Resonant structures in the positron annihilation spectra of small molecules

Energy-resolved measurements of positron-molecule annihilation show that positrons can attach to molecules via vibrational Feshbach resonances (VFR) [1]. Furthermore, thanks to a recent theory by Gribakin and Lee, it is possible to quantitatively predict the positron-molecule annihilation spectra for small molecules in which all the vibrational modes are infrared-active [2]. In this paper, we examine the annihilation spectra for a variety of small molecules and relate them to this theory. We find that fundamental-mode \emph{and} multi-mode VFR are necessary to explain the spectra of molecules such as methanol. We also present results for molecules such as water and CO$_2$ that do not have clearly identifiable VFR but appear to have structure to their spectra. \newline ~[1] L. D. Barnes, J. A. Young, and C. M. Surko, \textit{Phys. Rev. A} {\bf 74}, 012706 (2006). \newline ~[2] G. F. Gribakin and C. M. R. Lee, \textit{Phys. Rev. Lett.} {\bf 97}, 193201 (2006).