Jeremy S. Savage

Complete Solution of Electronic Excitation and Ionization in Electron-Hydrogen Molecule Scattering

The convergent close-coupling method has been used to solve the electron-hydrogen molecule scattering problem in the fixed-nuclei approximation. Excellent agreement with experiment is found for the grand total, elastic, electronic-excitation, and total ionization cross sections from the very low to the very high energies. This shows that for the electronic degrees of freedom the method provides a complete treatment of electron scattering on molecules as it does for atoms.

Electron scattering from molecular hydrogen in a spheroidal convergent close-coupling formalism

Electron scattering from molecules is a fundamental interaction of matter and is the mechanism behind many chemical reactions. In this work we rework the ab initio Convergent Close-Coupling scattering theory into prolate spheroidal coordinates-a natural system for diatomic molecules-to present total, differential, and ionisation cross sections of electron-H2 collisions.

Positron scattering on atoms and molecules

An overview is given of recent progress in the calculation of positron scattering on atoms and molecules using the convergent close-coupling method. Particular emphasis is given to those cases where positronium formation is one of the reaction channels, as well as the importance of demonstrating convergence with increasing orbital angular momentum of the bases used. Targets considered are atomic hydrogen, lithium, and molecular hydrogen. The last two decades have seen extraordinary progress in the field of electron, positron and photon scattering on atoms and ions. The problems of electron and photon scattering on atoms are very closely related. In the typical case of single photon absorption, the interaction proceeds by the resulting photo-electron scattering on the residual ion. For example, photon-helium scattering is essentially electron scattering on the singly charged helium ion. Positron-atom scattering is a little more interesting due to the possibility of positronium (Ps) formation. This is a rearrangement collision that considerably increases the complexity of the problem. Though it has been often claimed that positron-atom scattering is simpler than the corresponding electron-scattering problem due to the absence of exchange, in practice the introduction of the Ps-formation channel creates considerably more significant challenges. Historically, computational approaches to the problems have been subdivided into the low-, intermediate- and high-energy regimes. In addition, excitation and ionisation processes have also received different treatments. However, our interest in developing the convergent close-coupling (CCC) method has been to unify the approach to all such problems to be valid for the three projectiles across all energies and for the major excitation and ionisation processes. In developing the CCC method for excitation we took note of the techniques used specifically in their regimes of validity. At low energies the R-matrix close-coupling approach (1) has yielded outstanding results. At the higher energies the perturbative approach (2) has been particularly successful. The CCC method (3) combines the two techniques because it formulates close- coupling as coupled Lippmann-Schwinger equations in momentum space, which may be readily expanded in a perturbative series. Furthermore, the coupled equations may be solved in a distorted-wave formalism (4). However, unlike distorted-wave approximations the CCC results are independent of the choice of the distorting potential. In this sense the usage of such a potential is solely for numerical ease of solution. Following the pioneering implementation of the close-coupling method to ionisation processes (5), we developed an even simpler CCC approach (6). Rather than reconstructing the total wavefunction of the electron-atom system, we associated ionisation amplitudes with excitation of the positive-energy pseudostates. In other words, we extracted the required

Calculations of electron and positron scattering from vibrationally excited H2+ and H2

Electron and positron scattering from the vibrationally excited H2+ and H2 molecules were investigated using the adiabatic-nuclei convergent close-coupling method. Converged results are presented for a range of vibrationally excited states.

Convergent close coupling calculations for positron-magnesium scattering

While a two-centre convergent close-coupling approach to positron-magnesium scattering is developed, a single-centre method has been used to calculate total cross sections up to incident energies of 100 eV. The results agree very well with the measurements of Stein et al. [1] for positron energies above the ionisation threshold (7.6 eV). Similar accuracy is expected for energies below the positronium formation threshold (0.8 eV) where presently there are no experimental data to compare to. In this energy region we find a large p-wave resonance at 0.17 eV. Similar resonance behaviour was found in calculations by Mitroy and Bromley [2] at an energy of 0.1 eV.