Resonant positron annihilation in the small molecule limit*

Energy-resolved measurements of positron-on-molecule annihilation have established the existence of vibrational Feshbach resonances (VFR) in alkanes and other large molecules [1,2]. Large annihilation rates occur whenever the incident positron energy is close to a vibrational mode energy minus the binding energy. Recently, Gribakin and Lee developed a quantitative model which successfully describes this process in halogen substituted methanes [3]. In this paper, we further examine VFR for small molecules. Using a cold positron beam from a Penning-Malmberg trap, we measured the energy resolved annihilation spectra of CD$_3$Cl, methanol, H$_2$O, and CO$_2$ and compared them to the predictions of the model. The presence or absence of resonances in these molecules is also discussed. CD$_3$Cl is compared to previous measurements of CH$_3$Cl. Since both should have identical binding, this provides a stringent test of the model. \\ ~[1] L. D. Barnes, \textit{et al.}, \textit{Phys. Rev. A} \textbf{67}, 032706 (2003). \\ ~[2] L. D. Barnes, \textit{et al.}, \textit{Phys. Rev. A} \textbf{74}, 012706 (2006). \\ ~[3] G. F. Gribakin and C. M. R. Lee, \textit{Phys. Rev. Lett.} \textbf{97}, 193201 (2006).