Suppression of field emission from W(110) and W(111) by Cu overlayers

The total energy distribution and the Fowler-Nordheim preexponential factor have been measured for field emission from the (100) and (111) facets of tungsten in the absence and in the presence of overlayers of chemisorbed copper. It is found that at close to monolayer coverage on the (100) facet the adsorbate dramatically reduces the surface density of states at the Fermi energy, whereas on the (111) facet the surface density of states at the Fermi energy is little affected. Self-consistent, semirelativistic LMTO calculations of the k-resolved layer densities of states at the centre of the surface Brillouin zone have been carried out for a tungsten-vacuum interface with and without an ordered overlayer of copper. According to the calculations, a copper monolayer on W(100) strongly suppresses the surface density of states at the Fermi energy, whereas for a copper monolayer on W(111) no such suppression is predicted. The consistency of these results with the experimental data indicates the promise of the present method for calculating the electronic structures of metal-vacuum interfaces.     

Cu单原子修饰对Fe(111)表面CO吸附性能及电子性质调变的第一性原理研究

采用密度泛函理论方法研究了Cu单原子修饰对Fe(111)表面CO吸附性能和电子性质的调变作用,其中,Cu单原子修饰研究了吸附和取代两种方式。结果表明,CO在Cu修饰的Fe(111)面吸附能力都会变弱,一是Cu原子自身提供的位点对CO的吸附较弱;二是Cu会使其附近的Fe对CO的吸附变弱。分析电子性质表明,Cu作用于Fe表面后,会导致Cu附近Fe原子部分电子向Cu原子转移,进而削弱了Fe与吸附分子间电子交互作用而改变Fe原子的吸附能力。故Cu原子改性Fe表面可以很好地调变CO的吸附、解离及后续反应催化活性,这为进一步探究Cu改性Fe表面的合成气催化反应机理提供了基础信息。     

The proportional relationship between the efficiency of a DC ion source and atom mass

The efficiency of ion extraction from a DC ion source has been found by experiment to bear a close correlation to atomic mass. The factors influencing this efficiency are examined theoretically in terms of the mechanisms of extraction, that is, by gas flow, electric field and bulk diffusion. It is found that while charged material is extracted by the electric field, the transport mechanism to the region influenced by the extraction electrode is dominated by gas flow and is therefore dependent on atomic mass. Close agreement with theory and experiment can be seen.The practical application of this approximately linear dependence of ion yield and mass is demonstrated in the VG 9000 glow discharge mass spectrometer. The simple ratio of matrix ion current to contamination ion current gives a direct quantitative reading of weight-% impurity levels down to ppt levels.Presented in part at the 1989 European Winter Conference on Plasma Spectrochemistry, Reutte, Austria     

Coordinatively unsaturated W(IV)-bis(pyridine) complexes, a reactive source of W(IV)

Addition of 2 equiv of a sigma-donor ligand (L = pyridine, 4-picoline, or quinoline) to complexes of the type [W(NPh)(eta(4)-arene)(o-(Me3SiN)2C6H4)] (arene = CH3CH2C6H5 (3), CH3CH2CH2C6H5 (4)) gave the W(IV)L2 compounds, [W(NPh)(o-(Me3SiN)2C6H4)(C5H5N)2] (5), [W(NPh)(o-(Me3SiN)2C6H4)(p-C6H7N)2] (6), and [W(NPh)(o-(Me3SiN)2C6H4)(C9H7N)2] (7). Synthesis of compounds 5 and 6 by Na degrees reduction of [W(NPh)(o-(Me3SiN)2C6H4)Cl2] in the presence of 3 equiv of L (L = 5, pyridine or 6, 4-picoline) is also presented. Compounds 5, 6, and 7 display hindered rotation of the donor ligands about the W-N bonds, resulting from a steric interaction with the Me3Si groups of the diamide ligand. The coordinative unsaturation of 5 and 6 has also been explored. Compounds 5 and 6 readily react with either CO and PMe3 to generated the six coordinate complexes [W(NPh)(o-(Me3SiN)2C6H4)(C5H5N)2(CO)] (8a), [W(NPh)(o-(Me3SiN)2C6H4)(C6H7N)2(CO)] (8b), [W(NPh)(o-(Me3SiN)2C6H4)(C5H5N)(PMe3)2] (10a), and [W(NPh)(o-(Me3SiN)2C6H4)(C6H7N)(PMe3)2] (10b), respectively.     

