Interaction of Low-Energy Positrons with Atoms and Molecules

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Positrons, a form of antimatter, are the antiparticles of electrons. While our world is almost solely composed of matter, it is not understood why. Positrons (antiparticles) can be created using a variety of techniques, however, and they are important in a number of applications. One example is in medicine and drug design, where positron emission tomography (PET) is used to study metabolic processes. Positrons are also used to study and develop new materials, to form antihydrogen (stable neutral antimatter), and to understand a variety of astrophysical processes. This research seeks to establish a fundamental understanding of the interaction of positrons with ordinary matter, particularly in the form of molecules, which can be used to further these applications and to develop new ones. The project is also an excellent training ground for scientific and technical personnel; it involves student and post-doctoral researchers at all levels, from the planning of experiments to the dissemination of research results. This project focuses on studies of the interaction of low-energy positrons with atoms and molecules. Of interest is the excitation of vibrational Feshbach resonances, a process in which a positron, in collision with a molecule, excites a vibrational mode and becomes trapped on the molecule. A particular focus of the research is the understanding of the observed enhancements in annihilation rates as a function of incident positron energy and using these spectra to determine positron-molecule binding energies. Fundamental aspects of atomic physics involved in this research include understanding electron-positron correlations, positron binding to atoms and molecules, and the nature of the resonances in positron annihilation resulting from positron attachment. A new cold beam will allow studies of the role of vibrational-mode symmetry in determining the strengths of the annihilation resonances. Other research goals include understanding quantitatively the annihilation of positrons on molecules when intramolecular vibrational energy redistribution (IVR) is operative. These positron-IVR studies could potentially provide new insights into molecular dynamics, complementary to studies now done by the chemistry and biology communities using optical techniques.