C. Harel

Common translation factor method

The basis and workings of a very useful technique in the treatment of atomic collisions is explained, which is the introduction of a common translation factor in the framework of close-coupling expansions. A historical review of the subject is presented, together with a description of the properties of the factor, and a detailed illustration of its performance.

A simple model for cathodic electronic emission enhanced by low-energy ions in high-pressure arcs

The influence of slow ions on the electronic emission of a cathode in a high-pressure electric arc is investigated. As a first approach, a simple model is introduced, where solving the stationary Schrodinger equation is reduced to integrating a one-dimensional second-order differential equation. We have found that the electronic current may be enhanced through two mechanisms: a non-resonant process related to the lowering of the potential barrier and a resonant mechanism involving bound states of the ions. The non-resonant process is found to be dominant. The influence of various parameters such as ionic density, cathode temperature and charge states of the ions on amplification coefficients is analysed.

Enhancement of cathodic electronic emission by slow positive ions in high-pressure arcs

Some recent results indicate that the electronic current emitted by the cathode of a high-pressure electric arc may be enhanced by very close positive ions. This problem is addressed here by means of a 2D treatment of the Schrodinger equation, based on a finite element method. In addition, a diagonalization method has been implemented to study the specific role played by processes of electronic resonance. This approach is applied to the test case of an ionized hydrogen plasma in front of a copper cathode. By comparison with a previous 1D study, the present predictions show that a full 2D treatment is required in most cases.

Total and partial cross sections of electron transfer processes with hydrogen gas targets: Be4+ + H2

We report the implementation of programs to treat electronic transitions in ion-molecule collisions and the calculation of vibrationally-resolved partial cross sections. This includes the ab initio calculation of non-adiabatic couplings, the implementation of a block-diagonalization procedure, and the evaluation of vibrational transition probabilities. Using a two-state basis, we present some preliminary results on the Be4+ + H2(v = 0, L = 0) Be3+(3l) + H2+(v, ΣLM) reaction. These show that for nuclear velocities v < 0.2 a.u. vibration cannot be ignored, even though anisotropy effects are small. The reason is the strong dependence of the electronic transition amplitudes on the H–H distance.

Single- and double-electron capture in low-energy Ne10+-He collisions

We present a combined experimental/theoretical study of Ne10+-He collisions in the 50?E?150?keV impact energy range using improved-resolution recoil ion momentum spectroscopy. Total and differential cross sections for single-electron capture onto the levels n = 3,4,5 and 6 of Ne9+(nl) and double-electron capture onto series of doubly excited states 3, n and 4, n with n = 4, 5 and 6 of Ne8+ have been obtained. They are found to be in good agreement with close-coupling calculations. Population mechanisms for the dominantly populated double-capture channels have been deduced from this work. Special emphasis is brought to the analysis of the partial stabilization ratios to obtain a new insight into the velocity dependence of total stabilization ratios.

Quantal and semiclassical calculations of charge transfer cross sections in + H collisions for impact energies of

Total and n-partial cross sections for charge transfer in collisions are calculated for collision energies between and . A molecular expansion is employed, and semiclassical and quantal calculations are carried out including a common translation factor and a common reaction coordinate, respectively. The comparison between quantal and semiclassical cross sections and transition probabilities is discussed.

Electron capture by C4+, N5+ and O6+ from atomic hydrogen in the keV amu-1 energy range

Total electron capture cross sections by C4+(1s2), N5+(1s2) and O6+(1s2) from atomic hydrogen are calculated in the molecular approximation. The problem is treated in a one-electron approximation by using a model potential approach. Electronic momentum transfer is accounted for through the use of the common electron translation factor proposed by Errea et al. (1982). Comparison is made with experiment in the energy range 0.25-25 keV amu-1.

Plasmon dispersion effects for surface plasmon mediated proton neutralization at Mg surfaces

Abstract Transition rates for surface plasmon-assisted electron capture by H+ ions interacting with Mg surfaces have been calculated. We show that the plasmon dispersion and the representation of the initial metal state significantly change the transition rates with respect to previous calculations. In particular, the ion perturbation on the initial state greatly increases the transition rates at large ion–surface distances which is consistent with the results obtained by other authors for the Auger process.

Model potential treatment of C4+ + H2 collisions at low impact energies

We present cross sections for electron capture in C4+ + H2 collisions in the energy range 50 eV amu-1≤E≤ 6 keV amu-1, calculated using a model potential aproximation to treat the interaction of the active electron with C4+ and H2+ cores and an equivalent-electron bielectronic interpretation of the probabilities. The limitations of this method are analysed by comparison with the two-electron ab initio treatment. Double-electron-capture cross sections are also calculated using ab initio and model potential treatments. We propose a modified independent particle model to evaluate single-electron-capture cross sections.

Asymptotically Optimal Class of Translation Factors

This letter reports a comparison between two common translation factors (CTF) which have been claimed to be asymptotically optimal, and thus well suited for the calculation of partial cross-sections, alignment parameters, etc. As a quality criterion for the comparison, we employ the Euclidean and partial norms of the couplings between the states included and those discarded in the molecular expansion. Our main conclusion is that one can define a class of asymptotically optimal CTFs, whose properties are indicated. The actual form used in a calculation may then be chosen according to the numerical techniques available.