Interaction of Low-Energy Positrons with Atoms and Molecules

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The laws of physics indicate that for every matter particle there is a "twin" particle of antimatter. When an electron and its antiparticle (the positron) collide, they can convert into two photons, each carrying off the particle rest energy. The universe contains relatively little antimatter, and this asymmetry is currently not understood. Nevertheless, positrons play an important role in many areas of science and technology, including atomic and plasma physics, astrophysics, materials science, biology and medicine. An important medical application is Positron Emission Tomography (PET) used to image internal organs and measure metabolic activity. In many of these contexts, positron interactions with molecules are significant. This project studies how positrons bind (or "stick") to most molecules, albeit for short times, less than about a billionth of a second. Even though transient, these bound states can have important consequences for medical effects and imaging technologies. This project seeks to understand the nature of positron binding and its potential implications for the fate of positrons in biology and material systems, and also in settings of astrophysical relevance. The Principal Investigator and collaborators have recently developed a new cryogenic, trap-based positron beam with energy resolution of 7 meV FWHM resolution, a factor of 5 better than the previous state of the art. This device will be used to study the interaction of low-energy positrons with molecules with unprecedented precision. Of interest is the excitation of vibrational Feshbach resonances in two-body, positron-molecule collisions; a process in which a positron excites a vibrational mode and becomes trapped on the molecule. While positrons bind to atoms, there are no low-lying excitations to absorb excess energy, and so such resonances cannot occur in two-body collisions in the same manner as for molecules. One objective of the project is to conduct new experiments to gain an understanding of the greatly enhanced molecular annihilation rates as a function of incident positron energy. While a successful theory exists for the excitation of dipole-allowed fundamental resonances, multi-mode effects appear to be important. Their role in annihilation spectra is less well studied to date and less well understood theoretically. The second objective is to make more precise measurements of positron-molecule binding energies. Prediction of these binding energies presents a considerable challenge to theory, particularly in treating accurately electron-positron correlation effects. The new high-resolution beam is expected to provide more precise measurements of binding energies and hence more precise tests of theory.