An angle-resolved photoemission study of the chemisorption of chalcogens on Cu(100): II. Cu(100) + c(2 × 2)O

Starting with the three-step direct-transition model of ARPES for bulk materials, which was examined in the preceding paper, we propose a framework for describing changes in the photoemission spectra due to chemisorption. Normal emission ARPES data for Cu(100) with a c(2 × 2)O overlayer were obtained in the photon energy range hv = 11 to 34 eV. These spectra have been compared within the proposed framework with those obtained from clean Cu(100). Changes were found in the Cu emission features which could be explained by the relaxation of momentum conservation perpendicular to the surface in the optical excitation step and by the relaxation of momentum conservation parallel to the surface in the escape step. These changes include a photon energy dependent broadening of the d-band peak and the preferential attenuation of the sharp direct-transition feature associated with the sp-band. Some evidence for a surface resonance at the top of the d-bands has been obtained. Changes in the spectrum of scattered electrons were related to modifications of evanescent final states. A 1.3 eV wide band derived from the oxygen p_x,y-orbitals was deduced from spectra obtained at normal emission and along the $\text{Γ}\text{X}$ and $\text{Γ}\text{M}$ lines of the surface Brillouin zone. On the other hand, no emission was clearly detected from the oxygen p_z-orbitals. Oxygen induced emission above the Cu d-bands was observed and attributed to antibonding states. This emission was directed towards the bulk [011] directions.     

An angle-resolved photoemission study of the chemisorption of chalcogens on Cu(100) III. Cu(100) + p(2 x 2)S

Normal emission ARPES data for Cu(100) with a p(2x2)S overlayer were obtained in the photon energy range hv=11 to 34 eV. These spectra have been compared, within a framework proposed previously, with spectra from clean Cu(100). Changes were found in the Cu emission features, which could be explained by the relaxation of momentum conservation perpendicular to the surface in the optical excitation step and by the relaxation of momentum conservation parallel to the surface in the escape step. These changes include a broadening of the d-band peak and the strong attenuation of the sp-band peak. In addition, a prominent feature appears at about ?4.0 eV at normal emission upon sulfur chemisorption. We tentatively attribute its appearance at normal emission to a new surface umklapp process induced by the overlayer. Changes in the spectrum of scattered electrons were related to modifications of evanescent final states. The sulfur 3p_z^? and 3p_x.y-components could be separated at normal emission and are located at ?4.7 and ?5.4 eV, respectively. No dispersion of the sulfur 3p-bands was detected when the $\text{Γ}\text{X}$ and $\text{Γ}\text{M}$ symmetry lines of the surface Brillouin zone were probed. Sulfur-induced emission above the Cu d-bands was observed and attributed to antibonding states. This emission was directed towards the bulk [011] directions.     

Face-dependent bond lengths in molecular chemisorption: the formate species on Cu(111) and Cu(110)

Many previous structural studies of molecular adsorbates on metal surfaces indicate that the local coordination and bonding is closely similar to that in organometallic compounds, implying that the metallic substrate has no significant influence. Here we show that such an influence is detectable for one model system, namely, the formate species, HCOO, adsorbed on the atomically rough and smooth (110) and (111) surfaces of Cu, leading to a statistically significant difference (0.09±0.05 ?) in the Cu-O chemisorption bond length. The effect is reproduced in density functional theory calculations.     

Hydrogen chemisorption on silicon (100) 2 × 1

Changes in the valence band density of states (DOS) of a (100) silicon surface that accompany he chemisorption of atomic hydrogen onto that surface are deduced from a study of the changes in the L_2,3VV Auger lineshape. Complementary changes in the conduction band DOS are inferred from changes in L_2,3VV-core-level characteristic loss spectra (CLS). The chemisorbed hydrogen layer is identified as the dihydride phase from low energy electron diffraction measurements. Upon hydrogen adsorption the DOS at the top of the valence band decreases and new energy levels associated with the Si-H bonds appear lower in the band. Assuming that the Auger signal from the hydrogen covered sample consists of a superposition of a signal from silicon atoms bonded to hydrogen in the dihydride layer and an elemental-Si signal from the substrate, a N(E) difference spectrum with features due only to the dihydride is obtained by subtracting the background corrected, loss deconvoluted L_2,3VV signal for a clean (100)Si surface rom the corresponding signal for the hydrogen covered surface. Comparisons of the energy position of the major peak in this difference spectrum with that of the main peak in a gas phase silane Si-L_2,3VV spectrum, and of the corresponding Auger energy calculated empirically, indicate a hole—hole interaction energy of ~8 eV for the two-hole final state in the gaseous system and zero for the dihydride surface system. Hydrogen induced changes in the conduction band DOS are less apparent than those of the valence band DOS with only the possibility of a decrease in the DOS at the bottom of the conduction band being inferred from the CLS measurements. Electron stimulated desorption of hydrogen from the dihydride layer is adduced from changes in the Auger lineshape under electron beam irradiation of the surface. Hydrogen induced changes in the near-elastic electron energy loss spectra (ELS) are also reported and compared with previously published ELS results.     

