Valence Level Photoemission Research Articles

Electronic states at oxygen deficient WO 3(001) surfaces: a study by resonant photoemission

Electronic states associated with oxygen deficiency at WO 3(001) surfaces have been studied by photoemission with the photon energy tuned to a resonance maximum in the W 5d ionization cross-section profile. Mild ion bombardment followed by UHV annealing leads to a reduced surface where the predominant W 5d states lie “deep” within the bulk bandgap. After more energetic and prolonged bombardment and annealing, a quasi metallic surface is obtained with a significant density of states at the Fermi energy. The valence level photoemission measurements are compared with results from scanning tunnelling microscopy and shallow W 4f core level photoemission. The “deep” states are associated with pairs of W ions σ-bonded together in defect troughs that can be imaged in real space by STM. By contrast the metallic states are assigned to reduced (1 × 1) terrace areas where all the W ions lose on-top oxygen.

Assignment of rest-atom surface core-level shift through adsorption-site selectivity of methoxy on Si(111)-7×7

The formation of the methoxy species on Si(111)-(7\ifmmode\times\else\texttimes\fi{}7) upon methanol adsorption leads to a strongly selective extinction of the rest-atom feature in the valence-level photoemission spectrum. The Si $2p$ surface core level at lowest binding energy is likewise removed in this process, demonstrating that this feature is related to charge distribution around the rest atom, an assignment that has been subject to controversy in the literature. The formation of a Si-O bond upon methoxy adsorption leads to marked changes in the Si $2p$ spectrum that are discussed in terms of the characteristics of this chemisorption bond.

Electronic Structure and Orientation of Poly(p-phenylenevinylene) Monolayers on MoS2 and Highly Oriented Pyrolitic Graphite

We report investigations of monolayer films of poly(p-phenylenevinylene) (PPV) adsorbed on both MoS2 and highly oriented pyrolitic graphite (HOPG) surfaces. The monolayer was prepared by deposition of a precursor polymer using the Langmuir−Blodgett technique and subsequent conversion to PPV by thermal treatment in ultrahigh vacuum. Photoelectron spectroscopy and X-ray absorption spectroscopy were used to characterize the conversion and to determine the orientation of the PPV with respect to the substrate surfaces. The appearance of new features in both the X-ray absorption and the valence level photoemission spectra upon conversion can be interpreted in terms of the creation of π conjugation along the polymer backbone. Additionally, in the angle-resolved photoemission measurements of the fully converted sample on HOPG, a weak dispersion of the π-derived features is just discernible. The angular dependence of the X-ray absorption spectra shows that the PPV monolayer on MoS2 is highly oriented, with its phenylene and vinylene moieties lying preferentially parallel to the surface.

Theoretical study of the valence level photoemission spectrum of C 6 adsorbed on Ni, Pd and Pt metal surfaces

The valence hole spectral functions of C 6, NiC 6, PdC 6 and PtC 6 are calculated by the ab initio third-order algebraic-diagrammatic-construction (ADC(3)) Green function method using an extended basis set. The calculations for XC 6, (X = Ni, Pd, Pt) were performed, assuming the top site adsorption of the two ring forms of C 6. These are the benzene structure ( 1A 1g) possessing D 6h symmetry and a distorted cyclic form ( 1A' t) of D 3h symmetry. The changes in spectral features (spectral intensity and energy level separations) from the free molecule to the adsorbate are predicted. The possibility of using the adsorbate data to determine the geometric structure of the adsorbed C 6 molecule is discussed.

Valence-level photoemission spectroscopy and photon-stimulated ion desorption studies of CH 3Cl adsorbed on Al(111) surface using synchrotron radiation

The interaction of CH 3Cl adsorbed on Al(111) surface at ∼90 K has been investigated by means of valence-level photoemission spectroscopy (PES), work function change and photon-stimulated ion desorption (PSID) using synchrotron radiation. The one-to-one correspondence between the gas-phase and the condensed-phase CH 3Cl PES spectra suggests that the molecules are molecularly adsorbed without decomposition. Adsorption of CH 3Cl leads to a decrease in work function of Al(111) ( Δø ∼ − 0.6 eV at a monolayer), indicating a dipole pointing away from the substrate and Cl is in direct contact with the metal. In the H + PSID spectra, three weak thresholds are observed, at ∼9.6, ∼13.9 and ∼16.5 eV, and a major threshold appears at ∼18.9 eV. CH + 3 PSID spectra show a weak threshold at ∼13.2 eV and two major thresholds near 9.0 and 16.7 eV. The desorption threshold below 18 eV may be due primarily to the single-hole excitation or Rydberg transition, whereas the ion desorption above 18 eV may result predominantly from excitations of C 2s electron correlation states.

