Associative ionization (AI): A + B* → AB+ + e− and dissociative recombination (DR) share the same reaction channels, although the precise dynamics depend on the actual collision taking place. The large angular momentum put into the system by heavy particle collisions does indeed obliterate the subtle role of closed Rydberg channels, as these resonances are displaced by the centrifugal energy. However, the systematic study of AI processes at low energy provides us with additional information on the reaction channels, their branching into atomic products, and the preferred symmetries of DR. The effect of isotope substitution on total cross sections, together with the development of a new diagnostic tool to measure the internal energy of AI products, give an unambiguous identification of the reaction pathways.

Measurements are presented and discussed that deal with associative ionisation in the collision of two hydrogen or deuterium atoms, one of which is excited in the 2s, 3s or 4s state. Total cross sections are given for collision energies ranging from 5 meV to 4 eV. Also reported here are some cross section measurements of associative ionisation in the collision of a negative ion (H-, D- or O-) with a positive one (D+, H-2(+), D-2(+) He+, C+ or O+).

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.

Sums of products of the Coulomb wavefunctions over degenerate manifolds have been obtained in a closed form. These sums appear in many atomic and molecular problems, The sums have been obtained making use of the properties of the Coulomb Green function G ((r) over right arrow, (r) over right arrow', E) in the limit E --> E-n where E-n is the eigenenergy of the hydrogen-like atomic ion. This paper develops the method described in our previous paper.

The cross sections of the detachment of one and two electrons during the collision of two negative ions H − + H − , H − + Cs − , and Cs − + Cs − are calculated in a wide range of collision energies: from the energy threshold to approximately 100 keV. In adiabatically slow collisions, the detachment of electrons occurs as a result of one-or two-electron Auger decays whose rates are calculated in the approximation of asymptotically large separations between ions. For high collision energies, the cross sections of the electron detachment are calculated by the method of close coupling of states. The calculated cross sections are in good agreement with the results of experimental measurements made for the H − + H − collision.

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.

In this paper we show, on a few examples, how the merging beam method is applied to measure absolute cross sections of low energy rearrangement processes such as resonant electron transfer, transfer ionisation and associative ionisation.

Sums of products of the Coulomb wave functions over degenerate manifolds have been obtained in a closed form. These sums appear in many atomic and molecular problems. The sums have been obtained making use of the properties of the Coulomb Green's function G(r, r', E), in the limit E --> E-n, where E-n, is the eigenenergy of the hydrogenlike atomic ion. The closed Hostler-Pratt form of G in the coordinate representation has been used. The sums calculated are a consequence of the n degeneracy of the Coulomb atomic energy levels. This itself, as is well known, follows from the four-dimensional symmetry of the Coulomb problem for the hydrogen atom.

Numerical evaluations of ion - hydrogen atom collision's ionization cross sections with the Classical Trajectory Monte Carlo (CTMC) method are shown here to be valid in the keV energy region. Careful numerical integration of the Coulomb Three-Body (CTB) system's (A(q+), p(+), e(-)) classical trajectories is correctly assessing the "saddle electron" dynamics in the 1 to 10 keV energy region. Resulting ionization cross sections are comparable to those recently obtained experimentally, although they are still lower for the higher energies. Consequently, the method has be extended to calculate low energy hydrogen - hydrogen collision ionization processes, which have been measured recently.

The cross section of single-electron capture in slow H- + A(3+) --> H + A(2+)(n) collisions is calculated in the one-electron approximation by using the close coupling approach. It is found that the change of the electronic configuration, that takes place at every avoided crossing of the ionic and covalent energy terms, is a source of nonadiabatic transitions in the system. This effect leads to a substantial increase of the total cross section and produces an effective repopulation of individual covalent states.

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.

This paper reports on the measurement of the absolute cross sections of the following associative ionization processes:H-2(+) + H- --> H-3(+) + eH-2(+) + D- --> H2D+ + eD-2(+) + H- + D2H+ + eD-2(+) + D- --> D-3(+) + e.Measurements were carried out using merged beams and chopping the negative beam to evaluate the background. The measurements cover the barycentric energy range from 0.07 to 10 eV. The four cross sections were, within experimental error, equal both in magnitude and energy dependence.

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 Green's functions are used to describe the weakly bound electron of . A satisfactory agreement is obtained with the experimental cross section.

Total cross-section calculations are reported for the electron detachment from H- in collision with multiply charged ions by using a simple two-state model. In the collision energy range where this model is expected to be realistic, results are in fair agreement with both very recent experimental and theoretical results.

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.

We describe a new experiment dealing with the multiphoton ionization of metastable atomic hydrogen H(2s). The ion yield has been measured in a relative way but it provides absolute photoionization rates. In the present contribution, the wavelength is tuned in the vicinity of the 2s-3p transition and the laser intensity is in the 10(7)-5 x 10(10) W cm(-2) range. Experimental photoionization rates and resonance width are found to be in agreement with theoretical predictions.

Basically, the associative ionisation process: $$A + B \to A{B^ + } + e$$ does not differ from dissociative recombination, the former process turning into the latter by a reversal of time.

The formation of H- in the collision of excited (3s) atomic hydrogen with helium and neon is investigated. The ratio (sigma3s/sigma2s) of the cross sections for electron capture on H(3s) and H(2s) is measured through an analysis of the kinetic energy spectrum of H-, which, in the case of helium, also allowed the measurement of the ratio (sigma2s/sigma1s). A resonant cavity is used, in the adiabatic rapid passage configuration, to increase the laser power used to excite the 3s state from the metastable state. An excitation yield as high as 90% is obtained. The measurements cover the range 300-3000 eV and confirm the important growth of the cross section ratio at the low energy side.