Photoemission studies of Ar,Kr and Xe adsorbed on Al(111): Dipole moments,polarizabilities and spatial distributions

Submonolayers of rare gases adsorbed on Al(111) have been studied using photoemission techniques. Coverage-dependent core level binding energy shifts and work function changes have been measured; polarizabilities and dipole moments are deduced. Results indicate that adatom spatial distributions for Xe submonolayers adsorbed on Al(111) at 40 K are best described by a random 2-dimensional distribution, and there is negligible charge transfer from the Al substrate onto the Xe adatoms in the photoionization process.     

Adsorbate structure modeling based on electron energy loss spectroscopy and lattice dynamical calculations: Application to O/A1(111).

High-resolution electron energy loss spectroscopy (EELS) and lattice dynamical calculations based on pair interactions are used to investigate oxygen chemisorption on Al(111). The O/Al(111) System is complicated by the simultaneous formation of an oxygen overlayer and underlayer. Oxygen atoms at overlayer and underlayer sites near the Al(111) surface produce well-defined vibrational loss peaks in EELS spectra, however, dynamical coupling between the oxygen atoms and with the host lattice cause vibrational energies to shift with overlayer and underlayer concentrations. These shifts as well as structural parameters of the O/Al(111) complex can be deduced from a slab model of the surface lattice dynamics.     

Surface state energy shifts by molecular adsorption: CO ON Cu(111)

Molecular adsorption induced energy shifts of a surface state is observed for the Cu(111)/CO system. Angle resolved photoelectron energy spectra show that a zone center surface state shifts to higher energy relative to the Cu bulk bands. The effects is discussed in terms of charge transfer, molecular interaction range and absorption site.     

Adsorption sites in coadsorption systems determined by photoemission spectroscopy: K and CO coadsorbed on Rh(111)

The adsorption sites of coadsorbed K and CO on the Rh(111) surface have been determined using high-resolution core-level spectroscopy, low-energy electron diffraction and site-resolved photoelectron diffraction. For both a (2×2)-2CO–1K and a -6CO–1K structure, we find that the CO molecules occupy threefold hollow sites and the K atoms on-top sites, contrary to the adsorption sites of K (threefold hollow site) and CO (on-top site below 0.5 monolayers) if adsorbed alone on Rh(111). Deposition of K onto a CO precovered surface is found to induce large shifts towards lower binding energy of the C and O 1s core levels (0.7 eV for C 1s and 1.5 eV for O 1s). The major part of these shifts is shown to arise from the K-induced site change of the CO molecules. This finding may be of importance in the interpretation of XPS data of related co-adsorption systems. Finally, it is suggested that the C and O 1s binding energies provide useful fingerprints of the CO adsorption site also for co-adsorption systems.     

Observation of opposite 4f-surface core level binding energy shifts for W(111) and Ta(111)

Surface 4f core level binding energy shifts have been measured in photoemission from W(111) and Ta(111). The surface shift was found to change sign across the row of 5d-metals: for the topmost layer of Ta(111) a +0.40 eV shift toward higher binding energy is found, whereas for W(111) the shift is -0.43 eV toward lower binding energy. The shifts are shown to be dependent on surface crystallography. Chemical shifts are determined for saturation coverage of hydrogen.     

The coadsorption of potassium and co on the pt(111) crystal surface: A TDS,HREELS and UPS study

The interaction of CO with a potassium covered Pt(111) surface is investigated using thermal desorption (TDS), high resolution electron energy loss (HREELS) and ultraviolet photoelectron (UPS) spectroscopies. When submonolayer amounts of potassium are preadsorbed, the adsorption energy of CO increases from 25 to 36 kcal/mole, while substantial shifts in the site occupancy from the linear to the bridged site are observed. The CO stretching vibrational frequencies are shown to decrease continuously with either increasing potassium coverage or decreasing CO coverage. A minimum CO stretching frequency of 1400 cm^?1 is observed, indicative of a CO bond order of 1.5. The work function decreases by up to 4.5 eV at submonolayer potassium coverages, but then increases by 1.5 eV upon CO co-adsorption. The results indicate that the large adsorption energy, vibrational frequency and work function changes are due to molecular CO adsorption with a substantial charge donation from potassium through the platinum substrate and into the 2π¹_CO orbital.     

