Ultraviolet Photoemission Measurements Research Articles

Hydrogen on WO 3(001)

Ultraviolet photoemission measurements of the (001) surface of WO 3 show that after exposure to atomic hydrogen new states appear in the energy gap region at 2.3 and 0.9 eV below the Fermi energy. A peak at 2.2 eV was also produced by hydrogen ion bombardment. To investigate whether these surface effects were due to the formation of hydrogen tungsten bronze H x WO 3, comparison was made with spectra from the electronically similar sodium bronze. The form of the spectra were different both in the valence band and energy gap regions indicating that hydrogen adsorption or loss of oxygen from the surface rather than formation of H x WO 3 is the major effect.

Hydrogen on WO 3(001)

Ultraviolet photoemission measurements of the (001) surface of WO 3 show that after exposure to atomic hydrogen new states appear in the energy gap region at 2.3 and 0.9 eV below the Fermi energy. A peak at 2.2 eV was also produced by hydrogen ion bombardment. To investigate whether these surface effects were due to the formation of hydrogen tungsten bronze H x WO 3, comparison was made with spectra from the electronically similar sodium bronze. The form of the spectra were different both in the valence band and energy gap regions indicating that hydrogen adsorption or loss of oxygen from the surface rather than formation of H x WO 3 is the major effect.

Ultraviolet photoemission studies of CuCl

Ultraviolet photoemission measurements on evaporated films of CuCl were made at photon energies 7-27 eV. Spectral quantum yield and photoelectron spectra were obtained. Doublet features can be seen in the predominantly Cu-$3d$ and Cl-$3p$ valence-band spectra. In addition, a stationary final-state structure at 2.3\ifmmode\pm\else\textpm\fi{}0.2 eV above the vacuum level is found in the photoelectron spectra for photon energies above approximately 18.5 eV. This structure is interpreted as an Auger effect. The spectra provide information on the width of the gap between the valence and conduction bands of CuCl.

A photoemission study of liquid and solid thallium, lead and bismuth

Abstract Ultra-violet photoemission (hω⩽ 21.2 eV) measurements for Tl, Pb and Bi in the liquid and solid (frozen) states are reported. The spectra for the solid phase are consistent with other published data and are broadly similar to the spectra for the liquid phase. Some loss of detail in the spectra for Tl and Bi is attributed to changes of ionic structure on melting. Optical-density-of-states functions for the liquid phase are derived using the three-step model. Significant deviation from the prediction of weak scattering formalisms are found, features associated with the 6s and 6p levels are clearly distinguished. The calculated imaginary part of the dielectric constant, taking account of both Drude and interband transitions, is consistent with experiment.

Ultraviolet photoemission studies of molecular adsorption on oxide surfaces

This paper reviews the current status of ultraviolet photoemission (UPS) measurements of the interaction of molecules with the surfaces of metal oxides. A brief summary of the electronic, geometric and compositional properties of atomically clean oxide surfaces is given, followed by a survey of the various adsorbate/substrate systems that have been investigated by UPS. Essentially all of that work to date has been on single-crystal oxide substrates. A brief description is also given of the various theoretical methods that are being used to study molecule/oxide interactions.

A comparative study of amorphous and crystalline superconducting molybdenum films by ultraviolet photoemission spectroscopy

Ultraviolet photoemission measurements at two photon energies are carried out, to understand the strong enhancement of the superconducting transition temperature T c in the nitrogen stabilized amorphous molybdenum films. The rise of T c can be attributed to a 40% increase of the electronic density of states at the Fermi level N(E F) and a 15% softening of the phonon spectrum.

Chemical reactions and local charge redistribution at metal-CdS and CdSe interfaces

The surface electronic structure of CdS and CdSe has been studied during the initial stages of Schottky-barrier formation with metals. Surface photovoltage spectroscopy, low-energy-electron-loss spectroscopy, and ultraviolet photoemission measurements taken as a function of metal overlayer coverage show that interface chemical reactions and local charge redistribution dominate the Schottky-barrier formation for both of these semiconductors. Despite the reported contrast in Fermi-level pinning behavior of CdS and CdSe, intrinsic surface states play no role in determining their Schottky-barrier heights. The microscopic metal-semiconductor bonding causes work-function and band-bending changes which determine the macroscopic space-charge characteristics of the interface. A critical heat of reaction $\ensuremath{\Delta}{H}_{R}^{C}\ensuremath{\sim}0.5$ eV/(metal atom) characterizes the compound formation inferred from alterations in the interface dielectric properties.

