vector region studied here. The observed peaks are very narrow, due to their energy location above the bulk $d$-band continuum as well as the good angle and energy resolution of our spectrometer.

Study of the 4 f Levels in Rare-Earth Metals by High-Energy Spectroscopies

Abstract: The density of the $4f$ states in all metallic rare earths has been studied with bremsstrahlung isochromat spectroscopy (BIS) and x-ray photoemission spectroscopy (XPS). The final-state multiplet structures of the $4{f}^{14\ensuremath{-}n}$ and $4{f}^{n}$ configurations observed by BIS and XPS, respectively, are closely related. The elements showing interconfiguration fluctuations are found to have one $4f$ level in the immediate vicinity of the Fermi level. Correlation energies are extracted directly from the spectra and compared with recent theoretical calculations.

X-ray photoemission spectroscopy study of zirconium hydride

Abstract: X-ray photoemission spectroscopy (XPS) measurements are reported for ZrH/sub 1.65/ and Zr metal. The valence-band measurements are compared with available band-theory density-of-states calculations for the metal and hydride. The hydride spectrum differs significantly from the metal spectrum. Most important, a strong peak associated with hydrogen s electrons appears approximately 7 eV below the Fermi level. XPS measurements of Zr 4p core levels show a binding-energy shift of 1 eV between Zr metal and ZrH/sub 1.65/. It is argued that this shift results from charge readjustment in the vicinity of the Zr site. With the addition of hydrogen, net charge must be transferred from the Zr site to the hydrogen site. A charge-density analysis based on simplified cluster calculations is presented.

Effects of vibronic interaction and autoionization on the photoelectron spectrum of N2O

Abstract: High‐resolution and high‐sensitivity HeI photoelectron spectra (PES) are reported for the first four valence levels of N2O. The vibrational structure, including many new peaks, is completely assigned for the X 2Π, A 2Σ+, and C 2Σ+ electronic states, and in all three cases excitation of a single quantum of the bending vibration is observed. The bending mode appears as a result of vibronic coupling within and between different electronic states. The Renner–Teller splitting is resolved in the (0,1,0) band of the transition to the X 2Π state. The relative intensities of the vibrational bands in the X 2Π and A 2Σ+ states are compared to the intensities determined from the threshold photoelectron spectra (TPES) of Frey et al. [Chem. Phys. Lett. 54, 411 (1978)]. In both states, autoionization of intermediate neutral states that are nearly degenerate with the molecular ion state to which they decay increases the relative intensities of the higher vibrational bands in the TPES. This resonant autoionization process...

Systematic Organic UV Photoelectron Spectroscopy

Abstract: Photoelectron spectroscopy (PES) provides valuable information on the ionization energies of atoms and molecules. The ionization energy (IE) is given by the relation. hv = IE + T where hv is t h e energy of the radiation and T i s the kinetic energy of the electron. The IEs are directly related to the orbital energies (Koopmans' theorem). By employing UV radiation (HeI. 21.2 eV. or HeII. 40.8 eV). extensive data on the ionization of valence electrons in organic molecules have been obtained in recent years. These studies of UV photoelectron spectroscopy. originated by Turner, have provided a direct probe into the energy levels of organic molecules. Molecular orbital calculations of various degrees of sophistication are generally employed to make assignments of the PES bands. Analysis of the vibrational structure of PES bands has not only provided structural information on the molecular ions, but has also been of value in band assignments. Dewar and co-workers [1, 2) presented summaries of availabl...

Determination of excited state lifetimes and ionization potentials by dual beam visible lasers

Abstract: A two-laser multiphoton ionization (MPI) technique is used to measure the lifetime and ionization potential of the lowest excited Rydberg state of 1,4-diazabicyclo[2,2,2]octane (DABCO). In this method one laser causes a two-photon transition to the excited state, and a second laser, at variable time delays from the first, ionizes only the excited state molecules. Good agreement is found between the fluorescence lifetime of this two-photon state and the lifetime determined from the ionizing pulse delay. By scanning the ionizing laser wavelength at a fixed time delay, the ionization threshold is obtained. The dual-beam MPI threshold is sharper than the onset of the photoelectron spectrum. This is due to the fact that transitions between Rydberg levels and the ionization continuum (which determine the shape of the threshold in this technique) are more Franck-Condon allowed than the transition between the ground state of the molecule and the ground state of the ion (observed in the photoelectron spectrum). This two-year MPI method, which requires neither fluorescence, vacuum UV nor photoelectron equipment, should be applicable to a wide range of molecules. A simple qualitative picture of DABCO MPI dynamics has also been constructed from an analysis of the dependence of the fluorescence intensity and the delayed ion current on the laser light intensity. Single-laser MPI spectroscopy is shown to be limited by competition between ionization and dissociation or other quenching processes in the levels preceding the ionization continuum. The dual-beam MPI technique can be used in such a manner as to bypass dissociative states.