Interactions between sulfur and platinum in bimetallic surfaces: Reaction of S2 with Pt–Al alloys

The reaction of S2 with Pt–Al alloys has been investigated using core- and valence-level photoemission. The Pt–Al alloys display a Pt 5d band that appears at much higher binding energy (1.8–2.1 eV) than in metallic Pt. This is accompanied by positive shifts in the Pt 4f (0.3–1.3 eV) and Al 2s (0.1–0.3 eV) core levels. The electronic perturbations in Pt do not increase the reactivity of this metal toward sulfur. After exposing Pt–Al alloys to S2 gas, aluminum is extensively sulfided, while platinum remains in a metallic state. The Pt↔AlSx interactions are much weaker than Pt↔Al interactions. The chemisorption of S2 induces the segregation of aluminum toward the surface of the alloys. The behavior observed for the PtAlx surfaces in the presence of sulfur is different from that seen for Cu/Pt (111) and Ag/Pt (111) surfaces, where the admetals promote Pt↔S interactions and the formation of several layers of platinum sulfide. Changes in the nature of the bimetallic bond and in the relative stability of the metal sulfides are responsible for this difference in behavior.

Adsorption of Sulfur on Ag/Al2O3and Zn/Al2O3Surfaces: Thermal Desorption, Photoemission, and Molecular Orbital Studies

The interaction of S2 with Ag/Al2O3 and Zn/Al2O3 surfaces has been investigated using thermal desorption mass spectroscopy, core- and valence-level photoemission, and ab initio self-consistent-field calculations. The sticking coefficient of S2 on clean alumina is small (<0.1) at temperatures between 300 and 700 K. In the S2/Al2O3 system, there is an energy mismatch between the orbitals of the molecule and the bands of the oxide, and the reactivity of S2 on pure alumina is very low. The adsorption of sulfur on Ag/Al2O3 and Zn/Al2O3 surfaces induces strong perturbations in the electronic properties of the admetals and oxide support. In the Ag/Al2O3 and Zn/Al2O3 systems, the supported metal clusters (or particles) provide a large number of electronic states that are very efficient at donating charge into the S2(2πg) orbitals, inducing in this way the breaking of the S−S bond and the formation of admetal sulfides (AgSx and ZnSy). The larger the electron transfer from the supported metal into the S2(2πg) orbit...

Theoretical study of the valence-level photoemission spectrum ofC2H4sadsorbedon a Ni metal surface

In order to investigate the valence-level photoemission spectrum of ${\mathrm{C}}_{2}$ ${\mathrm{H}}_{4}$ adsorbed on a Ni metal surface, we calculated the valence-hole spectral functions of ${\mathrm{NiC}}_{2}$ ${\mathrm{H}}_{4}$ (\ensuremath{\pi} bonding) using the ab initio third-order algebraic-diagrammatic-construction (ADC(3)) Green-function method using an extended basis set. We obtained an overall good agreement with experiment. The changes in the spectral features such as the changes in the separations of the ionization energies from the gas phase to the chemisorbed phase are due to the metal ligand bonding exerted via the charge-transfer excitations and do not arise from the distortion of the molecule, which occurs upon chemisorption.

Electronic states and molecular symmetry of the higher fullerene C 80

Core and valence level photoemission and C 1s core excitation spectra are presented from solid C 80. Ten valence band features are identified in the binding energy range 0–15 eV, whose relative intensity depends strongly on the incident photon energy. C 1s core level photoemission comprised fine structure which was interpreted as reflecting one electron and collective electron excitations of the ionised C 80 system. The number of levels arising from C 1s to unoccupied π ∗ transitions observed in core excitation spectra and the nature of the filling of these levels upon Rb intercalation suggest that our C 80 samples contain predominantly molecules which possess D 2 symmetry.

Electronic and chemical properties of Pd in bimetallic systems: interaction of Pd with Rh(111)

The nature of the metal-metal interactions in a series of Pd Rh(111) surfaces has been examined using thermal desorption mass spectroscopy, core- and valence-level photoemission, CO chemisorption, and ab initio self-consistent field calculations. A monolayer of Pd supported on Rh(111) exhibits a Pd 3 d 5 2 binding energy ∼0.2 eV higher than that of the surface atoms of Pd(100), and desorbs at ∼1390 K. The desorption temperature of CO from this Pd Rh(111) system is 50–70 K lower than those seen on Pd(111) and Rh(111), indicating a weakening of 3–5 kcal mol −1 in the strength of the CO adsorption bond. These results are compared with data reported in the literature for bimetallic surfaces that contain Pd and Rh, and general trends are explained in terms of a simple model.

