Role of multimode excitations and inelastic escape channels in positron-molecule attachment

Experiments have shown that positrons, like electrons, can attach a variety of molecules via vibrational Feshbach resonances (VFR). Because of the positron's unique annihilation channel, one can probe particularly short-lived resonances by examining features in the positron energy-resolved annihilation spectra. We discuss here energy-resolved annihilation spectra for a variety of molecules and relate these results to theory [1]. In particular, we examine the role of multi-mode excitations, which are found to produce VFR in a number of small molecules [2] and may participate in the enhancement of single-mode resonances in large molecules [3]. Also discussed is how fluorine substitution in a large hydrocarbons produces a strong inelastic escape channel. This, in turn, limits resonance lifetimes and decreases the magnitudes of the annihilation resonances above the inelastic threshold [3]. \\ ~[1] G. F. Gribakin \& C. M. R. Lee, \textit{Phys. Rev. Lett.}, \textbf{97}, 193201 (2006). \\ ~[2] J. A. Young, C. M. Surko, G. F. Gribakin, \& C. M. R. Lee, \textit{Phys. Rev. A}, in press (2008). \\ ~[3] J. A. Young \& C. M. Surko, \textit{Phys. Rev. A}, \textbf{77}, 052704 (2008).