Photoemission study of oxygen adsorbed on coldly deposited silver films

UPS spectra of coldly deposited silver films differ from those of films deposited at room temperature by electronic states localized at surface defects with an energy about 4.2 eV below E_F. Changes after exposure at 140 K to oxygen only occur in the presence of these defects, demonstrating that oxygen is only adsorbed at defects. Raman vibrational spectroscopy shows that oxygen is adsorbed nominally as O₂^? and O₂^2?. Possible assignments of the oxygen related UPS structures are discussed.     

Electronic properties of clean and oxygen exposed GaAs (100) surfaces

Both Photoemission Yield Spectroscopy (PYS) and Auger Electron Spectroscopy (AES) have been used in the study of the electronic properties of the clean GaAs(100) surface prepared by IBA procedure and subsequently exposed to oxygen. For the clean GaAs(100)c(8 × 2) surface, the values of the work function and the absolute band bending were 4.20 ± 0.02 eV and −0.23 ± 0.06 eV, respectively, which confirms the pinning of the Fermi level E_F, and two filled electronic surface state bands localized in the band gap below the Fermi level were observed. After exposition of this surface to 10³ L of oxygen, the electronic surface state band localized just below the Fermi level E_F disappeared, and the work function and the absolute band bending increased by only 0.12eV, whereas for the higher oxygen exposures of 10⁴L and 10⁵L, only small increases in the values of the work function and the absolute bending by 0.04 eV and 0.03 eV, respectively, were observed.     

H2O interaction with clean and oxygen precovered Ni(110)

The interaction of water vapour with clean as well as with oxygen precovered Ni(110) surfaces was studied at 150 and 273 K, using UPS, ΔΦ, TDS, and ELS. The He(I) (He(II)) excited UPS indicate a molecular adsorption of H₂O on Ni(110) at 150 K, showing three water-induced peaks at 6.5, 9.5 and 12.2 eV below E_F (6.8, 9.4 and 12.7 eV below E_F). The dramatic decrease of the Ni d-band intensity at higher exposures, as well as the course of the work function change, demonstrates the formation of H₂O multilayers (ice). The observed energy shift of all water-induced UPS peaks relative to the Fermi level (ΔE_max = 1.5 eVat 200 L) with increasing coverage is related to extra-atomic relaxation effects. The activation energies of desorption were estimated as 14.9 and 17.3 kcal/mole. From the ELS measurements we conclude a great sensitivity of H₂O for electron beam induced dissociation. At 273 K water adsorbs on Ni(110) only in the presence of oxygen, with two peaks at 5.7 and 9.3 eV below E_F (He(II)), being interpreted as due to hydroxyl species (OH)^δ? on the surface. A kinetic model for the H₂O adsorption on oxygen precovered Ni(110) surfaces is proposed, and verified by a simple Monte Carlo calculation leading to the same dependence of the maximum amount of adsorbed H₂O on the oxygen precoverage as revealed by work function measurements. On heating, some of the (OH)^δ? recombines and desorbs as H₂O at ? 320 K, leaving behind an oxygen covered Ni surface.     

Electronic structure of PdO studied by photoemission (XPS,UPS)

In this paper we present the results of photoemission studies (XPS and UPS) performed on a polycrystalline surface of PdO. The electron density of states (EDOS) deduced both from XPS and UPS (He_I and He_II) are very similar. The valence band of PdO, which differs significantly from the Pd one, can be built up by four structures located at 0.5 eV, 2.2eV, 4.5 eV and 6.5 eV below E_F. The various electronic contributions (p or d) in the band are considered and, in order to explain our spectra, we discuss several hypothesis taking into account the possible existence of satellite lines or crystal field effects. Our XPS and UPS spectra show that the energy bands of PdO are narrow (～ 2–3 eV), moreover the energy shift of the core levels (|ΔE^F_B| = 2 eV) is important : these results suggest that the correlations between the d electrons may be important in PdO.     

