Positron Annihilation on Molecules: Effects Beyond the Gribakin-Lee Model

Annihilation at positron energies in the range of the molecular vibrational modes is dominated by large-amplitude vibrational Feshbach resonances (VFR) in which the positron attaches to the molecule.\footnote{G. F. Gribakin, J. A. Young, and C. M. Surko, {\it Rev. Mod. Phys.} {\bf 82}, 2557 (2010). } In small molecules, there is a quantitative description of the annihilation rates, $Z_{\rm eff}$.\footnote{G. F. Gribakin and C. M. R. Lee, {\it Phys. Rev. Lett.} {\bf 97}, 193201 (2006).} This talk focuses on other resonant annihilation phenomena that are less well understood. A broad spectrum of enhanced annihilation will be described that is observed in the annihilation spectra of many, if not most molecules. This spectral component, for example, dominates the spectrum in small molecules with relatively large binding energies such as CCl$_4$ and CBr$_4$. The relationship of this seemingly ubiquitous feature to a model of statistically complete resonant, multi-mode annihilation\footnote{G. F. Gribakin and C. M. R. Lee, {\it Eur. Phys. J. D} {\bf 51}, 51 (2009).} will be discussed.