Energy-resolved positron-molecule annihilation: Vibrational Feshbach resonances and bound states

This thesis describes a new technique for the measurement of positron annihilation rates for positrons incident on atoms and molecules. Positrons from a radioactive source are collected and cooled in a Penning-Malmberg trap and formed into a magnetically guided beam. The beam is then guided through a cell filled with the molecule to be studied while the gamma ray signal from annihilation events is recorded. As distinguished from previous work, these experiments allow tuning of the positron energy over the range between 50 meV and the threshold for positronium formation. The energy resolution of the beam is 25 meV. A key result in these measurements is the discovery of resonances in the annihilation rates for selected molecules, associated with the vibrations of the target. We attribute these resonances (in accordance with a model proposed by Gribakin [28]) to the presence of a Feshbach resonant state consisting of a positron bound to a vibrationally excited target molecule. The annihilation spectra for a number of molecules are presented including alkane molecules, fluoroalkanes, deuterated alkanes, ring hydrocarbons, noble gases, alkenes and alkynes. The goals of these measurements are to investigate the dependence of these resonant processes on the size, composition and shape of the target molecules, and to provide benchmarks for theoretical descriptions of this process.