Ann A Kernoghan

Positron scattering by atomic hydrogen

Cross sections for positron scattering by atomic hydrogen are calculated in the energy range 0 - 110 eV using a 33-state approximation which employs the 1s, 2s and 2p eigenstates of both positronium and hydrogen together with 27 hydrogen pseudostates. Cross sections are given for elastic scattering, H(2s) and H(2p) excitation, Ps(1s), Ps(2s), Ps(2p) and total positronium formation, ionization, and total scattering. The degree of agreement with an earlier 18-state calculation of Kernoghan et al, with sophisticated single-centre approximations, and with the available experimental data, strongly suggests that the main cross sections for positron scattering off ground-state atomic hydrogen are now known to quite a high degree of accuracy. Problems with the total cross section highlighted by Kernoghan et al are now resolved by new experimental data of Stein et al which give excellent agreement with theory for both positron and electron scattering. Suggestions are made for further experimental investigations.

Positron collisions with one- and two-electron atoms

Abstract The current status of coupled-state results for positron scattering by atomic hydrogen and the alkali metals is briefly reviewed. The extension of the coupled-state approximation to “two-electron” targets, i.e., He and the alkaline earths, is outlined and new calculations for positron scattering by He are presented.

An 18-state calculation of positron-hydrogen scattering

We report results for positron scattering by ground-state atomic hydrogen in the energy range 0 to 80 eV. The calculations have been performed in an 18-state Ps(1s, 2s, 3s, 4s, 2p, 3p, 4p, 3d, 4d)+H(1s, 2s, 3s, 4s, 2p, 3p, 4p, 3d, 4d) approximation where the pseudostates (denoted by a bar) have been taken from Fon et al. (1981). Cross sections are presented for elastic scattering, positronium formation, total scattering and ionization. The elastic scattering results are in good agreement with accurate variational numbers at low energies and with other sophisticated, but very different, theoretical approximations at higher energies. We estimate that the elastic cross section is now known to better than 10%. The cross section for positronium formation is dominated by capture into the 1s state and is in fairly good agreement with the measurements of Weber et al. (1994). The results for the total cross section are generally consistent with the upper and lower bounds of Zhou et al. (1994) but are a little larger than other theoretical estimates at the higher energies. There is also a general theoretical problem concerning the degree to which the total cross sections for electrons and positrons merge at energies above 31 eV. The calculated ionization cross section is in agreement with the measurements of Jones et al. (1993).

Cross-channel coupling in positron-atom scattering

AbstractCoupled-state calculations including positronium channels are reported for positron scattering by atomic hydrogen, lithium and sodium. Integrated cross sections and total cross sections are presented for all three atoms. For lithium differential cross sections are also given. Throughout, comparison is made between results calculated with and without inclusion of the positronium channels. S-wave cross sections for positron scattering by atomic hydrogen in the Ps(1s, 2s, 2p)+H(1s, 2s, 2p) approximation show the high energy resonance first observed by Higgins and Burke in the coupled-static approximation. This resonance has now moved up to 51.05 eV and narrowed in width to 2.92 eV. Other pronounced structure is seen in the S-wave cross sections between 10 and 20 eV; it is tentatively suggested that this structure may be due to the formation of a temporary pseudo-molecular collision complex. Results calculated in the Ps(1s, 2s,n

Positron scattering by potassium

overline {3s} ,overline {4s}

Positron Collisions with Atoms

n, 2p,n

Resonances above the ionization threshold in positron-hydrogen scattering

overline {3p} ,overline {4p} ,overline {3d} ,overline {4d}