C J Noble

Semiclassical calculations of charge exchange and excitation in Na+-Li and Li+-Na collisions using atomic-orbital expansions

Charge transfer and excitation cross sections are calculated for Na++Li to or from Na+Li+ collisions. Large bases consisting of travelling atomic orbitals on both centres are employed for a range of projectile velocities 0.1<or= nu <or=1.4 (au). For low velocities ( nu <0.25) good agreement is found with the recent molecular-orbital calculations of Allan and Hanssen (1985). For velocities lying beyond 0.25 the authors predict lower cross sections, partly because a common translational factor was used in the molecular-orbital calculations. The authors also find good qualitative agreement with the experimental data of Daley and Perel (1969), which shows the oscillatory structure to be present in the velocity dependence of the charge transfer.

Electron capture by fully stripped ions of helium, lithium beryllium and boron from atomic hydrogen

A two-state approximation based on atomic wavefunctions is used to calculate cross sections for electron capture by He2+, Li3+, Be4+ and B5+ from atomic hydrogen in the ground state. The velocity range covered is from v=0.44 to v=2.8 au which corresponds to a laboratory energy range of from 5 to 200 keV amu-1. Reasonable agreement is obtained with the experimental data for He2+, Li3+ and B5+.

Positron scattering by singly charged helium ions in the ground state

A coupled channel approximation has been employed to calculate cross sections for positron collisions with He+(1s) in the energy ranges 0-40.8, 40.8-47 and 50-250 eV. In the lowest energy interval only elastic scattering is possible whereas in the intermediate energy interval both elastic and inelastic cross sections are calculated. Above the capture threshold total cross sections have been calculated for the excitation of He+, for the formation of positronium and for ionization. The target basis set includes the n = 1 and 2 eigenstates on the He+ and Ps centres together with up to 23 pseudostates centred on the He+. The coupled equations were solved using a new minimum-norm extension of the least-squares method. The new approach provides accurate solutions to even large coupled sets of scattering equations. Where possible the results of the present calculations have been compared with previous investigations.

Theoretical studies of the interaction of He2+ with H(1s) and H+ with He+

Cross sections calculated using an atomic basis are presented for the charge exchange reaction He2++H(1s) to He+(nl)+H+ for states with n<or=3. Excitation and ionisation cross sections for He2+ impact on atomic hydrogen are calculated within a model using expansions into both atomic states and pseudostates. A two-centre expansion using the same basis is applied to the reaction H++He+(1s) to H+He2+ and results are obtained in harmony with experiment and with calculations using alternative basis sets.

Charge transfer in H+ + He' and He2+ + H collisions

Charge transfer and excitation cross sections for H++He+ and He2++H collisions have been calculated for centre of mass energies in the range 2 to 200 keV, using a two-centre atomic expansion model in which 1s, 2s and 2p orbitals are retained about each nucleus. For the He2++H reaction close agreement is found with experiment up to about 60 keV, but for the inverse reaction, H++He+, agreement with experiment is found only at energies below about 15 keV. A comparison with the results of previous atomic and molecule expansion models is made.

Charge transfer in H+-Na0 collisions: atomic orbital calculations

Cross sections have been computed for the charge transfer processes H++Na(3s) to H(nl)+Na+, with n=1, n=2 and n=3, and for H(1s)+Na+ to H++Na(3l), with l=0, l=1 and l=2. the laboratory energy range studied was 0.5<or=E<or=20 keV amu-1. A two-centre expansion in travelling atomic orbitals was employed. Comparison is made, where possible, with other calculations and with experiment. For H+-Na collisions, good agreement is found over the entire energy range with recent atomic orbital calculations by Fritsch (1984) and with the earlier measurements of Anderson et al. (1979). Agreement with molecular orbital calculations reported by Allan (1986) in an accompanying paper is also good for energies E<5 keV amu-1. The computed values of the Na 3s to 3p excitation cross section are given.

Charge transfer in Li3++H collisions

Charge transfer and excitation cross sections for Li3++H collisions have been calculated for laboratory kinetic energies in the range 1.4 to 200 keV amu-1, using a multi-state atomic expansion model. A comparison with the results of experiments and of previous atomic and molecular expansion models is made.

On the reduction of momentum space scattering equations to Fredholm form

In a momentum representation the close-coupling equations for atomic and molecular scattering problems are coupled singular integral equations. Various methods for removing the singularities from the corresponding Lippmann-Schwinger equations in nuclear physics have been proposed by Sloan (1965), Haftel and Tabakin (1970), Kowalski (1965) and Noyes (1968), and by Partovi (1965). Some of these techniques have not previously been tested for solving atomic scattering calculations. Differences between the methods are studied numerically by considering positron scattering by hydrogen atoms.

Charge exchange between Cs+ ions and related studies

The two-state approximation in the impact parameter formalism, using atomic basis functions with plane-wave translational factors, is used to compute cross sections for the charge exchange reaction Cs++Cs+ to Cs+Cs2+ for energies from 300 keV to 5 MeV. To test certain aspects of the model, calculations are presented for the reactions Li++Li+ to Li+Li2+, H++He to H+He+ and H++Li+ to H+Li2+, in the intermediate-energy regions.

The distorted-wave and coupled-channel approaches to the excitation of atomic hydrogen by proton impact

Single-centre coupled-channel models based on sufficiently large pseudo-state expansions are expected to describe the proton-hydrogen system accurately above those energies for which charge exchange is large. Neither the first-order, nor the second-order, distorted-wave approximations to such a model can provide total cross sections which are an improvement on those of the first Born approximation in the energy region from 50 to 200 keV.