A THEORETICAL STUDY OF POSITRON/ATOM AND POSITRON/MOLECULE SYSTEMS

A theoretical investigation of positron/atom and positron/molecule complexes is conducted using standard quantum mechanical techniques. This study addresses questions of stability and seeks to characterize these novel species. This work is divided into three major parts; the first is a Restricted Hartree-Fock (RHF) study for the first-row positron/anion systems {A(\u27-);e(\u27+)}, A = Li, Be, B, C, N, O and F. Both RHF wavefunctions and total energies are computed and it is found that all {A(\u27-);e(\u27+)} systems are stable with respect to A(\u27-) + e(\u27+) but not with respect to A + Ps. However, it is shown (using a cycle argument to provide estimates of electron-positron correlation) that the binding energy of the positron is -3.41, -3.50, -2.88, -1.39, -2.38, -0.62 and 1.61 eV for the systems {Li(\u27-);e(\u27+)}, {Be(\u27-);e(\u27+)}, {B(\u27-);e(\u27+)}, {C(\u27-);e(\u27+)}, {N(\u27-);e(\u27+)}, {O(\u27-);e(\u27+)} and {F(\u27-);e(\u27+)}, respectively. The corresponding positron/neutral-atom systems, {A;e(\u27+)} have been considered as well and all indications suggest that a neutral atom will not bind a positron at the RHF level. The second major part explores the viability of the Multiconfiguration Hartree-Fock (MCHF) method as a means of obtaining post Hartree-Fock results for positron/atoms and positron/molecules. Although accurate treatments of {H(\u27-);e(\u27+)} (e.g. using Hylleraas and Hylleraas-Configuration Interaction methods) have been reported, the application of these techniques to large atoms (Z \u3e 10) is very difficult. Attempts are made to achieve a \u27benchmark\u27 result for {H(\u27-);e(\u27+)}, a well known system, and the resulting wavefunctions are used to compute spin averaged two-gamma annihilation rates. The performance of the MCHF method was disappointing, capturing approximately 80% of the correlation energy and producing an annihilation rate equal to only one-third of the accurate result reported by Page and Fraser. Possible reasons for the poor showing and suggestions for improvement of the MCHF method are discussed. The study of positron/neutral molecule systems, {M;e(\u27+)}, M = CH(,3)F, CH(,2)F(,2), CHF(,3), CH(,3)Cl, CH(,2)Cl(,2), CHCl(,3), CH(,3)OH, CH(,2)FCl and CH(,3)CN, forms the third major section in the thesis. Positron binding in these neutral polar systems is explored and in particular the positron/dipole interaction. Both formal and numerical calculations have established that a molecule will bind an electron if its dipole moment exceeds the critical dipole moment, (mu)(,c) = 1.625 Debye and this result applies for a positron as well. . . . (Author\u27s abstract exceeds stipulated maximum length. Discontinued here with permission of author.) UM