L. F. Errea

State-selective electron capture in collisions of ground and metastable O2+ ions with H(1s)

An ab initio calculation of the electron capture cross sections for collisions of ground and metastable states of O2+ with H(1s) is presented. For impact energies between 0.125 and 3.4 keV amu−1, we find good agreement between the cross sections from the ground state ion with the mixed beam experimental data of Phaneuf et al (Phaneuf A, Alvarez I, Meyer F W and Crandall D H 1982 Phys. Rev. A 26 1892).

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.

Implementation of the sudden approximation eikonal method in ion - diatom collisions

We describe an implementation of the vibro-rotational sudden approximation eikonal (SEIKON) method to treat collisions between ions (atoms) and diatomic molecules at impact energies such that a semiclassical eikonal approximation can be applied to treat the relative motion of the colliding species, and a close-coupling molecular expansion is able to accurately represent electronic transitions. Our method, which does not make use of Franck - Condon-type approximations, allows us to evaluate vibrationally resolved charge-transfer and excitation cross sections, as well as total cross sections for transitions to the vibrational continuum. The implementation is illustrated with calculations of cross sections for single-charge transfer, vibrational excitation, and transfer dissociation in , and DT collisions. The validity of the Franck - Condon approximation for these reactions is studied.

Classical calculation of ionization and electron-capture total cross sections in H{sup +}+H{sub 2}O collisions

We report total cross sections for single ionization and electron capture in H{sup +} collisions with water molecules at impact energies 25 keV<E<5 MeV. Calculations have been carried out by applying the independent-particle model and the eikonal-classical trajectory Monte Carlo (CTMC) method. We have also estimated fragmentation cross sections by multiplying the partial cross sections by the branching ratios measured in the photoionization experiments of Tan et al. [Chem. Phys. Lett. 29, 299 (1978)].

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.

Evaluation of molecular integrals in a basis of travelling orbitals

All integrals which arise when momentum-transfer phase factors are introduced in the molecular approach to atomic collisions can be evaluated analytically, provided that an atomic basis set of GTOs is used for the energy calculations. The authors present general closed-form formulae which are numerically stable, and can be coded directly for the automatic computation of the integrals. They expect their analysis to encourage workers to study explicitly the effect of momentum transfer in cross section calculations, even for complex systems.

Ab initio calculation of nonadiabatic couplings using MELD

We present a numerical differentiation technique for the calculation of directional derivatives of electronic wave functions evaluated using a multireference configuration interaction method. The technique has been implemented in the set of programs MELD, and its application is illustrated by calculations of nonadiabatic couplings for the ArH+2 quasimolecule.

Convergence study of He2++H and He++H+ charge exchange cross sections using a molecular approach with an optimised common translation factor

A convergence study of cross sections is reported for electron transfer in He2++H and He++H+ collisions, using a molecular approach with up to ten (1 s sigma , 2 p sigma , 2 s sigma , 2 p pi ; 3 d sigma , 3 d pi , 3 p sigma , 4 f sigma , 4 f pi , 5 g sigma ) basis functions and a common translation factor (CTF) which was optimised according to the norm criterion. The energy ranges investigated are 0.75-16 keV amu-1 (He2++H) and 5-80 keV amu-1 (He++H+). Conclusions are drawn from the excellent agreement between these results and those of considerably more involved calculations with plane-wave translation factors, and the persisting small discrepancy between theoretical and measured sigma (2sHe+) cross sections is discussed.

Classical and semi-classical treatments of Li3+, Ne10++H(1s) collisions

We perform molecular close-coupling and impact-parameter classical trajectory Monte Carlo calculations of total and partial cross sections for capture and ionization in collisions of highly charged ions on H(1s). We first consider Li3++H(1s) as a benchmark to ascertain the complementarity of the methods, and then Ne10++H(1s), which has been scarcely studied up to now, and has recently become of interest for fusion plasma research.

Molecular treatment of excitation and charge transfer in Be++H collisions

A 28-term molecular expansion is employed to calculate total and partial cross sections for charge exchange and excitation in Be++H collisions in the range of impact energies 0.1-25 keV amu-1, which are of predictive value.