Shigehiko Yamamoto

Role of Ion Bombardment in Field Emission Current Instability

The role of ion bombardment in field emission current instability is clarified experimentally by varying the emission current level and pressure over a wide range. It is found that bombarding ions sputter-off adsorbed gas molecules on the emitter surface and this causes field emission current fluctuation. This current fluctuation caused by ion bombardment is larger than that caused by gas molecule migration when the product of the pressure and the emission current is higher than 7×10-12 PaA. The relative fluctuations turn out to be the logarithm of the product of the pressure and the emission current. The ions are mainly generated close to the emitter due to electron collisions with residual gas.

Formation Mechanism of a Monoatomic Order Surface Layer on a Sc-Type Impregnated Cathode

The electron emission of a Sc-type impregnated cathode is known to be enhanced by a monoatomic-order surface layer composed of Ba, Sc, and O. This layer reduces the work function much lower than those of conventional impregnated cathodes.1,2) It also makes the cathode resistant to gas contamination.3) In the present work, a basic impregnated cathode is coated with a thin tungsten film containing a certain amount of Sc2W3O12 (about 5–10 atomic percent) to produce a (W-Sc2W3O12) coated cathode. The supplying mechanism of free Sc atoms, and hence the monoatomic-order layer formation mechanism,4) is clarified by Auger electron analysis and X-ray diffraction analyses. Free Sc atoms are produced through the following surface chemical reaction between Sc2W3O12 and Ba atoms supplied from the substrate4) as Sc2W3O12+3Ba=3BaWO4+2Sc. This monoatomic layer formation improves the electron emission current density measured at 850°Cb (brightness temperature) about threefold compared to that of conventional Os coated impregnated cathodes.

Electron emission properties and surface atom behavior of an impregnated cathode coated with tungsten thin film containing Sc2O3

A new cathode has been developed which shows similar electron emission characteristics as a previously reported Sc2O3 mixed matrix impregnated cathode (Sc2O3 MM Cathode). Contrary to the Sc2O3 MM cathode, the new cathode is resistive to prolonged heating at high temperatures and to ion bombardment. This has been made possible by applying to a standard impregnated cathode a tungsten thin-film containing about 5 weight percent Sc2O3. The electron-emission property is found to be strongly linked to the surface atom composition as well as to the distribution of surface atoms.

Dissociative chemisorption of CH4 on a cesiated Pt(111) surface studied by supersonic molecular beam scattering techniques

We have clarified the dynamics and mechanism of CH4 dissociative chemisorption processes on a cesiated Pt(111) surface by means of molecular beam scattering techniques. A comparison is made of similar processes on a Pt(111) surface. It is found that the CH4 molecules with enough energy to overcome the activation barrier for dissociation can no longer dissociate on a cesiated surface of Pt(111). Time-of-flight and angular intensity distribution measurements of the reflected CH4 molecules have revealed a similarity in inelastic collision dynamics both on the Pt(111) and the cesiated Pt(111) surfaces. Even a low concentration of the Cs layer as 0.06 of coverage is found to modify the surface electronic structure such that the activation barrier is enhanced as a result of an increased Pauli repulsive potential component in the potential energy surface. It is also found from time-of-flight measurements that direct dissociation is the only pass way and precursor mediated dissociation is not possible both on the...

Time-of-flight measurement of CH4 molecules rainbow scattered from a LiF(001) surface

Time-of-flight spectra of CH4 molecules scattered from a LiF(001) surface have been obtained at various scattering angles by making use of a cross-correlation chopper blade for the [100] and [110] azimuthal directions. The incident translational energies of CH4 molecules are varied in the 190–500 meV range, while the target surface temperature is maintained at 300 K. The experimental results have been examined in relation to the washboard model [J. C. Tully, J. Chem. Phys. 92, 680 (1990)] which is modified here to take into account the speed distribution of incoming molecules. A qualitative agreement on their angular dependence has been obtained both in the mean speed and the energy spread of the scattered CH4 molecules, which reflects the effect of the strong corrugation of the sample surface. Their quantitative differences are partly explained by the translation-rotational excitation of CH4 molecules during collision.