Embedded atom method study of Cu deposition on Ag(111)

A new model is applied to the study of Cu deposition on Ag(111), It links elements from the embedded atom model and Monte Carlo simulations. In agreement with the experiment, the present results favor overpotential upon underpotential deposition. A diffusion coefficient of 1.93 × 10⁻⁶ cm² s⁻¹ is estimated for the system. The predicted structure of the adsorbed monolayer is more compact than the electrochemical observation, a fact which may be due to some simplifications of the model like neglecting the solvent and anion effects or the nature of the potentials employed.     

Cluster models of interaction of a fluorine atom with the (111) face of silicon

The AMI method has been used in calculating activation energies of desorption of surface complexes from the (111) face of silicon. For the desorption of a surface Si atom, the desorption of an FSi radical containing this Si atom when a fluorine atom implanted in the surface layer is present, and the desorption of the FSi radical when such a fluorine atom is not present, the values found for the activation energy are 8.0, 0.9, and 6.4 eV, respectively.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 28, No. 2, pp. 144–147, March–April, 1992.     

Sequential segregation of Sn and Sb in a Cu(111) single crystal

The sequential segregation of Sn and Sb to the surface of a Cu(111) single crystal was measured in the temperature range 400–1100 K by Auger electron spectroscopy. It was found that Sn with the higher diffusion coefficient first segregates to the surface and then is replaced by the slower‐segregating Sb. The results were fitted by a ternary segregation model yielding segregation energies (ΔG_Sn = 76.3 kJ mol^?1, ΔG_Sb = 95.9 kJ mol^?1), interaction parameters (Ω_SnCu = 3.8 kJ mol^?1, Ω_SbCu = 16.2 kJ mol^?1, Ω_SnSb = ?5.3 kJ mol^?1) and diffusion coefficients (D_0(Sn) = 1.8 × 10^?5 m² s^?1, E_Sn = 173 kJ mol^?1, D_0(Sb) = 6.0 × 10^?5 m² s^?1, E_Sb = 205 kJ mol^?1) for both species. The validity of the interaction coefficients and segregation energies was verified using the Guttman equations for equilibrium segregation in ternary systems. Copyright © 2003 John Wiley & Sons, Ltd.     

Decay of atomic systems in a strong electric field. Negative ion and hydrogen atom

An exact transcendental equation was obtained determining the energy levels of a particle bound by a short-range force and placed in uniform electric field. At high intensities there is a considerable difference between the values of decay probabilities calculated by previous theory and by this one. The results obtained were used to determine the electron binding energy and the polarizability of negative ions by employing experimental data on ion decay during a flight through the field region. Concrete calculations were carried out for the C^? ion.The calculation of the resonance energy E_r and the decay probability 2Γ of the quasistationary state n = S, n₁ = 3, n₂ = 0, and m = 1 of the hydrogen atom in a uniform electric field was performed by means of the direct integration of the time-independent Schrödinger equation for field intensities ranging from 0.8 × 10⁶V/cm to 1.7 × 10⁶V/cm. The calculated values fo E_r perfectly agree with those of the Rayleigh—Schrödinger perturbation energies calculated up to the fourth power in the field strength and differ substantially from the resonance energies obtained earlier by Hirschfelder and Curtiss. Our results concerning the rate of spontaneous ionization also differ substantially from the results of Hirschfelder and Curtiss and agree rather well with the results of Bailey, Hiskes and Riviere who used the general WKB method of Rice and Good.     

Adsorption of a single molecule of a diarylethene photochromic dye on Cu(111)

On the route to single (large) molecule unimolecular chemistry, the adsorption of a photochromic dithienylethene dye on Cu(111) at a submonolayer level has been studied by Ultra High Vacuum-Scanning Tunneling Microscopy at Low Temperature. This technique has shown that the observed adsorbed molecule's shape is compatible with an helical conformation but has also revealed a surrounding electronic corrugation due to the perturbed surface states. Careful examination of the standing wave pattern indicated that only a part of the molecule is indeed interacting with the metallic substrate. Geometric considerations were used to infer that the bridging ethene moiety could be responsible for the electronic scattering. Scanning Tunneling Spectroscopy has shown a substantial amount of charge transfer from the surface to the adsorbate. The hypothesis that this precise double bond is a reactive locus toward charge transfer processes is confirmed by the electrochemical results: this double bond is indeed reduced upon coulometric reduction on glassy carbon. Furthermore, the use of a copper cathode strongly facilitates the reduction since a +0.6 V shift was recorded.     

