The scattering of hydrogen from a cesiated tungsten surface

doi:10.1016/0039-6028(83)90117-6

Surface Science

Volume 131, Issue 1, 2 August 1983, Pages 17-33

https://doi.org/10.1016/0039-6028(83)90117-6 Get rights and content

Abstract

The reflection of protons from a partially cesiated tungsten surface is studied in the energy domain between 100 and 2000 eV and in the angular domain between 75° and 85° with respect to the surface normal. The study is performed by measuring the angular and energy distribution of the scattered negative ions. The reflection can take place along two paths. One path is reflection from the cesium surface layer, the other one is reflection from the tungsten substrate. A dependence of the final charge state on the path is observed. It is inferred that this phenomenon is due to incomplete neutralization of the protons scattered from the cesium layer. The energy loss of the reflected ions cannot be accounted for by using only the binary collision model. Also the electronic stopping of the atoms by the metal electrons is shown to be an important energy loss mechanism. Total conversion measurements of H⁺ to H^- combined with the measurements of the negatively charged fraction of the scattered particles, as reported in the proceeding paper, yield the particle reflection coefficient as a function of the angle of incidence. These reflection coefficients show that for angles of incidence less than 75° already more than 50% of the particles do not reflect from the surface. Total conversion efficiency measurements with H^- ions as primary ions show that the influence of the initial charge state on the total conversion is very small.