A LEED/UPS study on the interaction of oxygen with a Ni(111) surface

Exposure of a Ni(111) surface to oxygen leads at first to the formation of a chemisorbed overlayer which is characterized by a 2 × 2-superstructure and a maximum in the photoemission spectrum (hv = 40.8 eV) centered at 5.6 eV below the Fermi level E_F. The emission from the Ni d-states is nearly unaffected at this stage of interaction. After high oxygen exposures the epitaxial growth of NiO can be identified from the LEED pattern. The corresponding photoelectron spectrum is strongly altered and exhibits close agreement with the transition energies as calculated by Messmer et al. for a NiO₆^10- -cluster.     

Inverse photoemission of pyridine on silver (111)

Inverse photoemission, together with UPS and EELS, is used to obtain information on the position of the affinity level of pyridine adsorbed on Ag(111). The unresolved a₂ and b₁ affinity levels (AL)(1.20 and 0.62 eV above the vacuum level for free pyridine) are found at E_F+(2.9±0.2) eV (indicating a Coulomb relaxation of about 2.1 eV). The reported phase transition in the first adsorbed monolayer of pyridine had no influence on the position and halfwidth of the AL nor on the electronic excitation intensities. The assignment of UPS structure is discussed. Relaxation is different for unoccupied and occupied states. The second layer “initial state” model of photoionization is not confirmed.     

Electron spectroscopic investigations of amorphous and crystallized Fe82B12Si6 (Metglas 2605s)

The electron structure of amorphous and crystallized Fe₈₂B₁₂Si₆ has been investigated by means of UPS, XPS and EELS, with the aim to establish spectral features distinguishing the two phases. Our results show distinct redistribution in the valence density of states, extending several eV below the Fermi level.     

Photoemission study of electronic structure evolution across the metal-insulator transition of heavily B-doped diamond

We studied the electronic structure evolution of heavily B-doped diamond films across the metal-insulator transition (MIT) using ultraviolet photoemission spectroscopy (UPS). From high-temperature UPS, through which electronic states near the Fermi level (E_F) up to ∼5k_BT can be observed (k_B is the Boltzmann constant and T the temperature), we observed the carrier concentration dependence of spectral shapes near E_F. Using another carrier concentration dependent UPS, we found that the change in energy position of sp-band of the diamond valence band, which corresponds to the shift of E_F, can be explained by the degenerate semiconductor model, indicating that the diamond valence band is responsible for the metallic states for samples with concentrations above MIT. We discuss a possible electronic structure evolution across MIT.     

Camel's back induced stabilization of electron-hole liquids : GaP

A camel's back-like nonparabolicity of the longitudinal electron mass enhances the density of states and strongly stabilized an electron-hole-liquid. In GaP therefore the EHL density is doubled to 8.6 × 18¹⁸cm^?3 and the Fermi energy ratio E_F^h/E_F^e changes from 1.9 to 4.9. The theoretical binding energy agrees with the experimental E_B=17.5±3meV interpreting the luminescence at 2.30 eV as a superposition of liquid and plasma recombination radiation.     

The interaction of oxygen with gadolinium: UPS and XPS studies

The interaction of oxygen with evaporated Gd films at 300 K has been studied for the first time using AlK α XPS and Hel and Hell UPS. The characteristic changes in the Gd(6s5d) and O(2p) valence bands, Gd(4f), Gd(5p) and Gd(4d) core levels, and O(2s) and O(1s) core levels were studied. Evidence is presented for the initial formation of an intermediate oxidation state at low exposure (characterized by a new Gd valence band with a maximum in the DOS at ～ 2.5 ev below E_F and an ～ 0.6 eV shift in Gd(4f)) prior to formation of Gd₂O₃ where the Gd(6s5d) valence band disappears completely, as expected for Gd³⁺. In the higher exposure range there is little further increase in the oxide thickness, which is estimated as $\text{? 20}\text{A}\text{?}$, but there is a slow replacement of O by OH, as characterized by a second O(1s) feature at 532.3 eV and OH 1π and 3σ orbitals in UPS at ～ 6.7 and 11.5 eV. The interpretations are supported by parallel studies on bulk Gd₂O₃ and by Ar⁺ sputtering experiments. Comparisons are made to other rare earth oxidation studies.     