Potassium-benzene interactions on Pt(111) studied by metastable atom electron spectroscopy

Electron emission spectra obtained by thermal collisions of He(?)(2(3)S) metastable atoms with C(6)H(6)/Pt(111), C(6)H(6)/K/Pt(111), and K/C(6)H(6)/Pt(111) were measured in the temperature range of 50-200 K to elucidate the adsorption/aggregation states, thermal stabilities of pure and binary films, and local electronic properties at the organic-metal interface. For C(6)H(6)/Pt(111), the He(?)(2(3)S) atoms de-excite on the chemisorbed overlayer predominantly via resonance ionization followed by Auger neutralization and partly via Penning ionization (PI) yielding weak emission just below the Fermi level (E(F)). We assigned this emission to the C(6)H(6) π-derived states delocalized over the Pt?5d bands on the basis of recent density functional calculations. During the layer-by-layer growth, the C(6)H(6)-derived bands via PI reveal a characteristic shift caused by the final-state effect (hole response at the topmost layer). C(6)H(6) molecules chemisorb weakly on the bimetallic Pt(111) (θ(K)=0.1) and physisorb on the K multilayer. In both cases, the sum rule was found to be valid between the K?4s and C(6)H(6)-derived bands. The band intensity versus exposure plot indicates that the C(6)H(6) film grows on the K multilayer by the Volmer-Weber mechanism (island growth), reflecting the weak K-C(6)H(6) interactions. In case of K/C(6)H(6)/Pt(111), the K atoms are trapped on the topmost C(6)H(6) layer at 65 K, forming particlelike clusters. The surface plasmon satellite was identified for the first time and the loss energy increases with increasing cluster size. The K clusters are unstable above ～100?K due to thermal migration into the C(6)H(6) film. When the cluster coverage is low, the K?4s band extends below and above E(F) of the Pt substrate and the anomaly is discussed in terms of vacuum level bending around the cluster.     

First principles study of adsorption and dissociation of CO on W(111)

The adsorption and dissociation of carbon monoxide on the W(111) surface is studied with density functional theory. The CO molecule is found to adsorb in end-on configurations (alpha states) and inclined configurations (beta states). The dissociation of the most strongly bound beta state CO is found to have an activation energy of about 0.8 eV, which is lower than the energy required to desorb CO molecularly from the surface. The diffusion of CO and O on W(111) is predicted to be facile at room temperature, whereas C atoms are virtually immobile up to approximately 600 K, according to our calculations. Preadsorbed carbon atoms are shown to prevent the dissociation of CO by blocking the most strongly bound beta state adsorption site and by blocking the dissociation pathway. We predict that dissociation of CO on W(111) is a self-poisoning process.     

The pulsed arc cluster ion source (PACIS)

A new cluster source, the “PACIS”, has recently been developed [1, 2]: a pulsed high-current arc is fired between two electrically isolated electrodes. In a stream of carrier gas, the nascent metal plasma cools down, thus undergoes significant aggregation. After supersonic expansion into high vacuum, the resulting cluster ions will be subject to investigation. Here, we present the current state of the source development and further technical details. The integral intensity is estimated by film thickness measurements and yields a deposition rate of, e.g., up to 2 Å per shot for lead. About 10% of the emitted material turns out to be charged. Time-of-flight measurements show similar cluster ion mass spectra as they are known from laser vaporization. Already single shot mass spectra display completen-series which can be followed ton ≈ 40. For heavy clusters, a significant velocity slip is estimated from the beam velocities.     

O atom induced gradual deconstruction of the 23xradical3 Au(111) surface

He diffraction has been used to investigate changes in the surface morphology of reconstructed Au(111) when small quantities of O atoms are adsorbed. It is proposed that the electronegative oxygen removes charge from the surface, which causes the surface to revert to the (111) structure. The extent of this deconstruction is dependent on the initial O coverage and the surface temperature. These results further delineate and emphasize the delicate interplay of adsorbate coverage and surface structure for the oxygen-gold system, a topic of current high interest due to the remarkable and technologically relevant catalytic properties of gold interfaces and clusters spanning atomic through nanoscale dimensions.     

