J.J.C. Geerlings

The scattering of hydrogen from a cesiated tungsten surface

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.

H− Formation in proton-metal collisions

Abstract Negative hydrogen ion formation is studied by scattering protons from a cesiated tungsten (110) surface. The primary energy ranges from 50 to 400 eV. The angle of incidence is 70° with respect to the surface normal. A maximum conversion efficiency H − (H − + H 0 ) of 67% is measured. The measurements can be described in terms of the probability model. The perturbation of the H − ion by the metal is described within first order perturbation theory. A reasonable agreement between theory and experiment is obtained.

The velocity dependence of the negatively charged fraction of hydrogen scattered from cesiated tungsten surfaces

Abstract By scattering protons under grazing angles of incidence at a cesiated tungsten (110) surface, the dependence of the scattered H − fraction as a function of the normal and parallel velocity components is measured. The incident energy ranges from 400 to 2000 eV, the angle of incidence is 80° with respect to the surface normal. The experiments are done with a surface which is covered with half a monolayer cesium and a thick layer of cesium, corresponding to work functions of 1.45 and 2.15 eV, respectively. The measurements show a strong dependence on the normal as well as on the parallel velocity component. The normal velocity component dependence can be understood in terms of the probability model. We extended this model to incorporate the parallel velocity. The essence of this extension is that the finite velocity of the metal electron is taken into account. Good agreement between theory and experimental results is obtained.

Local work function effects in the neutralization of alkali ions scattered from cesiated surfaces

The charge state of Li, K and Cs ions scattered from cesium covered W(110) is measured as a function of the surface work function. The particle energy ranges from 100 to 2000 eV. It is shown that the local electrostatic potential has a significant influence on the neutralization process. In the case of potassium and cesium scattering the neutralization is due to a collective effect of the adsorbed cesium ions. In the case of lithium scattering it is found that the adsorbed cesium ions act as independent neutralization centres. The corresponding neutralization cross section is of the order of 600 a 0 2 .

Charge transfer in atom surface collisions: on the validity of the semi-classical approximation

Abstract The classical limit of the quantum-mechanical theory, describing resonant electron transfer in atom-metal collisions, is studied. It is shown that the charge-transfer process can be described in terms of a classical master equation, if the position of the atomic valence level depends on atom-surface distance z . For the particular case of H − formation on cesiated tungsten (110), the difference between the quantal calculation and the solution of the classical master equation is less than 5%.

Li- formation at low normal velocities

Li − formation is studied by scattering a Li + beam from a cesium covered tungsten (110) surface. The energy of the primary particles is varied between 100 and 1000 eV. It is found that the probability for Li − formation is a function of the velocity component of the scattered particles normal to the surface only. In the normal velocity range between 5×10 3 and 2×10 4 m/s the formation probability is exponentially dependent on the reciprocal normal velocity. The experimental results are explained by a simple semi-classical model.

Formation of H− by scattering H+ on a cesiated polycrystalline tungsten surface

We present measurements on the charge transfer and reflection of H+ ions which are scattered from a cesiated polycrystalline tungsten surface. The particle energy ranges from 400 to 2 keV, the angle of incidence with respect to the surface normal is varied between 65 and 90°. The measured values are compared with data obtained earlier for cesiated monocrystalline tungsten (110). The maximum differential H− fraction of scattered particles in the case of cesiated polycrystalline tungsten is 25%. This value is roughly a factor of 2 lower than that of cesiated monocrystalline tungsten (110). The maximum total conversion efficiency, that is the reflected H− current divided by the incident positive ion current, is 12%. This value is about a factor of 3 lower than that obtained for monocrystalline tungsten (110). The different behavior of the polycrystalline with respect to monocrystalline material cannot be explained theoretically by the difference in work function. Calculated values are a factor of 1.7 higher ...

Formation of negative hydrogen ions on a coadsorbed layer of cesium and hydrogen on W(110)

Negative ion formation on a W(110) surface which is covered with a coadsorbed layer of cesium and hydrogen is studied by scattering a proton beam from such a surface. The primary energy is 400 eV. The angle of incidence is 70° with respect to the surface normal. The hydrogen exposure ranges from 0 to 3000 L. The negative ion formation probability on a surface with 0.6 times the saturation cesium coverage is reduced by a factor of 4 by a hydrogen exposure of 3000 L. At small coverage the reduction is found to be proportional to the number of adsorbed hydrogen atoms. The formation probability on a surface which is covered with a thick cesium layer is hardly affected by a similar exposure. These phenomena are attributed to resonant electron transfer between a negative ion and an adsorbed hydrogen atom.

Formation of negative hydrogen ions on a cesiated W(110) surface; the influence of hydrogen implantation

The negative hydrogen ion formation process on a cesiated W(110) surface is studied by scattering a proton beam from such a surface. The primary energy ranges from 50 to 3000 eV. The angle of incidence with respect to the surface normal is 45° or 70°. A maximum negative‐ion formation probability of 0.67 is measured. This quantity does not depend on the angle of incidence. However, it is strongly influenced by the time the surface has been exposed to the beam. This effect is attributed to hydrogen implantation.

Charge and energy transfer in collisions of Cs+ ions with a cesiated W(110) surface

A beam of Cs+ ions with an energy of 500, 1000, or 2000 eV is scattered from a cesiated W(110) target. The angle of incidence is 45° or 75° with respect to the surface normal. The charge state and energy of the scattered particles are measured. The influence of hydrogen coadsorption on the final charge state is investigated. All scattered cesium particles are neutrals when the surface work function is smaller than 2.6 eV. The scattered particles have suffered a pronounced energy loss. From the measurements an extrapolation is made to conditions relevant for surface conversion negative ion sources.