Theoretical study of the valence level photoemission spectrum of C 2H 2 adsorbed on a Ni metal surface

In order to study the valence level photoemission spectrum of C 2H 2 adsorbed on an Ni metal surface, we calculated the valence hole spectral functions of NiC 2H 2 by the ab initio third-order algebraic-diagrammatic construction (ADC(3)) Green's function method using an extended basis set. We obtained an overall good agreement with experiment. In spite of the fact that the present minimal cluster model describes the π-bonding complex, it provides a sufficient picture of the ionization of ligand levels. It is shown that the changes in the spectral features such as the changes in the separations of the ionization energies in going from the gas phase to the chemisorbed phase, cannot be described simply by using a free-molecule approximation.

Electronic structure of cubic gallium nitride films grown on GaAs

The composition, surface structure, and electronic structure of zinc blende–GaN films grown on GaAs (100) and (110) by plasma-assisted molecular beam epitaxy were investigated by means of core and valence level photoemission. Angle-resolved photoelectron spectra (photon energy 30–110 eV) exhibited emission from the Ga 3d and N 2s levels, as well as a clear peak structure in the valence band region. These peaks were found to shift with photon energy, indicative of direct transitions between occupied and unoccupied GaN bands. By using a free electron final band, we are able to derive the course of the bands along the Γ-X and Γ-K-X directions of the Brillouin zone and to determine the energy of critical points at the X point. The relative energies of the Ga 3d and nitrogen 2s bands were also studied, and a small amount of dispersion was detected in the latter. The resulting band structure is discussed in relation to existing band structure calculations.

Zinc sulfide on GaP(110): Characterization of epitaxial growth and electronic structure

Epitaxial layers of ZnS were grown on cleaved GaP(110) surfaces by molecular beam epitaxy in an ultrahigh vacuum photoelectron spectrometer. The growth mode and the structure of the overlayer were studied by means of low-energy electron diffraction (LEED) and core as well as valence level photoemission using synchrotron radiation. The attenuation of substrate core-level intensities with ZnS deposition indicate layerwise growth. LEED demonstrates the growth of the cubic (zinc-blende) phase as expected for substrate-stabilized growth. A minor interface reaction is evident from changes in the appearance of the substrate (Ga 3d) and overlayer (S 2p) core levels with increasing thickness. S–Ga bonding was observed in a thin interfacial layer. The valence band offset for this lattice-matched heterojunction interface system was determined, and found to be of the straddling type (type I); its magnitude is in agreement with predictions based on the dielectric midgap energy model.

Influence of metal–metal bonds on electron spectra of MoO2and WO2

Core and valence level photoemission and electron energy loss (EELs) spectra of MoO2 and WO2 have been measured. Metal–metal bonding in these distorted rutile dioxides splits the metal 4d or 5d conduction band into two components, with a significantly bigger splitting for WO2 than MoO2. The O 2p bandwidth is also found to be bigger for WO2 then MoO2. Plasmon loss peaks below 2 eV show that the effective mass ratios for electrons not involved in σ metal–metal bonding are much greater than unity. The photoemission and EELS data both suggest that metal–metal π bonding is important in these compounds. Comparison between photoemission spectra of WO2 and oxygen-deficient Na0.65WO3 –y suggests that structure evident in the bandgap of the latter compound may be associated with metal–metal bonding allowed by oxygen deficiency.

Interaction of Zinc with Transition-Metal Surfaces: Electronic and Chemical Perturbations Induced by Bimetallic Bonding

The addition of zinc can induce significant changes in the chemical and catalytic properties of a transition-metal surface. The properties of a series of bimetallic surfaces that combine Zn with Rh or group 10 elements (TM = Ni, Pd, or Pt) have been examined using thermal desorption mass spectroscopy, core- and valence-level photoemission, CO chemisorption, and ab initio self-consistent-field calculations. The deposition of Zn on Rh(111) or polycrystalline surfaces of group 10 metals leads to the formation of alloys. These surface alloys decompose at high temperatures: 600−800 K, ZnNi; 650−850 K, ZnRh; 750−950 K, ZnPt; 850−1000 K, ZnPd. In the alloys, the core levels and valence d band of the transition metals exhibit positive binding energy shifts, while negative shifts are observed for the Zn 2p levels and 3d band. For CO/ZnRh and CO/ZnTM surfaces, the CO adsorption bond is weaker than for CO/Rh and CO/TM surfaces: 1−2 kcal/mol on ZnNi; 4−5 kcal/mol on ZnRh; 4−8 kcal/mol on ZnPt; and 12−16 kcal/mol on...

Electronic structure of ultrathin ordered iron oxide films grown onto Pt(111).