Spin-related electronic pathway through single molecule on Au(111)

Spin properties of organic molecules have attracted great interest for their potential applications in spintronic devices and quantum computing. Fe-tetraphenyl porphyrin (FeTPP) is of particular interest for its robust magnetic properties on metallic substrates. FeTPP is prepared in vacuum via on-surface synthesis. Molecular structure and spin-related transport properties are characterized by low-temperature scanning tunneling microscope and spectroscopy at 0.5 K. Density functional theory calculations are performed to understand molecular adsorption and spin distribution on Au(111). The molecular structure of FeTPP is distorted upon adsorption on the substrate. Spin excitations of FeTPP are observed on the Fe atom and high pyrrole groups in differential conductance spectra. The calculated spin density distribution indicates that the electron spin of FeTPP is mainly distributed on the Fe atom. The atomic transmission calculation indicates that electrons transport to substrate is mediated through Fe atom, when the tip is above the high pyrrole group.     

Micro-machined planar field asymmetric ion mobility spectrometer as a gas chromatographic detector

A planar high field asymmetric waveform ion mobility spectrometer (PFAIMS) with a micro-machined drift tube was characterized as a detector for capillary gas chromatography. The performance of the PFAIMS was compared directly to that of a flame ionization detector (FID) for the separation of a ketone mixture from butanone to decanone. Effluent from the column was continuously sampled by the detector and mobility scans could be obtained throughout the chromatographic analysis providing chemical inforrmation in mobility scans orthogonal to retention time. Limits of detection were approximately I ng for measurement of positive ions and were comparable or slightly better than those for the FID. Direct comparison of calibration curves for the FAIMS and the FID was possible over four orders of magnitude with a semi-log plot. The concentration dependence of the PFAIMS mobility scans showed the dependence between ion intensity and ion clustering, evident in other mobility spectrometers and atmospheric pressure ionization technologies. Ions were identified using mass spectrometry as the protonated monomer and the proton bound dimer of the ketones. Residence time for column effluent in the PFAIMS was calculated as approximately 1 ms and a 36% increase in extra-column broadening versus the FID occurred with the PFAIMS.     

Quasiclassical study of Eley-Rideal and hot atom reactions of H atoms with Cl adsorbed on a Au(111) surface

Using quasiclassical methods and a potential energy surface based on total energy calculations, we have found that H atoms react with Cl atoms adsorbed onto a Au(111) surface to produce HCl via Eley-Rideal (ER), hot atom (HA), and Langmuir-Hinschelwood (LH) pathways. We observe two ER mechanisms. At small normal incidence energies reaction results from a more or less direct collision with Cl, leading to a large amount of product vibration (nu=8), and relatively cold rotation and translation. In the second mechanism, more dominant at near-normal incidence and/or large incident energies, the H atom passes near Cl, recoils from the metal, and is pulled into orbit about Cl. This leads to broader product state distributions, and a more even distribution of the 3.0 eV of available energy among the product degrees of freedom, similar to products formed via the HA pathway. Overall, ER processes tend to contribute less than 10% to the reactivity, and most of the HCl is formed via HA processes. There is an increase in HCl formation with surface temperature for both the ER and HA mechanisms, but this increase is relatively weak. We observe typically about 12% H atom sticking, which would lead to HCl formation via a LH process in the experiments, above 140 K. We observe a weak forward scattering due to the direct ER component, as in the experiments. However, unlike the experiments, we observe a dip in our product angular distributions about thetaf=0 degrees, which we ascribe to our quasiclassical approximation. While we tend to see more energy in the hot products than in the experiments, our product translational, rotational, and vibrational distributions are in relatively reasonable agreement with those measured. One major disagreement with experiment is that there is apparently a significant sticking of the H atom at low temperatures, leading to a large LH component. In addition, the ER and HA components increase much more strongly with temperature than in the calculations. It is possible that electon-hole pair excitations in the metal strongly relax both the H atom and the excited HCl molecules formed.     

On the scattering of a light pulse on a single atom perturbed by collisions with inert gas atoms

The excitation of the strontium-like systemS ₀ —P ₁ by the light pulse of duration comparable or larger than relaxation times is considered. The influence of the different pulse envelopes on the time evolution of the density matrix components and on energy spectrum is analyzed. In the case of large detuning and small Rabi frequency the secular approximation is adopted.     

Surface stress-charge response of a (111)-textured gold electrode under conditions of weak ion adsorption

We report a cantilever bending investigation into the variation of surface stress, f, with surface charge density, q, for (111)-textured thin films of gold in aqueous NaF and HClO 4. The graphs of f(q) are highly linear, and the surface stress-charge coefficients, d f/d q, are -1.95 V for 7 mM NaF and -2.0 V for 10 mM HClO 4 near the potential of zero charge. These values exceed some previously published experimental data by a factor of 2, but they agree with recent ab initio calculations of the surface stress-charge response of gold in vacuum.