F. Brouillard

Experimental-study of the Mutual Neutralization of H+ and H- Between 5-ev and 2000-ev

The cross section for the mutual neutralisation process: H++H- to H+H has been measured over the energy range 5-2000 eV, using merged beams and coincident detection of the products. The results obtained differ significantly from those published by other investigators. In particular, the energy dependence is free of structures and similar to that predicted by theory. The cross section is also in excellent agreement with recent calculations.

One-electron charge exchange between and positive multiply charged ions. 1. Collisions

The cross section for one-electron transfer is calculated in this paper. The collisional system is treated as a three-electron one and matrix elements for the formation of excited helium in both singlet and triplet states (1S, , P and , P, D) are obtained. Close-coupling calculations were done for the 27 (1S, , P and , P, D) final states covering the range 1 - of the relative collision velocity. An approximation is used for the effective potential of and Coulomb Greens functions are used to describe the weakly bound electron of . A satisfactory agreement is obtained with the experimental cross section.

Transfer ionization in slow H++H- collisions

The transfer ionization reaction H-a(-) + H-b(+) = H-a(+) + H-b(1s) + e, on which we had previously carried out experiments and calculations, is reconsidered here at higher collision energies and interpreted as ionization of weakly bound electron of the H- ion, accompanied by a simultaneous resonant exchange of the 1s core electron. The ionization of H- is treated as being strongly coupled to the dominant mutual neutralization channels H-a(-) + H-b(+) = H-a (1s) + H-b (nlm), and the cross sections for all relevant reaction channels are calculated by using the molecular-orbital close-coupling scheme.

Improvement of the merged-beam method and its application to the study of the reaction: H-+He+->H+He

It is shown that two parallel ion beams react at a rate that is independent of their density profiles when made to oscillate against each other in a two-dimensional scanning motion. An experimental set-up that makes use of this principle is described. Absolute cross sections obtained in this way are in good agreement with those obtained with the beams merging in the usual (static) mode. Cross sections for single-charge transfer between H-(D-) and He+ in the energy range 5-4000 eV are presented and compared to other existing data.

Animated Crossed Beams for the Measurement of Electron Impact Ionisation

Measurements of electron impact ionisation of highly charged ions are to be undertaken using the so-called animated crossed beams method. An account is given of the capability of this method and of some results obtained with singly charged ions.

The Formation of H+ From H- Ions By Electron-impact

Cross sections of the reaction H-+e to H++3e have been measured for interaction energies between 17.8 and 1000 eV. Comparison with the only other measurement shows present results to be much smaller. The discrepancy ranges from a factor four at low energies to a factor 12 at high energies. The influence of positive ion trapping in the electron beam has been carefully investigated in the course of this study as it could explain the large discrepancy.

Electron capture and excitation in slow H+ + He*(n) collisions

The electron-capture and excitation processes in slow collisions of protons with He*(1s3l) are studied using the close-coupling method within the semiclassical approximation. The Stark splitting of electron-capture states on H is explicitly taken into account and the coupling-matrix elements between these states and the initial angular-momentum states on He are calculated analytically. The cross sections for excitation (de-excitation) and single-electron capture to specific spherical hydrogen states have been calculated in the relative velocity range 2 x 10(6)-1.3 x 10(8) cm s(-1). The cross section values for both types of processes in the considered velocity range are found to be large (10(-14)-10(-13) cm(2)) due to the large values of electron-exchange couplings at large internuclear distances. The excitation (de-excitation) processes are controlled by two-step exchange (capture and re-capture) transitions rather than by direct coupling among the states centred on He.

Transfer ionization in He2+-H- collisions: measurement of the exothermicity and theoretical interpretation

The energy imparted to the ions in the transfer ionization reaction He2+ + H- --> He+ + H+ + e has been measured in a merged beam experiment using two different techniques (magnetic analysis and time of flight). The result obtained in both cases is of the order of 6 eV and confirms that the transfer ionization proceeds in two steps: a single electron transfer at large internuclear separation followed by a resonant Penning ionization at small distances. With this view, a simple calculation of the cross section is presented, with results that are in good agreement with the available experimental data.

Electron capture for and into excited states

A survey is presented of some achievements and problems pertaining to electron capture involving excited states in the incident or exit channel. The focus is on hydrogen. Principal experimental techniques are mentioned with their limitations. Results for the simplest processes are examined in an attempt to assess the reliability of present basic theory.

Associative ionization in the collision of H(1s) with H(3s) and D(1s) with D(3s)

The absolute cross section for associative ionization in H(1s) + H(3s) and D(1s) + D(3s) collisions has been measured, in the energy range 0.006-3.6 eV, in a merged beam apparatus. The 3s state is populated by exciting metastable atoms, with CW laser radiation, in a static field. For both isotopes, the cross section exhibits an E-1 behaviour at low energy, and a faster decrease above the ionization threshold of the 3s state. In the intermediate energy region, isotope-dependent oscillations of the cross section are observed, which we attribute to interferences between several reaction pathways.