Ultraviolet photoemission measurements of the band structure ofTiOx(0.93≤x≤1.15)

We have measured the ultraviolet photoemission spectra for vacuum-fractured Ti${\mathrm{O}}_{0.93}$ and Ti${\mathrm{O}}_{1.15}$ and find clear evidence for an energy gap between the $\mathrm{O}(2p)$ and $\mathrm{Ti}(3d)$ bands, in disagreement with recent Hartree-Fock calculations of Jennison and Kunz. Exposure to ${\mathrm{O}}_{2}$ depopulates the $\mathrm{Ti}(3d)$ band near the surface, suggesting an explanation for the absence of emission from that band in published x-ray photoemission spectra.

Molecular Geometries of Acetylene and Ethylene Chemisorbed On Cu, Ni, Pd, and Pt Surfaces

Ultraviolet photoemission measurements of the valence orbital electronic structure of acetylene and ethylene chemisorbed on Cu(100) or Cu(111), Ni(111), Pd(111), and Pt(111) are presented. We compare the measured energy levels of these chemisorbed species to those of the free molecule and use a similar comparison of the relative changes in ground state energy levels of distorted free molecules calculated with a SCF-LCAO (Self Consistent Field—Linear Combination of Atomic Orbitals) method to determine the molecular geometries of these chemisorbed species. The limitations and accuracies of such an approach are discussed. From the determined geometries we identify two trends in the structure of these chemisorbed molecules on these surfaces: first, increasingly greater molecular distortions occur with increasing atomic number of the substrate atom, and secondly, greater molecular distortions occur for ethylene than for acetylene on the same metal. These trends are consistent with a π-d bonding interaction and can be accounted for by the electronic structure of the substrate and of the molecule, respectively. With the exception of ethylene on Pd or Pt, we determine molecular geometries characteristic of small rehybridization. The molecular geometry of ethylene on Pd or Pt is generally characteristic of rehybridization to an sp <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> configuration.

Surface electronic and chemical structure of [formula omitted]CdSe: Comparison with CdS

Surface photovoltage spectroscopy, Auger electron spectroscopy, LEED, X-ray and ultraviolet photoemission measurements are reported for (112̄0) CdSe under a variety of ultrahigh vacuum conditions. As with CdS, all surface electronic features can be related to chemical contamination, Ar + bombardment-induced lattice defects, or bulk trap states. Oxygen adsorption on CdSe and CdS produce qualitatively different electronic features which are attributed to different bonding at surface vacancy sites. Changes in surface atomic order show no direct effect on measured electronic features. Furthermore, CdSe exhibits no intrinsic surface state features which can account for its Schottky barrier formation with metals.

Photoemission from ferrocene, decamethylferrocene, and decamethylferrocene-bis(7,7,8,8-tetracyano-p-quinodimethane)

We present the first vacuum ultraviolet photoemission measurements of condensed ferrocene, decamethylferrocene, and the highly conductive charge transfer complex decamethylferrocene-7,7,8,8-tetracyano-p-quinodimethane, at photon energies from 7.7 to 21.2 eV. We compare the electronic spectra of these compounds and discuss substituent effects. The first ionization potentials are 6.1, 5.1, and 5.3 eV, respectively. The charge transfer complex is a semiconductor with its highest occupied states 0.8 eV below EF. The electron scattering lengths in ferrocene and decamethylferrocene are ?60 Å for electrons 0.5 eV above the vacuum level and decrease to ?2.5 Å as the energy is increased to 16 eV. From this result we estimate the electron affinity of solid ferrocene and decamethylferrocene to be about 1 eV.

Electronic structure of7,7,8,8−tetracyano−p−quinodimethane(TCNQ)

The electronic structure of $7,7,8,8\ensuremath{-}\mathrm{tetracyano}\ensuremath{-}p\ensuremath{-}\mathrm{quinodimethane}$(TCNQ) is calculated using a complete neglect of differential overlap (CNDO) parametrization similar to that recently introduced by Lipari and Duke for aromatic molecules. Comparison with various other theoretical methods is presented and it is shown that the present CNDO parametrization gives the best agreement with ab initio (i.e., Hartree-Fock) calculations. New ultraviolet photoemission data for solid TCNQ are presented and shown to be in good agreement with the gas-phase spectrum, in contrast to the results of others. The polarization energy of TCNQ is found to be 1.08 eV. The present CNDO parametrization gives excellent agreement with gas phase, and our solid-phase ultraviolet photoemission measurements. Furthermore, the optical spectrum is calculated and shown to be in agreement with the observed inelastic electron scattering data in the prediction of both the dipole-allowed transitions and the monopole-allowed transitions.

