摘要:

The scattering of thermal energy atomic and molecular beams from macroscopically rough surfaces has been found to display scattered flux distributions that are typically skewed towards the incoming, beam i.e., backscattered. Such an observation is consistent with the results reported in the literature describing flow-conductance measurements, which are less than the conductance calculated assuming the Knudsen (diffuse) scattering law. Two types of surfaces were studied in the present work: shot-blasted surfaces and gold-blacked surfaces. Both these surfaces displayed scattered flux distributions of a backscattered nature with only subtle differences noted between the two. The conditions required for backscattering are contrasted with the conditions which appear to be necessary for Knudsen scattering and the possibilities are considered for direct application of these observations to a description of the environment existent in typical vacuum systems.

ISSN:0022-5355    

DOI:10.1116/1.1492975

出版商:American Vacuum Society    

年代:1971

数据来源: AIP

摘要:

The coupling of an atom (or molecule) to a metal converts originally discrete levels which fall in the energy bands of the metal to virtual levels (resonances) with fractional occupancies determined by their positions with respect to the Fermi level. The calculation of these resonances and occupancies is a major problem in the theory of adsorption: It is discussed here in terms of a model Hamiltonian of Anderson's type. The concept of a surface molecule, i.e., a local molecular structure whose molecular orbitals correspond to the resonances, arises with transition metal adsorbents. This concept is used for Na and S on Ni(100) in the pyramidalNi4Xlocal geometry. The experimental binding energy is used to predict the electronic structure and resonances for Na, and the experimental resonance to predict the structure and binding energy of S.

ISSN:0022-5355    

DOI:10.1116/1.1316326

出版商:American Vacuum Society    

年代:1971

数据来源: AIP

ISSN:0022-5355    

DOI:10.1116/1.1316347

出版商:American Vacuum Society    

年代:1971

数据来源: AIP

摘要:

The surface of a degeneratep-type GaAs crystal, cleaved in ultrahigh vacuum to expose the (110) plane, has been examined as a function of cesium coverage using several methods. Electron stimulated desorption (ESD) of ions upon bombardment of the surface by 100-eV electrons is found to be extremely sensitive to trace quantities of adsorbed impurities. The work function-coverage relation forCs+deposited from a Cs zeolite ion gun was determined using a retarding potential method; the energy dependence of electron reflection in the range 0–10 eV was found to differ markedly between the clean and cesiated surfaces.

ISSN:0022-5355    

DOI:10.1116/1.1316348

出版商:American Vacuum Society    

年代:1971

数据来源: AIP

摘要:

An electron-impact desorption (EID)–flash-filament system incorporating a gas-phase and a surface-ion spectrometer, and a means of vapor-depositing clean surfaces has been used to study the adsorption behavior of hydrogen on polycrystalline nickel surfaces with several surface treatments. The desorption activation energy, coverage, and sticking coefficient varied from 98 kJmole−1,5×1014 molecules cm−2and 0.19, respectively, for freshly deposited nickel films, to 50 kJmole−1,2×1014 molecules cm−2and 0.002, respectively, for nickel foils with no prior cleaning treatment. The results indicate a low number (less than 1% of the surface sites) of active sites for EID ofH3+and a preferential filling of these sites by surface diffusion.

ISSN:0022-5355    

DOI:10.1116/1.1316350

出版商:American Vacuum Society    

年代:1971

数据来源: AIP

摘要:

The coadsorption of hydrogen and carbon monoxide on the (111) face of a platinum single crystal has been studied by a combination of thermal flash desorption and electron impact desorption (EID) techniques in ultrahigh vacuum. Several compositions over the range from 0%–100% CO have been studied, over a total pressure range from5×10−9to5×10−7 Torr. The observed adsorption behavior is complex, indicating multiple binding sites for both gases. In addition, the EID studies show the formation of a surface species containing both H and CO. This same species has also been observed when formaldehyde was adsorbed directly from the vapor.

ISSN:0022-5355    

DOI:10.1116/1.1316351

出版商:American Vacuum Society    

年代:1971

数据来源: AIP

摘要:

The relative energies of chemisorption of molecules on a clean lead surface have been computed with an extended Hückel molecular orbital method. The calculations were carried out with several organic molecules in order to investigate the formation of monomolecular layers and their possible use as thin (10–20 Å) insulating Josephson tunneling barriers. Calculating the adsorption energy at various locations on the Pb surface yields a measure of the surface coverage and the most suitable organics for forming the monolayer. Nonoxidative adsorbates are found not to chemisorb on Pb while those which are oxidative do bond. The bonding energy of the acetoxy radical is found to be 4 eV and the ethyl radical energy is 0.6 eV.

ISSN:0022-5355    

DOI:10.1116/1.1316353

出版商:American Vacuum Society    

年代:1971

数据来源: AIP

摘要:

Two basic approaches to the solution of surface problems exist in both theoretical and experimental research on metals. In the first approach, the bulk properties of metals and changes caused by surface processes are studied and the interpretation is based on the nearly-free-electron model of the metal. This approach was successful in the explanation of bulk properties. However, because of the complexity and asymmetry of the system (crystal and gas molecule), one is usually forced to use oversimplified models, which sometimes fail in the elucidation of all the observed effects. Another approach is based on the assumption that the surface interaction (resulting in the formation of a quasichemical compound) concerns a limited number of participants (as in a chemical reaction). In the simplest case: gas particle and a surface atom. New experimental evidence (field-ion-emission microscopy, field-electron-emission microscopy, ion-neutralization spectroscopy) has been found supporting the idea that the properties of a surface atom in the moment of the interaction can be roughly approximated by the properties of an individual atom. The type of approach which should be used in a particular case depends both on the experimental method and on the problems in which we are interested.

ISSN:0022-5355    

DOI:10.1116/1.1316355

出版商:American Vacuum Society    

年代:1971

数据来源: AIP

摘要:

The interactions between hydrogen and nitrogen adsorbed species on a (100) tungsten crystal have been investigated by flash desorption methods. When the crystal covered with a monolayer of hydrogen is exposed to gaseousN2, the nitrogen slowly replaces the two chemisorbed hydrogen β states by means of a slight lowering of the hydrogen desorption energy. This displacement process occurs via slow thermal desorption of hydrogen in the presence of gaseousN2when the surface temperature is ≳300 K no such replacement is observed at 273 K. Coverage measurements indicate that there is a stoichiometric ratio between N atoms adsorbed and H atoms displaced from the crystal at ∼300 K. Although virtually no hydrogen will adsorb on the nitrogen-covered surface at 300 K, several weakly bound hydrogen states are populated at 100 K on this surface. No chemical difference between theβ1andβ2hydrogen species was detected, suggesting that both states originate from adsorbed atoms.

ISSN:0022-5355    

DOI:10.1116/1.1316357

出版商:American Vacuum Society    

年代:1971

数据来源: AIP

摘要:

The interaction of hydrogen sulphide,n-propyl mercaptan, and dimethyl disulphide with the (111) and (100) faces of nickel has been studied by LEED at temperatures up to 700 K. During interaction and subsequent annealing,in vacuobelow10−9 Torrat temperatures up to 1200 K, series of diffraction patterns were observed. Four of the patterns on (111) have been attributed to coincidence nets arising from structures which involve rearrangement and reorientation to (100) type layers. The patterns are described and the interpretations discussed.

ISSN:0022-5355    

DOI:10.1116/1.1316358

出版商:American Vacuum Society    

年代:1971

数据来源: AIP