Inelastic collision processes of CH4 and C2H6 molecules at highly corrugated surfaces studied by the molecular beam scattering technique

Inelastic collision dynamics of alkane (CH4 and C2H6) molecules at highly-corrugated surfaces is studied by the molecular beam scattering technique. For the azimuthal direction such as the [110] or the [100] of the impinging molecules on a LiF(001) surface, a wide variety of surface corrugation can be realized. Angular distributions of the reflected molecules realized in our experiment are found to qualitatively agree with those predicted by such simple classical collision models as hard cube [R. M. Logan and R. E. Stickney, J. Chem. Phys. 44, 195 (1966)] and washboard [J. C. Tully, J. Chem. Phys. 92, 680 (1990)] models. The surface corrugation experienced by C2H6 molecules is found much less than that experienced by CH4 molecules, due to the effectively large size of a C2H6 molecule. The surface corrugation decreases as the kinetic beam energy of the impinging molecules increases. This is caused by the difference in energy dependence of the repulsive potential energy surface of Li and F ions which indepe...

Impregnated cathode coated with tungsten thin film containing Sc2O3

An impregnated cathode of a novel structure is proposed, fabricated, and evaluated. A thin tungsten film 100–400 nm in thickness containing various amounts of Sc2O3 is coated on a standard impregnated cathode composed of a porous tungsten body in which electron emissive materials are impregnated. The electron emission property measured with a diode configuration is found to be dependent on Sc2O3 content and surface atom distribution. Surface atom distribution is depicted by means of Auger electron spectroscopy. For high electron emission enhancement it is necessary for Sc2O3 content to be 2.5–6.5 wt. % and for a layer of the order of a monolayer in thickness composed of Ba, Sc, and O to develop on the cathode surface.

Molecular beam study of CH4 oxidation on a Pt(111)–(2×2)-O surface

The interaction of CH4 with a Pt(111)–(2×2)-O surface has been investigated by a supersonic molecular beam scattering technique. CH4 irradiation is found to completely remove oxygen atoms from the surface via CH4 oxidation reaction, CH4+O→CO+H2. The reaction is found to be translationally activated. The oxidation probability of CH4 on Pt(111)–(2×2)-O is found roughly an order of magnitude greater than the initial dissociative chemisorption probability of CH4 on Pt(111), especially under a low incident kinetic energy condition. From the angular intensity distribution and the time-of-flight distribution measurements of scattered CH4, no significant difference in the collision dynamics of CH4 both on Pt(111) and on Pt(111)–(2×2)-O is recognized.

Moiré-like Distribution of Local Tunneling Barrier Height of the Monolayer Graphite Adsorbed on Pt(111) Surface

The Moire-like distribution of the local tunneling barrier height (LBH) in the surface of monolayer graphite (MG) adsorbed on Pt(111) is discussed on the basis of atom-resolved LBH imaging. The Moire-like LBH distribution is explained by the local work function distribution induced by the lateral variations of inter-layer interaction between C and Pt atoms, which results from the lattice mismatch between MG and Pt(111). Further, the tip-sample distance dependence of the LBH image is discussed. When the tip-sample separation is improperly small, we find that the apparent LBH is influenced by the surface deformation induced by the interaction between the tip and the sample, resulting in the reversal of the Moire-like contrast. This is also explained in the framework of the inhomogeneous inter-layer interaction.

Rainbow scattering of CH4 and C2H6 molecular beams from a LiF(001) surface: Dependence on incident kinetic energy and molecular anisotropy

Angular intensity distributions of CH4 and C2H6 molecules scattered from a highly corrugated LiF(001) surface have been measured in high resolution. CH4 molecules reveal clear rainbow scattering, while C2H6 molecules do not. This difference is attributed to the structural anisotropy between these molecules; a CH4 molecule is more spherically symmetric than a C2H6 molecule. The microscopical repulsive potential corrugation of the LiF(001) surface experienced by these molecules is found to decrease as the incident kinetic energy increases. This counterintuitive lessening of the corrugation is a consequence of the potential surface arising from two very different sized substrate ions.