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.     

Spin-resolved photoemission study of in situ grown epitaxial Fe layers on W(110)

Spin- and angle-resolved photoemission spectroscopy of in situ grown epitaxial Fe layers on W(110) shows bulk-like behavior for more than two atomic Fe layers. For about ten and more atomic layers of Fe we find a spin polarization to be about -100% near the Fermi energy and +80 % between 1 eV and 3 eV binding energy. For the bilayer of Fe drastic changes in the spin-resolved spectra and a 20 % enhancement of the spin polarization compared to the bulk value are observed. The monolayer of Fe is ferromagnetically ordered with a spin polarization reduced by about 50%. A switching of the easy magnetization axis from [001] to [11̄0] is observed in the spin polarization with decreasing Fe layer thickness near d = (65±5) Å.     

Spin-resolved photoemission from Ag(111): Theory and experiment

Using circularly polarized synchrotron radiation between 14 and 24 eV, spin-resolved normal photoemission spectra have been measured from an unreconstructed Ag(111) surface. Corresponding spectra were calculated by means of a fully relativistic one-step theory of photoemission together with the bulk band structures for real and complex potentials, using two different local approximations for the exchange-correlation potential. Experiment and theory employing anX potential agree well with regard to existence and positions of peaks. Relative peak heights match except for an observed enhancement at photon energies, at which two or more direct interband transitions may resonate.     

Angle-resolved photoemission study of Ag nanofilms grown on fcc transition-metal (111) substrates

A comparative study of the electronic structures of Ag nanofilms on the pseudomorphic metastable fcc Fe(111) and bulk-like fcc Co(111) substrates has been carried out to investigate their quantized electronic structures. The photoemission spectra of both Ag nanofilms exhibit the fine structures derived from the quantized sates of Ag sp valence electron. The nanofilm-thickness dependences of the binding energy of the quantized states are reproduced by the calculated results based on the phase accumulation model. From the angle-resolved photoemission measurements, the effective masses of the quantized electronic states along the direction parallel to the nanofilm surface were directly determined. We discuss the electronic hybridization effect between quantized states in Ag nanofilm and 3d-derived electronic states in transition-metal substrates.     

Core-level photoemission study of 2D ordered Bi/Si(100) interfaces

A high resolution core-level photoemission investigation of 2D ordered Bi layers grown on Si(100)-(2×1) is presented. We study the Si 2p and Bi 5d core-levels at room temperature as a function of coverage and in the reconstructed phases. The different Bi structural configurations around the monolayer coverage and in the (2×n)-reconstructed phase are derived from the core-level lineshape evolution. By following the Fermi level pinning, the presence of Bi-induced occupied electronic states close to the Si mid-gap is suggested.     

Spin-resolved photoemission of surface states of W(110)-(1 x 1)H

The surface electronic states of W(110)-(1 x 1)H have been measured using spin- and angle-resolved photoemission. We directly demonstrate that the surface bands are both split and spin-polarized by the spin-orbit interaction in association with the loss of inversion symmetry near a surface. We observe 100% spin polarization of the surface states, with the spins aligned in the plane of the surface and oriented in a circular fashion relative to the Smacr; symmetry point. In contrast, no measurable polarization of nearby bulk states is observed.     

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.     

Resonant photoemission from SmS(100)

A strong, sharp resonance enhancement of 4f photoemission has been observed on SmS(100) surfaces for photon energies in the region of the 4d-4f transitions at about 126 eV. The discrete final state reached via the excitation hν+4d¹⁰4f⁶→4d⁹4f⁷ autoionizes primarely via a super Coster-Kronig transition of the type 4d⁹4f⁷→4d¹⁰4f⁵ + unbound electron. Other decay channels, e.g, Sm 5p emission, as well as a surface induced binding energy shift in the Sm³⁺ final state are identified and discussed.     

Mechanistic study of the reaction of methyl peroxy radical (CH3O2) with formaldehyde (CH2O)

ABSTRACT

A direct dynamic study on the reactions of CH₃O₂?+?CH₂O was carried out over the temperature range of 300–1500?K. All stationary points were calculated with the M06-2X/6-311++G(d,p) level of theory and identified for local minimum. The energetic parameters were refined at QCISD (T)/cc-pVTZ and CCSD (T)/cc-pVTZ levels of theory. Three channels were explored and a reaction of hydrogen abstraction from CH₂O by CH₃O₂ was identified as dominant channel which involves the formation of a prereactive complex in the entrance channel. The rate coefficient of the dominant channel was calculated with TST and TST/Eck and the Eckart tunnelling effect is only important over the lower temperature region. The calculated rate coefficient of the dominant channel has positive temperature dependence and agrees reasonably with the available literature data.     

Angle-resolved photoemission study of NbC(100): Theory and experiment

The electronic band structure of NbC_0.83 has been investigated with angle-resolved photoelectron spectroscopy using HeI, NeI and ArI radiation. Experiments were carried out on the (100) surface with the emission azimuth lying in the (001) and the (0 1) symmetry planes. The experimental data are compared with theoretical spectra for NbC_1.0(100). The results show that most of the structures observed in the experimental spectra can be well explained by the bulk energy band structure of NbC_1.0. The large differences observed when comparing the spectra obtained in the two emission planes are discussed.     