Photoemission studies of crystalline palladium and the metallic alloyPd0.775Cu0.06Si0.165in its glassy and crystalline forms

We report x-ray and ultraviolet photoemission measurements of the levels and valence bands of palladium and of the metallic alloy ${\mathrm{Pd}}_{0.775}$${\mathrm{Cu}}_{0.06}$${\mathrm{Si}}_{0.165}$ in both its glassy and crystalline forms. The x-ray photoemission measurements show that there is little significant difference between the relative peak positions of the various levels, and that only the copper core level widths in the glass alloy are broader than those in the crystal. The anomalously large width of the $4p$ levels, which is often more sensitive to the chemical environment than the chemical shifts, is also the same for all three samples. A large variation with angle was observed in the peak positions of the $3d$ core levels of the crystalline palladium sample. In the valence band the ultraviolet photoemission spectra of both forms of the alloy are quite similar. However, the valence band for the alloy is narrower than that for the pure palladium, and the density of states at the Fermi level is smaller. The results of these measurements are compared with the various theories of metallic glass stability, and it is suggested that chemical bonds do not contribute to the enhanced glass-forming tendency or stability against crystallization of this alloy. The results are consistent with the nearly-free-electron model of metallicglass stability.

The effect of disordering on the spectral properties of partial monolayers of H on Si(111)

Ultraviolet photoemission measurements are reported for a H covered Si(111) surface for which the H coverage ranged from a fraction to a full monolayer. These measurements reveal striking differences depending whether the Si(111) substrate is kept at room temperature (RT) or 150°C. In particular, the 150°C sample UPS spectral series shows monotonic growth with little line shape change while the RT series shows significant line shape modification with coverage. These results are interpreted as island growth at 150°C and disordered adsorption at RT. Theoretical model calculations are carried out of the electron density of states of a fractional monolayer of H chemisorbed to Si(111) that reproduce the essential features of the RT data and confirm the role of disordering there.

The density of states of liquid copper

Ultraviolet photoemission measurements (h(cross) omega =21.2 eV) on clean liquid copper samples are presented. On melting, the 3d band structure is observed to change in shape in agreement with recent multiple-scattering calculations.

The Antisymmetric Gap and the Total Width of the Valence Band of Binary Compound Crystals

AbstractSoft X‐ray emission, X‐ray photoemission, and ultraviolet photoemission measurements of binary compounds show an antisymmetric gap within the valence band. The ratio of this antisymmetric gap to the total width of the valence band is shown to be a simple function of the Phillips‐van Vechten ionicity parameter of the compound. This relation is valid as well for tetrahedrally as for octahedrally coordinated compounds and for such elemental compounds (Se, Te) which contain two kinds of bonds. The relation is useful to determine the ionicity parameter of a compound and to test empirical band structure calculations which are fitted t o optical transition energies. Furthermore, also a relation for the estimation of the total valence band width in terms of the Fermi energy and the Penn gap is given.

Detailed x-ray photoemission study of the valence band of Cu, Ag and Au

Detailed X-ray photoemission studies ( hv = 1253.6 eV) of the valence band structure of the noble metals Cu, Ag and Au are reported. The experimental results are compared to ultraviolet-photoemission measurements and theoretical calculation of the band density of states.

Soft-X-RayL2,3Spectrum and Electronic Band Structure of Chromium

The soft-x-ray ${L}_{2,3}$ emission and absorption spectra from pure chromium are shown and the effects of satellite emission and self-absorption are assessed. Features of the ${L}_{3}$ spectrum are discussed in terms of the energy-band structure. Results are evaluated by comparing the ${L}_{3}$ spectrum with the x-ray $K$ and $M$ spectra, with ultraviolet photoemission measurements, and with band-structure calculations. There is good agreement between theory and experiment as to the width of the occupied states, the position of the Fermi energy, and the position of most maxima and minima in both the occupied and vacant portions of the density of states. Some disparities are also observed but they involve primarily the fine features of the structure.