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Theoretical calculation of cesium deposition and co-deposition with electronegative elements on the plasma grid in negative ion sources
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Neutral beam injection (NBI) based on negative hydrogen ion acceleration is one of the most important facilities to heat the magnetic confinement fusion plasma and drive the plasma current [1,2]. Cesium (Cs) injection is an efficient way to enhance the extracted current in a negative ion source[3,4]. This method is the so-called ‘surface production’, and another is the ‘volume production’ [3].
We studied the work function of cesium deposition and co-deposition with the electronegative element on the plasma grid (PG) using the first-principles calculations. The impurity particles may exist in the background plasma and vacuum chamber wall, and the work function of the PG will be affected. The results indicate that the minimum work functions of pure cesium deposition on Mo (110), W (110), and Mo (112) are reached at a partial monolayer. They are 1.66 eV (σ = 0.56 θ), 1.69 eV (σ = 0.75 θ), and 1.75 eV (σ = 0.88 θ), respectively. An appropriate co-deposition model consisting of cesium with electronegative elements can further decrease the work function. The coverage of cesium and electronegative elements are both 0.34 θ in all the co-deposition models. The F-Cs co-deposition model where the Cs atom and F atom are aligned along the surface normal obtains the lowest work function. They are 1.31 eV for F-Cs on Mo (110), and 1.23 eV for F-Cs on W (110), respectively. The change in work function is linearly related to the change in dipole moment density with a slope of −167.03 VÅ. For pure cesium deposition, two factors control the change in dipole-moment density, one is the electron transfer between adsorbates and the substrate, and another one is the restructuring of surface atoms. There are two additional factors for the co-deposition model. One is the intrinsic dipole moment of the double layer, the other is the angle between the intrinsic dipole moment and the surface. The latter two factors play important roles in increasing the total dipole moment.
Hydrogen ion scattering from alkali/graphene surface: Alkali core states effects
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We present a theoretical study of the frontal collision of protons with a graphene surface to which an alkali-adatom is added. Na and Cs adsorbates are studied in a low coverage limit. We analyze how the presence of adsorbates affects the charge exchange when the binary collision between the H⁺ protons and the adatoms or different carbon atoms close to the impurity occurs. The charge transfer process in the scattering of protons in the K and C atoms of a graphene surface to which a K adatom is added has already been analyzed and discussed in our previous works. We find that the inner states of the alkali atoms adsorbed on graphene introduce important differences in the charge transfer depending on their positions with respect to the valence band of graphene. For a better grasp of this, in this work we select Cs and Na as adatoms due to Cs has core levels that can resonate with the valence band of graphene, while Na does not. The interacting system is described by the Anderson Hamiltonian which takes into account the electronic repulsion at the projectile site; the ion charge fractions after the collision are calculated by using the non-equilibrium Green-Keldysh functions formalism. Also, we describe the adsorption of the alkali atom on the surface using the single impurity Anderson model and introduce the Green's functions required to calculate the adatom spectral density. In the scattering of H⁺ from Cs, the charge fractions as a function of the incident energy, behave similarly to the scattering from K. For the Na adatom, the dependence is different, showing a monotonous increase in the high energy regime. The results of this work allow us to infer the signals in the charge exchange, from the localized features of the density of states on the alkaline adsorbate during the scattering process, due to the peculiarities of the electronic band structure of graphene.
Effective Work Functions of the Elements: Database, Most probable value, Previously recommended value, Polycrystalline thermionic contrast, Change at critical temperature, Anisotropic dependence sequence, Particle size dependence
2022, Progress in Surface Science
As a much-enriched supplement to the previous review paper entitled the “Effective work functions for ionic and electronic emissions from mono- and polycrystalline surfaces” [Prog. Surf. Sci. 83 (2008) 1–165], the present monograph summarizes a comprehensive and up-to-date database in Table 1, which includes more than ten thousands of experimental and theoretical data accumulated mainly during the last half century on the work functions ( $ϕ^{+}$ , $ϕ^{e}$ and $ϕ^{-}$ ) effective for positive-ionic, electronic and negative-ionic emissions from mono- and polycrystalline surfaces of 88 kinds of chemical elements (₁H–₉₉Es), and also which includes the main experimental condition and method employed for each sample specimen (bulk or film) together with 490 footnotes. From the above database originating from 4461 references published to date in the fields of both physics and chemistry, the most probable values of $ϕ^{+}$ , $ϕ^{e}$ and $ϕ^{-}$ for substantially clean surfaces are statistically estimated for about 600 surface species of mono- and polycrystals. The values recommended for $ϕ^{e}$ together with $ϕ^{+}$ and $ϕ^{-}$ in Table 2 are much more abundant in both surface species and data amount, and also they may be more reliable and convenient than those in popular handbooks and reviews consulted widely still today by great many workers, because the latter is based on less-plentiful data on $ϕ^{e}$ published generally before $\sim$ 1980 and also because it covers no value recommended for $ϕ^{+}$ and $ϕ^{-}$ . Consequently, Table 1 may be more advantageous as the latest and most abundant database on work functions (especially $ϕ^{e}$ ) for quickly referring to a variety of data obtained under specified conditions. Comparison of the most probable values of $ϕ^{e}$ recommended for each surface species between this article and other literatures listed in Tables 2 and 3 indicates that consideration of the recent work function data accumulated particularly during the last $\sim$ 40 years is very important for correct analysis of these surface phenomena or processes concerned with either work function or its changes. On the basis of our simple model about the work function of polycrystal consisting of a number of patchy faces (1–i) having each a fractional area (F $_{i}$ ) and a local work function ( $ϕ_{i}$ ), its values of both $ϕ^{+}$ and $ϕ^{e}$ are theoretically calculated and also critically compared with a plenty of experimental data. 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In particular, a significant amount of interest has revolved around the influence of surface adsorbates on the ion fractions. Experimentally, the ion survival probability has been found to be very sensitive to the surface structure and dynamics conditions [2–8]. On the theory side, the survival probability is extremely sensitive to the modeling of the potential barrier for the active electron [9–15].
The survival probability of the H⁻ ions colliding with a 0.25 ML Na covered Cu(111) surface is studied by both theory and experiment. We find that the survival probability is influenced by the projectile direction on surface. The reason is the different bonding between the Na atoms and their Cu neighbors along different surface directions. A stronger bond leads to a larger survival probability.
Dynamics conditions for channeling during H<sup>-</sup> scattering on Cu(111)
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The dynamics conditions for channeling effects during H⁻ scattering on Cu(111) surfaces, including: critical angle, distance of closest approach to surface, reflective angle shift, energy loss, and channeling dip depth, are studied with Molecular Dynamics. Calculations show that the critical angle exponentially decreases with incident energy, and the distance of closest approach to surface decreases not only with the incident energy, but also with the incident angle. For the reflective angle shift and the energy loss, we find that there is a range of very small incident angles where both decrease slightly with the incident angle; beyond this range, they increase rapidly with both incident angle and incident energy. For large angles of incidence, the channeling dip depth exponentially increases with the incident angle. The H⁻ ion injected through the middle point of two nearest neighbor Cu atoms on the first layer provides the maximum depth. The reason is a lower surface potential barrier for the injection through this point. The target atomic string density on the surface also affects the dynamics processes. Projecting along an atom string with higher atomic density, leads to a larger critical angle, a larger distance of closest approach to surface, a smaller reflective angle shift, a smaller energy loss, and a shallower channeling dip depth.
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The scattering of hydrogen from a cesiated tungsten surface

Abstract

Surface Sci.

Surface Sci.

Surface Sci.

Surface Sci.

Phys. Rev.

Max-Planck Institut für Plasmaphysik, Report IPP 9/32