Photoemission study of oxygen chemisorption on tin

Photoemission measurements have been made at photon energies from 5–12 eV and at 21.2 eV on evaporated Sn films and the same films with varying room temperature exposure to oxygen. For hν ? 9 eV the quantum yield for Sn with exposures of as much as 4000 L oxygen (1 L = 1 Langmuir = 10^?6 Torr sec) differs only slightly from the clean metal. For hν ? 9 eV no change in yield is observed with oxygen exposure. The energy distribution of photoemitted electron (EDC's) from Sn with increasing exposure to oxygen above ? 20 L are characterized by the growth of two peaks which were not present in the EDC's for the clean metal, located 2.9 ± 0.1 eV and 4.8 ± 0.1 eV below the Fermi level. We associate this structure with the presence of SnO₂. No sharp resonance which could be associated with adsorbed oxygen was seen. Uniformly reduced emission from metallic Sn states and a Fermi level as sharp as for the clean metal is observed in the EDC's at all oxygen exposures. In addition, no change in work function with oxygen exposure was detected. The effects of oxygen saturate for exposures ? 4000 L. We suggest that under the conditions used in this experiment, the oxygen penetrates beneath the surface forming SnO₂ and leaving metallic Sn on the surface.     

Photoemission and electronic structure of crystalline Co3B and amorphous Co - P - B

We present an XPS and UPS study of crystalline Co, Co₃B and Co₇₈P₁₄B₈ glassy metal. For Co₃B the electronic distribution curves (EDC) of the valence band and the previous specific heat and magnetic results are interpreted in a qualitative model where : i) the s-p cobalt-boron bonding states lie in the low part of the band, the upper levels being mainly cobalt d states and ii) a large density of states of the majority spin band is present at the Fermi level. For amorphous Co₇₈P₁₄B₈ the EDC shows that E_F is located in a high density of states region, it is suggested that the phosphorus p states ae centred at 7.5 eV. The photoemission and the magnetic results can be also interpreted in the framework of the previous model.     

Chemisorption of NO and NH3 on cobalt: Studies by UPS,XPS, and work-function measurements

Chemisorption of NO and NH₃ on cobalt has been studied by UV and X-ray photoelectron spectroscopy (UPS and XPS) and work-function measurements. The 25°C data for NO are consistent with dissociative chemisorption below ～5 L exposure followed by molecular chemisorption at higher exposures; complete dissociation occurs after heating to 500°C. The UPS of molecularly chemisorbed NO exhibit peaks at ～2.7, 10.2 and 14 eV below E_F, corresponding to ionization of the 2π*, (1π + 5σ), and 4σ MO's. The UPS data for NH₃ exhibit peaks at 7.9 and 11.4 eV below E_F, consistent with ionization of the 3a₁ and 7e MO's of NH₃ and/or MO's of the fragments NH₂ and/or NH.     

Core and valence level photoemission studies of iron oxide surfaces and the oxidation of iron

The core and valence level XPS spectra of Fe_xO (x ~ 0.90–0.95); Fe₂O₃ (α and γ); Fe₃O₄; and FeOOH have been studied under a variety of sample surface conditions. The oxides may be characterized by a combination of valence level differences and core-level effects (chemical shifts, multiplet splittings, and shake-up structure). Fe^II and Fe^III states are distinguishable, but octahedral and tetrahedral sites are not. The O 1 s BE cannot be used to distinghuish between the oxides since it has a nearly constant value. Fe 3d valence level structure spreads some 10 eV below E_F, much broader than suggested by previous UPS and photoelectron-spin-polarization (ESP) measurements for Fe_xO and Fe₃O₄. Fe surfaces (films, foils, (100) face) yield predominantly Fe^III species when exposed to high exposures of oxygen or air, though there is evidence for some Fe^II also. At low exposures the Fe^II/Fe^III ratio increases.     

