| The advent of the buffer gas trapping technique and subsequent production of pulsed trap-based beams with an energy spread of 25 meV or better, tunable from 0.1 to 100 eV, have opened up new areas of study of low energy positron scattering. This thesis discusses the techniques for making absolute scattering measurements in a strong magnetic field using a trap-based beam. This procedure has resulted in the first state-selective vibrational and electronic excitation cross sections for positrons. This thesis reviews some of these data as well as presents new state-selective electronic excitation cross section measurements for the diatomic molecules, N2 and CO and state-selective vibrational excitation cross sections for CF4. A central topic of the dissertation is the adaptation of this technique to study the unique-to-positron process of positronium formation (i.e., the bound state of a positron and an electron). This process is important, for example, to applications of positron scattering and annihilation in material science and biophysics. The commercial importance of this process notwithstanding, positronium formation remains a serious challenge to incorporate theoretically, while experimentally studies of the process, even in simple targets, are in considerable disagreement regarding the magnitude and energy dependence of positronium formation cross sections. A detailed study of the ionization and positronium formation in noble gas atoms and some diatomic molecules is presented and compared to other experimental and theoretical work where available. |