Energy level shifts of CO chemisorbed and condensed on RH(111)

Energy level shifts of CO in different phases, gas, solid and chemisorbed on Rh(111) are compared. Between gaseous and solid CO a shift of the 4σ and 5σ derived molecular orbitals of about 2.7 eV to lower binding energies due to polarization is observed. A larger 1π – 4σ separation of the CO molecules condensed and chemisorbed on Rh(111) is observed with respect to the gas phase. A comparison of the binding energies of the three molecular orbitals in the different phases allows to estimate the bonding shift of the 5σ level to 2.7 eV.     

Thermal decomposition of CO on a stepped Ni surface

The adsorption-desorption behaviour of CO on the stepped Ni(S) [5(111) × (11̄0)] and the smooth Ni(111) plane are compared by using LEED, thermal flash desorption and AES. Above room temperature flash desorption from the Ni(111) face yields a single α peak characteristic of molecularly adsorbed CO whereas from the stepped surface in addition to the a peak α second desorption peak (β2) appears around 550°C which is assigned to associative CO desorption. If carbon and oxygen are separately chemisorbed on Ni(111) associative desorption of CO leads to a desorption peak around 350° C. It is concluded that steps lower the activation energy for CO decomposition but increase the activation energy for associative desorption.     

Surface interactions of Au(I) cyclo-trimer with Au(111) and Al(111) surfaces: A computational study

A plane-wave density functional theory (DFT) study on surface interactions of a cyclo-[Au(μ-Pz)]₃ monolayer (denoted as T), Pz = pyrazolate, with Au(111) and Al(111) surfaces (denoted as M′) has been performed. Structural and electronic properties at the M′–T interfaces are determined from individually optimized structures of M′, T and M′–T. Results show that the gold pyrazolate trimer (T) binds more strongly on the Au(111) surface than on Al(111). Charge redistribution has been observed at both M′–T interfaces, where charge is “pushed” back towards the Au(111) surface from the trimer monolayer in Au(111)–T system, while the opposite happens in the Al(111)–T system where the charge is being pushed toward the trimer monolayer from the Al(111) surface. Considerable changes to the work function of Au(111) and Al(111) surfaces upon the trimer adsorption which arise from monolayer vacuum level shifts and dipole formation at the interfaces are calculated. The interaction between cyclo-[Au(μ-Pz)]₃ with metal surfaces causes band broadening of the gold pyrazolate trimer in M′–T systems. The present study aids better understanding of the role of intermolecular interactions, bond dipoles, energy-level alignment and electronic coupling at the interface of metal electrodes and organometallic semiconductor to help design metal–organic field effect transistors (MOFETs) and other organometallic electronic devices.     

铝和银在铱（111）宽范围吸附的稳定性、原子构型及电子特性研究

利用第一性原理密度泛函理论研究了铝和银在铱的111面的宽范围吸附特性。基于密度泛函理论计算了覆盖度在0.11ML到2.00ML的结构稳定性、原子构型及平均结合能。对于铝原子在铱111面的吸附,最稳定的结构是铝原子覆盖度为0.5ML位于密堆六方空位（hcp-hollow）,相应的结合能为-4.68eV;对于亚层铝原子的吸附,最稳定结构是铝原子覆盖度为1.00ML时位于octahedral位置,相应的结合能为-5.28eV。对于覆盖度为2.00ML的满覆盖度混合结构的表层及亚层吸附,最稳定结构是Al位于六方密堆及八方密堆位置,相应的结合能为-4.70eV。这意味着当铝原子以满覆盖度吸附在铱的111面上时,趋向于在铱的111面的亚层形成化学键,而非吸附于表层。相比于铝吸附在铱111面,银的吸附特性呈现出很大的不同,面心位置更为稳定,在覆盖度为0.25ML时其结合能为3.89eV,略微高出密堆六方位置处3.88eV的结合能。     

Tailoring confining barriers for surface states by step decoration: CO /vicinal Cu(111)

The influence of CO adsorption on the Shockley type surface state on vicinal Cu(111) surfaces is investigated using angle resolved photoemission. As the steps are decorated with CO the surface state shifts to higher binding energies, which is opposite to the known behavior on flat Cu(111). This is described within a one-dimensional potential model in which clean steps represent repulsive barriers and decorated steps become attractive wells. From the coverage dependence the integrated CO well potential can be quantified. It is U(CO)a = -2.9 eV A on both Cu(332) and Cu(221) surfaces. Density functional calculations reveal that this attractive potential is due to the very local charge transfer from the Cu step atom to the adsorbed molecule.     