Evidence for two oxygen chemisorption sites on the Cr(100) surface at room temperature

The interaction of oxygen with the clean Cr (100) surface has been investigated by work function measurements, angle resolved photoemission spectroscopy and low energy electron diffraction. At low coverages (≈0.15 monolayer (ML)) the adsorbed oxygen is characterized by a work function decrease, a feature at 6.8 eV binding energy in UPS and a weak diffuse C(2 × 2) diffraction pattern. This form of oxygen labeled form a is quite stable at temperatures below 500°C. Upon further exposure at room temperature (⪆0.20 ML) a second form of oxygen labeled form b and characterized by a work function increase and peaks at 3.8 and 5.9 eV at normal photo-emission is identified. It is proposed that oxygen in forms a and b is chemisorbed in the fourfold hollow and on-top sites respectively.     

The chemisorption of CO on Cu(100) studied with angle resolved photoelectron spectroscopy

Using angle resolved UV photoelectron spectroscopy, coupled with the continuum of polarized light available with synchrotron radiation, we make an interpretation of the photoelectron spectrum of CO adsorbed on Cu(100). We point out that the bonding of CO on Cu as observed by photoemission is considerably different from the bonding of CO on Ni. These differences do not seem to be caused by bonding orientation differences, however, as the CO molecular axis is found to be very close to the surface normal, as was the case for CO on Ni(100). No evidence is found for a second phase of CO on Cu(100).     

Oxygen chemisorption on Cr(110): II. Evidence for molecular O2(ads)

Oxygen chemisorption and dissociation on Cr(110) at 120 K have been studied using high resolution electron energy loss spectroscopy (HREELS), electron stimulated desorption ion angular distribution (ESDIAD), low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Dissociative adsorption dominates although vibrational and stimulated desorption data provide evidence for a coexisting minority molecular binding state. An O₂(ads) vibrational frequency of 1020 cm⁻¹ and a six beam ESDIAD pattern are suggestive of super-oxo O₂(ads) bonding at six local sites each with the O-O molecular axis tilted away from the surface normal. These results are compared with data for chemisorbed oxygen on other transition metal surfaces.     

The chemisorption of sulphur on W(100)

The interaction of sulphur vapour with a W(100) surface is studied in detail with Auger Electron Spectroscopy (AES), LEED, work function difference (Δ?) measurements and thermal desorption spectroscopy (TDS). The dissociative adsorption of S occurs on the W surface without reconstruction. Several LEED structures are observed which indicate repulsive adatom interactions. TDS shows that the desorption energy of atomic S decreases from about 8 eV at θ = 0.1 ML to about 3 eV near saturation in close vicinity of 1 ML. Above $\text{θ =}\text{3}\text{4}$ ML, S₂ desorbs in addition to S in a high temperature peak which saturates at about 1 ML. Sulphur in excess of about 1 ML is desorbed in two low temperature peaks of which the lower one consists not only of S and S₂ but also of S₃ and S₄.     

Molecular cluster theory of CO chemisorption on a nickel(100) surface

Self-consistent Hartree-Fock-Slater molecular cluster models for the chemisorption of carbon monoxide on a (100) transition metal surface are presented. Energy levels and charge distribution for the CO: Ni₅ cluster in C_4v symmetry are obtained, and the variation of binding energies with height of the CO molecule above the surface of nickel is studied in detail. Comparison is made with experimental binding energy spectra and with the multiple-scattering results of Batra and Bagus. The redistribution in energy of free-atom valence levels is studied by means of local-densities-of-states diagrams.     

Adsorbate band formation: The chemisorption of CO on Pd (100)

In ordered overlayers of adsorbed gases on metal surfaces, high coverage situations can lead to overlap between orbitals on adjacent species and hence to adsorbate band formation. We conclusively demonstrate the existence of this effect in chemisorption by examining the dispersion of the 4α level in the u.v. photoelectron spectrum of the CO/Pd (100) system. The results agree well with a first principles extended tight binding calculation of the two-dimensional band structure.     

Sulfur chemisorption on W(100): Antiphase boundaries with point defects

On the basis of LEED data recorded for sulfur chemisorbed on the (100) surface of tungsten in the 0.5–0.67 coverage range, partially ordered superstructures have been calculated within the kinematic approximation. They reveal c(2 × 2) domains separated by antiphase boundaries with point defects. The superstructures are modelled with statistical occupancies of the adsorption sites. Various possibilities for the registry of the adiayer in this coverage range are discussed.     

