R P McEachran

Relativistic Theory of H2

The Dirac equation for H2+ is reduced to a form similar to that of the Schrodinger equation with some perturbation terms. A first‐order perturbation calculation for the 1sσg state then reveals that these relativistic effects lower the electronic energy by a small amount of order 0.2α2 for most internuclear separations and thereby make the molecule slightly more stable. The formalism is readily generalized to the isoelectronic sequence of H2+, and the ratio of the relativistic correction to the nonrelativistic energy is found to be of order Z2α2. Consequently, the relativistic correction becomes progressively more important as Z increases.

Positron scattering from noble gases

The elastic scattering of positrons from noble gases has been formulated using a polarized-orbital approximation. In principle all multipole moments of the positron-atom interaction are included and the polarized orbital is calculated by a perturbed Hartree-Fock scheme. The scattered wave is readily obtained from a potential scattering problem and is used to calculate Zeff and the angular correlation as well as the scattering cross sections of interest. Numerical results are presented for positron-helium scattering in the elastic region.

Forward angle scattering effects in the measurement of total cross sections for positron scattering

Measurements of total scattering by positron impact have typically excluded a significant portion of the forward scattering angles of the differential cross section. This paper demonstrates the effect that this can have on measurements of the total cross section. We show that much of the apparent disagreement between experimental measurements of positron scattering from atoms and molecules may be explained by this excluded angular range. It is shown that this same effect may also lead to an anomalous energy dependence of some cross sections.

Elastic scattering of electrons from neon and argon

The authors continue their earlier study on the effects of polarisation and exchange in low-energy elastic scattering of electrons from noble gases. They present detailed results for argon and conclude that including only the dipole part of the polarisation potential and treating exchange exactly produces the most reliable results. The behaviour for neon is also very similar to that for helium. Values for phaseshifts, differential, total elastic and momentum transfer cross sections are given for neon and argon and compared with recent experimental and theoretical results.

Total, elastic, and inelastic cross sections for positron and electron collisions with tetrahydrofuran

We present total, elastic, and inelastic cross sections for positron and electron scattering from tetrahydrofuran (THF) in the energy range between 1 and 5000 eV. Total cross sections (TCS), positronium formation cross sections, the summed inelastic integral cross sections (ICS) for electronic excitations and direct ionization, as well as elastic differential cross sections (DCS) at selected incident energies, have been measured for positron collisions with THF. The positron beam used to carry out these experiments had an energy resolution in the range 40-100 meV (full-width at half-maximum). We also present TCS results for positron and electron scattering from THF computed within the independent atom model using the screening corrected additivity rule approach. In addition, we calculated positron-impact elastic DCS and the sum over all inelastic ICS (except rotations and vibrations). While our integral and differential positron cross sections are the first of their kind, we compare our TCS with previous literature values for this species. We also provide a comparison between positron and electron-impact cross sections, in order to uncover any differences or similarities in the scattering dynamics with these two different projectiles.

Positron scattering from argon

The authors have extended their work on the polarised-orbital approximation to the calculation of scattering and annihilation processes for the positron-argon system in the elastic-scattering region. They present results for the phaseshifts; the elastic, diffusion and annihilation cross sections; and the angular correlation of photons produced during annihilation. As in previous calculations the results are in very good agreement with various experimental measurements. The authors have also investigated the angular distribution of the scattered positron and the possibility of forming a bound state of the system. The model predicts that there is no bound state of the positron-argon system.

Positron scattering from krypton and xenon

The elastic scattering of positrons from noble gases has been formulated in a frozen-core version of the polarised-orbital approximation. This approximation has been applied to the scattering of positrons from krypton and xenon. The authors present results for phaseshifts and elastic, diffusion and annihilation cross sections in the low-energy (elastic scattering) region, as well as the angular correlation of the photons produced during annihilation. Good agreement is obtained between the elastic scattering cross section and recent experimental results for xenon below the positronium formation threshold. The agreement is less satisfactory in the case of krypton.

Elastic scattering of electrons from krypton and xenon

Results are given for the low-energy elastic scattering of electrons from krypton and xenon. The calculations were carried out by treating exchange exactly and including only the dipole part of the polarisation potential. Values are given for the phaseshifts, differential, total elastic and momentum transfer cross sections. Comparison are given with recent experimental and theoretical work for these processes.

Positron scattering from neon

The authors have used the polarised-orbital approximation for positron scattering from noble gases as formulated recently by McEachran et al. (1977) to calculate scattering and annihilation processes in neon. They present results for phaseshifts, elastic, diffusion and annihilation cross sections as well as the angular correlation of the photons produced during annihilation in the low-energy (elastic scattering) region. Very good agreement is obtained between these elastic scattering cross sections and recent experimental measurements.

Modelling single positron tracks in Ar

In this study, we present a complete set of positron interaction cross sections for scattering from Ar, for incident energies ranging from 0 to 10 keV. Experimental data have been critically reviewed from previous experiments performed at the Australian National University and University College London. Differential and integral cross sections, including the effect of positronium formation, have been calculated by using two different optical potential methods. The results of these calculations, in combination with experimental cross sections and experimental energy-loss spectra, have been established as input parameters for an event-by-event Monte Carlo simulation procedure to generate single positron tracks in argon. The reliability of this method to obtain energy deposition models at the nano-scale is also discussed.