铝和银在铱（111）宽范围吸附的稳定性、原子构型及电子特性研究

利用第一性原理密度泛函理论研究了铝和银在铱的111面的宽范围吸附特性。基于密度泛函理论计算了覆盖度在0.11ML到2.00ML的结构稳定性、原子构型及平均结合能。对于铝原子在铱111面的吸附,最稳定的结构是铝原子覆盖度为0.5ML位于密堆六方空位（hcp-hollow）,相应的结合能为-4.68eV;对于亚层铝原子的吸附,最稳定结构是铝原子覆盖度为1.00ML时位于octahedral位置,相应的结合能为-5.28eV。对于覆盖度为2.00ML的满覆盖度混合结构的表层及亚层吸附,最稳定结构是Al位于六方密堆及八方密堆位置,相应的结合能为-4.70eV。这意味着当铝原子以满覆盖度吸附在铱的111面上时,趋向于在铱的111面的亚层形成化学键,而非吸附于表层。相比于铝吸附在铱111面,银的吸附特性呈现出很大的不同,面心位置更为稳定,在覆盖度为0.25ML时其结合能为3.89eV,略微高出密堆六方位置处3.88eV的结合能。     

Ar adsorptions on Al (111) and Ir (111) surfaces: a first-principles study

We investigate the adsorptions of Ar on Al (111) and Ir (111) surfaces at the four high symmetry sites, i.e., top, bridge, fcc- and hcp-hollow sites at the coverage of 0.25 monolayer (ML) using the density functional theory within the generalized gradient approximation of Perdew, Burke and Ernzerhof functions. The geometric structures, the binding energies, the electronic properties of argon atoms adsorbed on Al (111) and Ir (111) surfaces, the difference in electron density between on the Al (111) surface and on the Ir (111) surface and the total density of states are calculated. Our studies indicate that the most stable adsorption site of Ar on the Al (111) surface is found to be the fcc-hollow site for the (2 × 2) structure. The corresponding binding energy of an argon atom at this site is 0.538 eV/Ar atom at a coverage of 0.25 ML. For the Ar adsorption on Ir (111) surface at the same coverage, the most favourable site is the hcp-hollow site, with a corresponding binding energy of 0.493 eV. The total density of states (TDOS) is analysed for Ar adsorption on Al (111) surface and it is concluded that the adsorption behaviour is dominated by the interaction between 3s, 3p orbits of Ar atom and the 3p orbit of the base Al metal and the formation of sp hybrid orbital. For Ar adsorption on Ir (111) surface, the conclusion is that the main interaction in the process of Ar adsorption on Ir (111) surface comes from the 3s and 3p orbits of argon atom and 5d orbit of Ir atom.     

Nickel carbonyl adsorption on the Ni(111) surface

Overlayers formed by the adsorption of Ni(CO)₄ in CO on the Ni(111) surface at 100 K were characterized using high resolution electron energy loss spectroscopy and thermal desorption spectroscopy. At temperatures below 135 K, molecular nickel carbonyl adsorbs on the CO saturated Ni(111) surface as suggested by several observations. Vibrational transitions characteristic of molecular Ni(CO)₄ are dominant. The energy dependence of both the elastic and inelastic electron scattering cross sections are dramatically altered by Ni(CO)₄ adsorption. All of the mass spectrometer ionization fragments typical of molecular Ni(CO)₄ are observed in the narrow thermal desorption peak at 150 K. The inelastic scattering cross sections for both adsorbed nickel carbonyl and adsorbed CO on the Ni(111) surface suggest that a nonresonant dipole scattering mechanism is dominant.     

Sodium adsorption on aluminum (100) and (111) surfaces: ELEED,Auger, and contact potential measurements

The adsorption of Na on sputter-cleaned Al(100) and Al(111) single-crystal surfaces has been investigated under ultrahigh vacuum conditions. Sodium was deposited with an apertured ion source permitting quantitative dosage determination. Low energy electron diffraction patterns show well-ordered coherent structures corresponding to $\text{1}\text{2}$ monolayer on both surfaces, and $\text{1}\text{3}$ monolayer on Al(111). A hexagonal pattern corresponding to $\text{7}\text{8}$ monolayer occurs at a nonconforming dosage on Al(100), indicating a reduction in effective sticking coefficient. This result is verified by Auger intensity measurements, which show saturation beginning at $\text{1}\text{2}$ monolayer on both surfaces. Changes in contact potential with exposure were inferred from shifts in the low-energy cutoff of secondary electrons. The initial dipole moment is in good agreement with that on transition metal substrates, while the saturation value of the contact potential corresponds closely to the difference between reported work functions for bulk Al and bulk Na. A true minimum in the work function was not observed. Present results are compared with results obtained from transition metal substrates. Essential differences are attributed to the more metallic character of the bonding in the Al:Na system.     