It has recently been shown that well-ordered iron oxide films can be prepared epitaxially onto Pt(111) surfaces. We have investigated a one-monolayer-thick film with FeO stoichiometry and a ${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$(111) multilayer. Moreover, we prepared well-ordered multilayers with \ensuremath{\alpha}-${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$ stoichiometry. Core and valence level photoemission data clearly reveal that the ${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$ and \ensuremath{\alpha}-${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$ multilayers on Pt(111) are chemically identical to thick single crystals and polycrystalline powder samples. The x-ray-absorption near-edge structures (XANES) at the O K edge arise from the hybridization between O 2p and Fe 3d states as well as Fe 4sp states, showing that the latter also are important for the bonding interaction of the oxides. Polarization-dependent XANES measurements at the O K edge reveal a FeO bond length in the monolayer of 2.23\ifmmode\pm\else\textpm\fi{}0.22 \AA{}, which almost corresponds to the bond length in the bulk FeO (2.15 \AA{}). The electronic structure of the iron oxides derived from the set of spectra presented will be described in terms of the ligand-field theory taking into consideration the exchange interaction between the iron 3d electrons.

The soft x-ray photochemistry of physisorbed SiF4. I. Reactions of the molecular species through desorption and dissociation

We present evidence that demonstrates photolysis of SiF4 adsorbed on Ge(100) at 30 K. Silicon 2p soft x-ray photoemission spectroscopy (PES) indicates that upon irradiation, the molecularly adsorbed SiF4 dissociates into SiFn species (where n=0,1,2,3) and desorbs as molecular SiF4. Also, the Si 2p PES from undissociated molecules exhibits a number of distinct kinetic-energy shifts. These are attributed to anisotropic adsorption in which different molecular sites have different apparent Si 2p binding energies. A structure of the adsorbate layer is proposed to account for the varying core hole screening. Examination of the gas phase during irradiation confirms molecular desorption and shows the system to have a significant neutral molecular desorption yield. Changes in the valence-level photoemission structure and signal intensity are consistent with the observed fragmentation and desorption, both of which lead to disappearance of the molecularly adsorbed species.

Ethene adsorbed on Cu(110): a combined photoemission and photoelectron diffraction study

The absorption geometry of ethene on Cu(110) has been investigated using photoelectron diffraction (PhD) in the scanned energy mode as well as angle-resolved valence level photoemission. Two best fit Phf geometries have been found, both of which are compatible with the photoemission data: the molecule is a adsorbed either in an atop site on the close-packed Cu rows at a perpendicular height of 2.08 ± 0.02 Å with a CC bond length of 1.32 ± 0.09 Å, or in a short bridge site on the Cu rows at a perpendicular height of 2.09 ± 0.02 Å with a CC bond length of 1.53 ± 0.13 Å. In each case the molecular plane is parallel to the surface, but the CC axis can be twisted azimuthally out of the [1 1 − 0] direction by as much as 24.

Photoemission Studies of Adsorbates on Metal Surfaces

A brief review is presented of the general field of synchrotron radiation photoemission of adsorbates on well characterized metal surfaces, contrasting some of the earliest work of the 1970s with more recent experiments, and highlighting some of the likely future developments that may derive from the use of third-generation synchrotron radiation facilities. Four general types of study are identified: shallow core-level photoemission either for the fingerprinting of the chemical state, or for quantitative structural studies via photoelectron diffraction, and valence-level photoemission either to study the two-dimensional band structure of adsorbate-induced surface-localized states, or to investigate the orientation of molecular adsorbates through the use of symmetry selection rules.

Scanning-tunneling-microscopy and photoemission study of an alkali-metal-induced structural phase transition: Si(111)-(7 x 7) into Si(111)-Na(3 x 1).

The structural phase transition from the clean Si(111)-(7\ifmmode\times\else\texttimes\fi{}7) reconstruction to the Na-stabilized (3\ifmmode\times\else\texttimes\fi{}1) reconstruction of this surface is studied using scanning tunneling microscopy as well as core- and valence-level photoemission. By combining the laterally integrating technique of photoemission with the highly spatially resolving technique of scanning tunneling microscopy, the kinetics of the phase transition is explained on an atomic scale. In addition to the information on the kinetic pathway of the phase transition, we present a simple model of the dynamics, which explains the time dependence of the transition rate from the (7\ifmmode\times\else\texttimes\fi{}7) to the Na(3\ifmmode\times\else\texttimes\fi{}1) reconstruction of the Si(111) surface. It is demonstrated that samples can be prepared with both surface reconstructions coexisting on the surface in any desired ratio. These coexisting islands show the properties of the pure reconstructions, which demonstrates a strong lateral modulation of the position of the Fermi level in the semiconductor band gap at the surface.