First-principles study on the electronic structures of iron phthalocyanine monolayer

The electronic band structure and magnetic properties of iron phthalocyanine (FePc) monolayer were investigated by using the first-principles all-electron full-potential linearized augmented plane wave energy band method. It is found that the ferromagnetic FePc monolayer is energetically more stable than the paramagnetic one. The exchange interaction, which splits the majority and minority bands, influences strongly on the electronic structure near the Fermi level (E_F). Magnetic moment of the central Fe atom is calculated to 1.95 μ_B. The range of the positive polarization of Fe site is larger in the out-of-plane than in the in-plane direction. The FePc ligand remains paramagnetic. The presence of states at E_F indicates the metallic character of FePc monolayer both for the paramagnetic and ferromagnetic states. However, the large density of states at E_F of the majority spins in the ferromagnetic state is expected to cause a phase transition to insulating antiferromagnetic state from the metallic ferromagnetic one.     

H,O原子在过渡金属表面吸附能的规则性

用Allan的简化d带模型描叙过渡金属的表面电子态,用广义相移法计算吸附原子在表面的吸附能,结果表明,不仅很好地描绘了H,O在一个周期的过渡金属表面吸附能的变化趋势,而且所算得的吸附原子感应的分离能级也同紫外光发射谱的实验符合得很好;同时还指出,简单气体在过渡金属表面的吸附能呈现规则性变化主要决定于费密能级E_F与吸附原子的有效能级ε_a之差(E_F-ε_a),其次是转移矩阵元ν和能带宽度w_b。 关键词：     

Photoemission (XPS,UPS) studies of Pd-Si metallic glasses

The valence bands of glassy $\text{Pd}{}_{\mathrm{100?x}}\text{Si}{}_{\mathrm{x}}\text{(15?x?21)}$ and pure Pd were studied by XPS and UPS. The valence band spectra of the alloys show a strongly reduced density of states at the Fermi energy E_F compared to Pd. From the measured relative photoelectric cross sections for the different excitation energies we conclude that the electron states near E_F of the glassy alloys have mainly d-character. This is in good agreement with recent measurements of the low-temperature specific heat, the magnetic susceptibility and the optical reflectivity.     

Comparative photoemission study of the electronic properties of L-CVD SnO2 thin films

In this work we present the results of comparative XPS and PYS studies of electronic properties of the space charge layer of the L-CVD SnO₂ thin films after air exposure and subsequent UHV annealing at 400 °C, with a special emphasis on the interface Fermi level position.From the centre of gravity of binding energy of the main XPS Sn 3d_5/2 line the interface Fermi level position E_F − E_v in the band gap has been determined. It was in a good correlation with the value estimated from the offset of valence band region of the XPS spectrum, as well as from the photoemission yield spectroscopy (PYS) measurements. Moreover, from the valence band region of the XPS spectrum and PYS spectrum two different types of filled electronic band gap states of the L-CVD SnO₂ thin films have been derived, located at 6 and 3 eV with respect to the Fermi level.     

Valence band of molybdenum by photoelectron spectroscopy

Experimental photoelectron spectra of a clean polycrystalline Mo surface excited by monochromatized Al K _α X-rays are presented. The spectra are compared with valence bands obtained by UPS and by band structure calculations within the 5 eV region below the Mo Fermi level. All results mentioned above display peaks at 0.3, 1.7, 2.8 and 4 eV belowE _F. The energy distribution of the valence band does not vary with photon energy and electron emission angle for the four different polycrystalline Mo surfaces compared. It is concluded that the four peaks representing the Mo valence band are predominantly of bulk origin.     

Cluster model calculation and photoemission studies of molecularly adsorbed NO on Ni

Self-consistent scattered wave cluster model calculations for the NONi system are presented with NO bonded N-down in a fourfold coordination site. The results suggest that the charge is transferred from metal into the single occupied 2π¹ molecular orbital of NO which is thus significantly perturbed. The interaction involves the metal s, p, and d-derived states. We calculate total and local “density of states” defined within the framework of the cluster model and compare the results with Ultraviolet Photoemission experiments recorded at 80 K. At 300 K partial dissociation occurs as judged from UPS and XPS. Increased photoemission observed at about 2 eV below the Fermi level in the molecularly adsorbed state is attributed to the alteration of density of states due to the interaction of the 2π¹ molecular orbital of NO with the substrate. A detailed analysis of the nature of the relevant molecular orbitals is also given.