Energy loss on of low energy ion N^＋q grazing the A1（111） surface

The total energy loss of N^＋q ions （for v 〈 Bohr velocity） grazing on the Al（111） has been simulated without any ＇fit＇ parameter and compared with the experimental data. The energy loss due to the charge exchange, happening before the N^＋q hits the Al（111） surface, is studied. The present simulation shows that the energy loss strongly depends on the charge state of the projectile and the lattice orientation of Al（111） surface. The calculated total energy loss agrees with experimental data very well.     

Mechanistic study of the CO oxidation reaction on the CuO (111) surface during chemical looping combustion

Cu-based oxides oxygen carriers and catalysts are found to exhibit attractive activity for CO oxidation, but the dispute with respect to the reaction mechanism of CO and O₂ on the CuO surface still remains. This work reports the kinetic study of CO oxidation on the CuO (111) surface by considering the adsorption, reaction and desorption processes based on density functional theory calculations with dispersion correction (DFT-D). The Eley–Rideal (ER) CO oxidation mechanism was found to be more feasible than the Mars-van-Krevelen (MvK) and Langmuir–Hinshelwood (LH) mechanisms, which is quite different from previous knowledge. The energy barrier of ER, LH, and MvK mechanisms are 0.557, 0.965, and 0.999 eV respectively at 0 K. The energy barrier of CO reaction with the adsorbed O species on the surface is as low as 0.106 eV, which is much more active in reacting with CO molecules than the lattice O of CuO (111) surface (0.999 eV). A comparison with the catalytic activity of the perfect Cu₂O (111) surface shows that the ER mechanism dictates both the perfect Cu₂O (111) and the CuO (111) surface activity for CO oxidation. The activity of the perfect Cu₂O (111) surface is higher than that of the perfect CuO (111) surface at elevated temperatures. A micro-kinetic model of CO oxidation on the perfect CuO (111) surface is established by providing the rate constants of elementary reaction steps in the Arrhenius form, which could be helpful for the modeling work of CO catalytic oxidation.     

Ni(111)表面一氧化碳和氢共吸附的密度泛函理论研究

本文用密度泛函理论(DFT)的总能计算研究了一氧化碳和氢原子在Ni(111)表面上p(2×2)共吸附系统的原子结构和电子态，结果表明CO和H原子分别被吸附于两个对角p(1×1)元胞的hcp和fcc位置.以氢分子和CO分子作为能量参考点，总吸附能为2.81 eV，相应的共吸附表面功函数φ为6.28 eV.计算得到的C—O，C—Ni和H—Ni的键长分别是1.19?, 1.96?和 1.71?，并且CO分子以C原子处于hcp的谷位与金属衬底原子结合.衬底Ni(111)的最外两层的晶面间距在吸附后的相对变化分别是 关键词： Fisher-Tropsch反应 催化作用 Ni(111) p(2×2)/(CO+H) 共吸附     

Normal modes of CO adsorbed on metal surfaces

A normal mode analysis of experimental data from helium atom scattering (HAS) and electron energy loss spectroscopy (EELS) for vibrations of CO molecules adsorbed in on-top and bridge sites on Ni(100) is presented, using a refined force-constant analysis. The similar case of CO adsorbed on Pt(111) is reinvestigated, revealing a misassignment of normal modes in previous work. Finally, available experimental data for CO on Pt(111) is used to construct a trial potential energy surface.     

Molecular orbital study of co chemisorption and oxidation on a Pt(111) surface

A molecular orbital study was made, using an atom superposition and electron delocalization (ASED) technique, of the structures and energy levels of CO on Pt(111) surface. CO is predicted to be preferentially adsorbed at a height of 2.05 Å from the surface at a 1-fold position with the carbon end down. The calculated binding energy (1.7 eV) is in good agreement with the recent experimental result (1.5 eV) of Campbell et al. Calculated binding energies for bridging (1.3 eV) and high coordinate (1.1 eV) sites are predicted to be smaller in magnitude. Calculated results are used to discuss the ordering of energy levels of adsorbed CO. The interaction between CO (adsorbed) and O (adsorbed) has been studied to estimate the energy of activation for the oxidation of CO on Pt(111) surface. The calculated activation energy (1.6 eV) is in reasonable agreement with the recent experimental result (1 eV) of Campbell et al. The Langmuir-Hinshelwood mechanism is found to be favored. We predict CO₂ bonds vertically.