Surface oxidation of nickel metal as studied by X-Ray photoelectron spectroscopy

Abstract: X-ray photoelectron spectroscopy has been used to study the oxidation of polycrystalline nickel metal. The results indicate that the oxidation process takes place in three stages; associative adsorption of molecular oxygen, followed by the combination of oxygen atoms with surface nickel atoms and, ultimately, the formation of bulk oxide. At room temperature only the first two stages can be detected. For exposures below 1 L the O 1s photoelectron spectrum is considered to be characteristic of an associatively adsorbed oxygen species, but for exposures above this value evidence for the formation of a monolayer of “NiO” is suggested by the development of an O 1s peak at 529.9 eV. Incorporation of oxygen into the nickel lattice is observed at temperatures >500°K. The activation energy for this place-exchange process was estimated at 1.80±0.06 eV.

Substrate effects on the electronic structure of metal overlayers-an XPS study of polymer-metal interfaces

Abstract: The electronic structures of copper, nickel, and chromium overlayers on polystyrene and polyvinyl alcohol have been investigated with x-ray photoemission spectroscopy. At submonolayer coverages, the peak position and width of the metallic-core levels vary significantly from one substrate to the other. Most of these variations can be accounted for in terms of changes in the atomic and extra-atomic relaxation energies during the photoemission process. Much of this change is brought about when the metal atom deposited on polyvinyl alcohol interacts with the substrate oxygen and forms a metal-oxygen-polymer complex. The presence of this complex is verified by changes in the photoemission line shapes of the substrate carbon and oxygen atoms.

Photoemission and Auger spectroscopy studies of the interaction of oxygen with zinc

Abstract: Photoemission spectroscopy (u.p.s. and X.p.s.) and Auger Spectroscopy (X-ray induced) have been used to study the oxidation of clean evaporated zinc films. At 300 K, two oxygen species coexist throughout the adsorption process. The major species [O(1s) b.e. 530.0 eV] is characteristic of oxide and is considered representative of a process wherein penetration of Zn layers occurs. The minor species [O(1s) b.e. ≃ 531.8 eV] represents oxygen confined more to the surface region and is electronically dissimilar from oxide. Though performed under u.h.v. conditions, we cannot rule out the possibility that this species is hydroxyl, since for adsorption at 77 K we demonstrate that some molecular H2O appears on the surface at high exposures. Adsorption to saturation at 77 K, followed by warming to 300 K produces less of the oxide phase and more of the minor oxygen species compared to 300 K adsorption. Adsorption at higher temperatures than 300 K increases the amount of oxide formation.Oxygen uptake values are estimated from peak relative intensities. The reliability and model dependent nature of such estimates is discussed. The results indicate that saturation coverage at 300 K is greater than or equal to 2.5 monolayers.The results of the present study are compared with other recent studies and it is concluded that the mechanism of oxide growth is still not fully understood, though it is clearly different from Ni or Fe since for Zn the growth of oxide nuclei is not preceded by chemisorption and induction periods as it is for Ni and Fe.

Photoelectron and Auger spectroscopy of CuCl

Abstract: The x-ray-photoelectron spectrum of the valence band of CuCl is compared with recent band-structure calculations to obtain information about the electronic structure and the energy gap. The gap at room temperature is found to be larger than 2 eV; too large to expect a decrease to zero at high pressures. A drastic change in the chemical structure of the material is therefore necessary to cause the recently reported high-temperature superconductivity at moderate pressures. In addition to the one-hole density of states possibly obtained from photoelectron spectroscopy, we also studied with Auger spectroscopy the two-hole density of states. From this we obtain the hole-hole Coulomb repulsion energies as well as some information about the hybridization.

Photon-correlation spectroscopy in the near ultraviolet.

Abstract: We describe the first reported photon-correlation spectroscopy experiments in the ultraviolet (363.8 nm) and briefly discuss the use of this technique to study the short-wavelength relaxation of the internal modes of large, flexible macromolecules, in particular, DNA.