Vibrational Feshbach Resonance Mechanism for Positron Annihilation on Molecules

Thanks to a concerted experimental [1,2] and theoretical [3,4] effort, the hypothesis of the role of molecular vibrational Feshbach resonances in providing enhanced positron annihilation rates in molecules [5], has largely been confirmed and accepted. Recent experiments have provided a wealth of information on the two crucial aspects of the resonant annihilation mechanism, namely the positron-molecule binding, and its coupling to the vibrational degrees of freedom. In small molecules with simple vibrational spectra of infrared-active modes (e.g., methyl halides), resonant annihilation is described remarkably well by a theory [4] which contains only one free parameter, the positron binding energy. Application of this theory to other molecules highlights the role of overtones and combinations (methanol), as well as infrared-inactive vibrational excitations (acetylene, ethylene, etc.). In this talk I will review current theoretical understanding of positron-molecule resonant annihilation and discuss some outstanding questions for future research. \begin{enumerate}\setlength{\itemsep}{-3pt}\setlength{\itemindent}{-12pt} \item S. J. Gilbert, L. D. Barnes, J. P. Sullivan, and C. M. Surko, Phys. Rev. Lett. {\bf 88}, 043201 (2002); L. D. Barnes, S. J. Gilbert, and C. M. Surko, Phys. Rev. A {\bf 67}, 032706 (2003). \item L. D. Barnes, J. A. Young and C. M. Surko, Phys. Rev. A {\bf 74}, 012706 (2006). \item G. F. Gribakin and P. M. W. Gill, Nucl. Instrum. and Methods B {\bf 221}, 30 (2004). \item G. F. Gribakin and C. M. R. Lee, Phys. Rev. Lett. {\bf 97}, 193201 (2006). \item G. F. Gribakin, Phys. Rev. A {\bf 61}, 022720 (2000). \end{enumerate}