Resonances in Positron-molecule Interactions

The development of cold, trap-based beams has enabled high-resolution, energy-resolved studies of positron scattering and annihilation processes [1]. This talk focuses on three topics in this area. For hydrocarbon molecules such as alkanes (C$_n$H$_{2n+2}$), giant enhancements in annihilation rates are observed due to vibrational Feshbach resonances. The dependence of the rates on positron energy provides evidence that positrons bind to these molecules and a measure of the binding energies [1]. Recent results include evidence for a second, ``positronically excited'' bound state and new data for the methane series, CH$_3$X, where X is a halogen. Other ``resonance-like features'' are sharp increases in the near-threshold electronic excitation cross sections for CO and N$_2$ [2], and in the vibrational excitation cross sections for CO, CO$_2$ and CH$_4$ [3, 4]. Outstanding questions and the relationship of these observations to available theoretical predictions will be discussed.\\ 1. C. M. Surko, G. F. Gribakin, and S. J. Buckman, \textit{J. Phys. B} \textbf{38}, R57 (2005).\\ 2. J. P. Marler and C. M. Surko, \textit{Phys. Rev. A} \textbf{72}, 062713 (2005).\\ 3. J. P. Marler and C. M. Surko, \textit{Phys. Rev. A } \textbf{72}, 062702 (2005).\\ 4. J. P. Marler, G. F. Gribakin and C. M. Surko, \textit{Nuclear Instrum. and Meth. B}, in press (2006).