Angle-resolved photoemission study of Pd(111) and Co/Pd(111)

Electronic structure of epitaxial Co films on Pd(111) has been investigated as a function of the Co overlayer thickness, by using angle-resolved photoemission spectroscopy (ARPES). It is found that experimental band structures for Pd(111) and work functions for Co/Pd(111) agree reasonably well with the results of band structure calculations. Compared to Pd(111), valence band ARPES spectra of Co/Pd(111) support the existence of the induced magnetic polarization of Pd atoms near the interface, suggesting large hybridization between Co 3d and Pd 4d states near the interface.     

Adsorption and reaction of bromine with Ag(110): A photoemission study

The adsorption of bromine on the (110) surface of silver has been studied by ultraviolet (hυ = 21.2 and 40.8 eV) photoelectron spectroscopy in the temperature range of 100–300 K. Four different adsorption and reaction states could be detected. For fractional monolayer coverages Br₂ adsorbs dissociatively on the Ag(110) surface. The chemisorption of bromide leads to new emission features at about 3 and 5.2 eV below E_F, which are assigned as occupied antibonding structures (3 eV) and as bonding Br4p_{x, y} orbitals (5.2 eV). At 100 K, further bromide adsorption leads to the formation of an AgBr layer with molecular adsorbed bromine on top of this corrosion layer. The He I spectrum is dominated by structures at 3.5, 5.8 and 7.5 eV which are due to emission from the π_g, π_u and σ_g molecular orbitals of Br₂. The buildup of the AgBr layer is clearly demonstrated by desorbing the molecular bromine at about 150 K. The resulting spectrum of the AgBr layer shows peaks at 2.5 and 3.4 eV with p- and mixed-in d-character and peaks at 4.1, 5.2 and 6.1 eV which are primarily d-like. Heating of the AgBr layer up to 300 K results in a transformation from a 2D layer into a 3D agglomeration of larger AgBr clusters on top of a Br/Ag(110) chemisorption layer.     

FC(O)O自由基与NO反应机理的理论研究

用密度泛函理论(DFT)的B3LYP方法,在6-311G(d,p)基组水平上研究了FC(O)O自由基与NO反应的微观机理,全参数优化了反应过程中各反应物、中间体、过渡态和产物的几何构型,在CCSD(T)水平上计算了它们的能量,振动分析结果证实了中间体和过渡态的真实性,从对FC(O)O与NO的反应机理的研究结果看,FC(O)O与NO反应为4条反应通道多步反应过程,其反应的主要通道是FC(O)O+NO→M3→TS6→M5→FNO+CO₂,其主要产物是自由基FNO和CO_{2关键词： 密度泛函理论 反应机理 反应通道}     

A photoemission study of the interaction of Ni(100), (110) and (111) surfaces with oxygen

Photoelectron spectroscopic studies of the oxidation of Ni(111), Ni(100) and Ni(110) surfaces show that the oxidation process proceeds at 295 and 485 K in two distinct steps: a fast dissociative chemisorption of oxygen followed by oxide nucleation and lateral oxide growth to a limiting coverage of 3 NiO layers. The oxygen concentration in the 295 K saturated oxygen layer on Ni(111) was confirmed by ¹⁶O(d,p) ¹⁷O nuclear microanalysis. At 295 and 485 K the oxide growth rates are in the order Ni(110) > Ni(111) > Ni(100). At 77 K the oxygen uptake proceeds at the same rate on all three surfaces and shows a continually decreasing sticking coefficient to saturation at ~2.1 layers (based upon NiO). An O 1sb.e. = 529.7 eV is associated with NiO, and O ls b.e.'s of ~531.5 and 531.3 eV can be associated, respectively, with defect oxide (Ni₂O₃) or (in the presence of H₂O) with an NiO(H) species. The binding energies (Ni 2p, O 1s) of this NiO(H) species are similar to those for Ni(OH)₂. Defect oxides are produced by oxidation at 485 K, or by oxidation of damaged films (e.g. from Ar⁺ sputtering) and evaporated films. Wet oxidation (or exposure to air) of clean nickel surfaces and oxides, and exposure of thick oxide to hydrogen at high temperature results in an O 1s b.e. ~531.3 eV species. Nuclear microanalysis ²H(³He,p) ⁴He indicates the presence of protonated species in the latter samples. Oxidation at 77 K yields O 1s b.e.'s of 529.7 and ~531 eV; the nature of the high b.e. species is not known. Both clean and oxidised nickel surfaces show a low reactivity towards H₂O; clean nickel surfaces are ~10³ times less reactive to H₂O than to oxygen.     

Angle-resolved photoemission studies of C (2 × 2) oxygen overlayer on Ni (100)

We report angular-resolved photoemission experiments with 44 eV p-polarized radiation on a C (2 × 2) oxygen overlayer on Ni as a function of polar angle θ. A photoemission calculation using a single step model has been performed. We show that with our present expemental equipment, it is very difficult to discriminate the p_z and p_x,p_y states of oxygen, because of hybridization with Ni s-p levels, except at some high symmetry points for which partially polarized light may give a measurable distinction.