Theory of the Structures of Intermediates in Positronium Chemistry

Abstract

Different mechanisms for positron annihilation in a chemical medium are distinguished by different intermediates. All meas ured annihilation properties (lifetime, angular correlation, etc.) are in fact properties of that intermediate which exists just prior to annihilation. The structures of these inter mediates have been studied quantum mechanically as an adjunct to laboratory measurement of annihilation proper ties. The quantum mechanical studies involve calculating approximate wave functions for one-positron, many-electron systems. Some scattering states have been studied in this way for atoms, but all molecular studies so far focus on bound positrons or positronium atoms The questions of whether such bound states exist for particular molecules, and of the properties of those bound states which do exist, have been addressed in several studies. The most successful of these (1) permits one to examine positron/positronium -molecule interactions at a more fundamental level than was hitherto possible. P ositron/positronium (Ps) chemistry takes place somewhere between the beginning of the initial positron/Ps-reactant molecule encounter and annihilation. What happens in the intermediate region of the reaction coordinate is of great interest to the chemist. If we can understand this region at the level of positron/Ps interactions with individual molecules, we can map out the whole of positron/Ps chemistry. Our knowledge of this intermediate region is presently very hazy. The experimental literature contains many speculations about structures of intermediates. Frequently these speculations are quite sensible and are widely and uncritically accepted by the community, but are incorrect. A