摘要:

The basic concepts in the theory of chemisorption of atoms on metals will be reviewed. The sharp atomic levels of the free atom are broadened into virtual levels by electron tunneling between the atom and the solid. For rapid tunneling, correlation effects on the adatom are weak and a self-consistent field molecular orbital approach is appropriate. The adatom density of states can vary from a Lorentzian level to states split above and below the valence band of the solid, corresponding to the bonding and antibonding orbitals of a surface complex formed between the adatom and its surroundings. The Anderson model treats only Coulomb interactions on the adatom, while the CNDO method of quantum chemistry has been recently employed to include Coulomb interactions in the solid. For strong intra-atomic Coulomb interaction on the adatom, correlation effects are important and the localized magnetic moment on the adatom as predicted by the unrestricted Hartree–Fock approximation is quenched. Correlations have been treated by extended valence bond methods, multiple scattering theory, and configuration mixing techniques. Semiquantitative agreement with experiment is obtained in a number of systems.

ISSN:0022-5355    

DOI:10.1116/1.1317718

出版商:American Vacuum Society    

年代:1972

数据来源: AIP

ISSN:0022-5355    

DOI:10.1116/1.1317720

出版商:American Vacuum Society    

年代:1972

数据来源: AIP

摘要:

Adsorption of Cs and O on W(112) has been studied with systematic variations of the composition and the sequence of deposition of the adsorbates. The results suggest that Cs and O form a W-O-Cs double layer, regardless of the sequence of deposition. The differences in structure of Cs on clean W and O/W are compared. It is found that Cs and O affect the structures of each other. The change in work function of the surface at various Cs and O coverages are reported. Throughout the coverage range of the present study, there is no apparent evidence of cesium oxide formation.

ISSN:0022-5355    

DOI:10.1116/1.1317721

出版商:American Vacuum Society    

年代:1972

数据来源: AIP

摘要:

This paper first develops a general theory of the screening change induced in an inhomogeneous electron gas by a small perturbing charge. This theory is applied to the screening of a point charge embedded in a metal surface. The first stage of this program which is reported here, uses the jellium model for the unperturbed metal surface and treats both the unperturbed and perturbed systems in the Thomas–Fermi approximation. Self-consistency, which is maintained in the present paper, leads to results which differ significantly from earlier results obtained with infinite barriers, which are by their nature not self-consistent.

ISSN:0022-5355    

DOI:10.1116/1.1317722

出版商:American Vacuum Society    

年代:1972

数据来源: AIP

摘要:

Soft x-ray appearance potential spectra have been obtained for dilute coverages of transition metals on transition metal surfaces. The appearance potential spectrum of a given core level is the differential excitation probability as a function of sample potential. For transition metals this produces a strong positive threshold peak whose width is related to the unfilled portion of thedband. The titanium-nickel system is of particular interest since, in their pure form, titanium and nickel have nearly empty and nearly filled 3dbands respectively. In preliminary studies of this system, we find the2p32peak of nickel in dilute concentration on a titanium surface to be identical to the titanium2p32peak obtained from dilute coverages of titanium on a nickel surface. The width of these peaks is intermediate between the clean Ti and Ni values. This result, which suggests a common band model for an alloy of stoichiometry near that of TiNi, is contrasted with recent experimental and theoretical studies of dilute transition metal alloys.

ISSN:0022-5355    

DOI:10.1116/1.1317723

出版商:American Vacuum Society    

年代:1972

数据来源: AIP

摘要:

Total energy distributions of field emitted electrons from the tungsten (110) and (100) planes as a function of coverage by hydrogen and deuterium have been recorded utilizing a spherical deflection energy analyzer. The elastic tunneling resonance spectrum gives a plot of the “local density of states” in the adsorbate. The inelastic tunneling spectrum reveals those discrete excitation energies available in the adsorbate–substrate complex. These spectroscopic data have been used to infer the chemical nature of the binding states which have been observed in the flash desorption spectrum of hydrogen from tungsten.

ISSN:0022-5355    

DOI:10.1116/1.1317724

出版商:American Vacuum Society    

年代:1972

数据来源: AIP

ISSN:0022-5355    

DOI:10.1116/1.1317725

出版商:American Vacuum Society    

年代:1972

数据来源: AIP

摘要:

Solids with electron band structures given by a simple analytic function of wavenumber (E∼k2for free electron bands;E∼kfor tightbinding bands) possess localized electron states at their surfaces which are easily treated within the complexkvector formalism developed especially by Heine and collaborators. These surface states, which will be called Cambridge surface states, can occur not only in energy gaps caused by Bragg reflections at Brillouin zone faces but also in spin-orbit split gaps and hybridization gaps within the zone. In this work a theory of tunneling and field emission of electrons from such surface states is developed. The field emission energy distribution fromd-band metal surface states is calculated using Harrison’sd-band pseudopotential theory, and the results are favorably compared with experimental data. Since the existence conditions for Cambridge surface states depend crucially upon the boundary conditions at the surface, the act of measurement via a tunneling process can sufficiently alter the boundary conditions that “surface states” are either created or destroyed by “looking” at them. In particular the strong coupling ion neutralization tunneling experiments of Hagstrum are very susceptible to this effect.

ISSN:0022-5355    

DOI:10.1116/1.1317726

出版商:American Vacuum Society    

年代:1972

数据来源: AIP

摘要:

A time-of-flight technique is used to study the release of excited neutrals and ions by electron-stimulated desorption ESD for the system CO on W. The method ensures the detection of the excited neutral particles without interference from photons produced by the electron beam. Use of a channel electron multiplier allows the counting of individual particle arrivals, insuring high sensitivity and allowing the use of low electron current densities. Energy distributions are obtained from the flight-time distributions for both kinds of particles, using a (100) single-crystal sample. The release of excited neutrals for this system in the temperature range 130 to 270 K is demonstrated.

ISSN:0022-5355    

DOI:10.1116/1.1317727

出版商:American Vacuum Society    

年代:1972

数据来源: AIP

摘要:

A UHV system, containing a heatable (up to>2600 °K) W ribbon target, a Finkelstein ion source (Ar+, 2 keV) with magnetic mass separator, an electron source (300 eV), a quadrupole secondary ion mass filter with a counting channel electron multiplier and a quadrupole gas analyzer, is used for the study of the adsorption and desorption ofH2,O2, and CO on W by simultaneous, i.e., fast interchanging “static” SIMS [secondary (ion induced) ion mass spectroscopy] and EID [electron induced (ion) desorption]and by flash-filament measurements. Different binding states can be distinguished. The emission cross sections of the adsorbate-specific ions by ion bombardment depend far less upon the gas exposure (and therefore upon the coverage) than those corresponding to electron bombardment.

ISSN:0022-5355    

DOI:10.1116/1.1317728

出版商:American Vacuum Society    

年代:1972

数据来源: AIP