Influence of band width on the scattered ion yield spectra of a He + Ion by resonant or quasi-resonant charge exchange neutralization

The influence of the band structure, especially the bandwidth, on the scattered ion yield spectra of a He+ ion by the resonant or quasi-resonant neutralization was theoretically examined using quantum rate equations. When calculating the scattered ion yield spectra of He+ to simulate the experimental data, we observed that the band structure, especially the bandwidth, had a strong influence on the spectra at relatively low incident He+ ion energies of less than several hundred eV. Through many simulations, it was determined that theoretical calculations that include bandwidth calculation can simulate or reproduce the experimentally observed spectra of He+-In, He+-Ga, and He+-Sn systems. In contrast, simulations not including bandwidth simulation could neither reproduce nor account for such spectra. Furthermore, the calculated ion survival probability (ISP) at low incident ion energies tended to decrease with increasing bandwidth. This decrease in ISP probably corresponds to the relatively small scattered ion yield usually observed at low incident ion energies. Theoretically, such a decrease indicates that a He+ ion with a low incident energy can be easily neutralized on the surface when the bandwidth is large

Surface analysis of thermionic dispenser cathodes

Surface analysis of thermionic Ba dispenser cathodes with a W matrix and Re, Ir, and Os/Ru surface coatings was performed by means of low-energy ion scattering and Auger electron spectroscopy. It was found that the Ba-O complex responsible for the low work functions of the cathodes has similar properties on the different coatings: the atomic Ba/O ratio is close to unity on all cathodes, with the O atoms positioned in a plane below the Ba atoms. The bonding between the Ba and O atoms as observed in the low-energy Auger spectra is also almost identical on all cathodes. However, a significant difference in the absolute Ba-O coverage was observed for the various cathodes. This density is determined by the strength of the bonding of the O atoms in the Ba-O complex with the substrate atoms. The results are discussed with respect to previous experimental and theoretical investigations of cathode surfaces

Dynamic behavior of thermionic dispenser cathodes under ion bombardment

We have investigated the surface coverage and electron emission of thermionic dispenser cathodes during 3 keV Ar+ ion bombardment, thereby simulating the bombardment of the cathodes by residual gases that takes place in cathode-ray tubes as used in television sets. During the ion bombardment at the operating temperature of 1030¿°C, a dynamic equilibrium is established between the sputter removal and resupply mechanisms of the Ba and O atoms that form the dipole layer on the cathode substrate. We demonstrated that the performance of the cathodes under ion bombardment is governed by the O removal and resupply rates. It was found that the Ba resupply rate is almost an order of magnitude higher than the O resupply rate, but that the Ba can only be present on the surface bound to O atoms. Therefore, the Ba/O ratio is approximately equal to unity during the ion bombardment. Based on the investigations of the removal and resupply processes, we proposed a model that accurately describes the surface coverage and electron emission during the ion bombardment, including the dependence of the ion flux and cathode temperature

Electron-beam growing and purification of W crystals

High-purity W single crystals have been grown by the electron-beam floating zone melting technique. The structural quality of these crystals was subsequently improved by the application of a strain-annealing technique. X-ray diffraction methods revealed a near-perfect crystallographic structure. Well-ordered clean W surfaces free from all contaminants were obtained by a two-step heating procedure. Low-energy ion scattering and Auger electron spectroscopy confirmed that the cleaning procedures removed all impurities and that the crystal faces expose only W in the outermost atomic layers. Thin layers of Re were deposited on a W(110) single crystal by magnetron sputtering and investigated by ion scattering and Auger electron spectroscopy

A round robin experiment of elemental sensitivity factors in low-energy ion scattering

In a round robin experiment a set of five polycrystalline, metallic samples is studied by low-energy ion scattering (LEIS) in five different laboratories, The energy range is 0.6-3.5 keV and He and Ne ions are used. Even though different experimental setups are used the evaluated elemental sensitivity factors agree within +/-20%. Reproducibility within single laboratories is better than 10%. In an additional study carried out in three laboratories the surface composition of an alloy, Cu55Pd45, was determined, using in situ calibration standards, These surface composition measurements agreed within +/-3 at% demonstrating that quantitative composition determination is possible using this procedure. (C) 1998 Elsevier Science B.V. All rights reserved