Molecular cluster calculations for the interpretation of CO chemisorption on a Ni(100) surface

Self-consistent Hartree-Fock-Slater molecular cluster calculations for the chemisorption of carbon monoxide on a Ni(100) surface are presented. In earlier calculations of this type the CO molecule has been assumed to be chemisorbed in a hollow position of C_4v symmetry. A recent EELS experiment shows however that in the most stable configuration CO is linearly bonded to the Ni atoms, i.e. a top position of the CO-molecule. This experiment indicates also that there exists an additional bridge bonding of the CO molecule to the two nearest neighbour Ni atoms. The variation of the energy levels, binding energies and charge distribution with the height of the CO molecule above the nickel surface is calculated for the top position using the NiCO and Ni₅CO clusters and for the bridge bonding configuration using the Ni₂CO cluster. The CO 1π level is found to be split by about 0.8 eV in bridge bonding geometry. For both hollow and top positions the 1π and 5σ levels are separated by about 0.5 eV. The energy separation to the 4σ level is about 3 eV, which is in good agreement with experimental data. Theoretical ionization energies relative to the Fermi energy for top position geometry at a bond distance of 3.5 au between the carbon atom and nickel surface were found to be 25.7, 11.7, 8.7 and 8.2 eV for the 3σ, 4σ, 5σ and 1π levels which should be compared with the experimental data of 29.0, 10.8, 8.4 and 7.8 eV, respectively. The corresponding ionization energies for a bond angle of 99° in bridge bonding were 23.7, 12.1, 7.3, 7.0 and 7.9 eV. The two last values represent the 1π level which is split into two levels in this geometry. The variation of the C-O stretch vibrational frequencies with the height of the CO molecule above the surface for the top-position geometry is estimated from the 5σ and 2π gross orbital populations and from the CO σ and π overlap populations.     

Hydrogen chemisorption on the 100 (2 × 1) surfaces of Si and Ge

This paper combines a theoretical study of the Si(100) surface having a monolayer of atomic hydrogen chemisorbed to it with an experimental study of the analogous Ge(100) and Ge(110) surfaces. In the theoretical work the underlying (100) silicon surface is taken to be reconstructed according to the Schlier-Farnsworth-Levine pairing model with the hydrogen located on the unfilled tetrahedral bonds of this structure. Self-consistent calculations of the electronic potential, charge density, spectrum, and occupied surface density of states are carried out. The force on the hydrogen atoms is then calculated using the Hellman-Feynman theorem. This force is found to be close to zero, confirming that the hydrogen atoms are indeed at the equilibrium position for the chosen silicon geometry. Features in the calculated photoemission spectrum for the Si(100) 2 × 1 : H surface are discussed in terms of related features in the photoemission spectrum of Si(111) : H, but are found not to agree with the previously measured photoemission spectrum of Si(100) 2 × 1 : H. Measured photoemission and ion-neutralization spectra for Ge(100) 2 × 1 : H agree in their major features with what is calculated for Si(100) 2 × 1 : H, however, suggesting that the Ge(100) 2 × 1 : H surface is reconstricted according to the pairing model. Similarly, measured spectra for clean Ge(100) 2 × 1 agree with calculations for the row dimerized Si(100) surface.     

Hydrogen chemisorption on Pd(100) studied with he scattering

He scattering from the clean Pd(100) surface yields extremely weak diffraction beams relative to the specular, corresponding to a very small maximum corrugation amplitude of ~ 0.04 Å. Hydrogen adsorption at a temperature of 110 K leads to the formation of a c(2 × 2) ordered phase at a coverage of 0.5 monolayers and a (1 × 1) phase at saturation coverage. The maximum corrugation amplitude of the c(2 × 2)H is ~0.13 Å; surface charge density calculations using overlapping atomic charge densities indicate a normal distance of the hydrogens to the topmost Pd layer d_n ? 0.65–0.70 Å corresponding to a H-Pd bonding distance of ~ 2.05 Å in the fourfold hollow sites. The result that the maximum corrugation amplitude of the (1 × 1) hydrogen phase, with ~ 0.025 Å, even smaller than that of the clean surface may indicate a movement of the hydrogens closer to the topmost metal layer, when the coverage is increased from 0.5 monolayers to saturation.     

Sorption,chemisorption and desorption of hydrogen by neodymium overlayers and Nd/Cu ultra thin alloy films on Cu(100)

The various modes of hydrogen uptake exhibited by Nd overlayers and Nd/Cu ultra thin alloy films on Cu(100) have been investigated by LEED, UPS, XPS and thermal desorption measurements. Ultra thin Nd overlayers exhibit very low sticking probabilities for H₂ (~10⁻⁴) — far lower than the values characteristic of thick Nd films. This behaviour is associated with the unusual surface structure adopted by the rare earth when present as a very thin film. Codeposition of Nd and H₂ leads to the formation of sorbed hydrogen and is accompanied by valence charge transfer from Nd to H. The kinetics of H₂ desorption from alloy films and Nd overlayers are markedly different: this feature provides a sensitive test for the overlayer → alloy transformation.     

Geometry of acetylene chemisorption on Ni(100) investigated by leed intensity analysis: The c(2 × 2) structure

Results from LEED dynamical calculations performed on the Ni(100) c(2 × 2)C₂H₂ structure, produced by adsorption of C₂H₂ at about 273 K and 1.5 L exposure, are reported. Among the several model geometries tried, the most favoured one appears to be a distorted acetylene molecule with the midpoint of the CC bond (1.20 Å long) placed above the fourfold hollow site at a vertical distance 2.02 Å from the topmost Ni layer and with the CC axis itself tilted by 50° with respect to the surface normal in the [011] direction. In this geometry the NiC distance is 2.2 Å.