Resonances in Positron Annihilation on Molecules -- Which Bells Ring?

Positron collisions with molecules can result in the excitation of high-Q vibrational Feshbach resonances -- temporary positron-molecule bound states that exhibit greatly enhanced annihilation rates.\footnote{G. F. Gribakin, J. A. Young, and C. M. Surko, {\it Rev. Mod. Phys.} {\bf 82}, 2557 (2010).} A simple theory agrees well with data for annihilation spectra as a function of incident positron energy for selected molecules, such as methyl halides, in which infrared-active vibrations dipole-couple the incident positron to the bound state.\footnote{G. F. Gribakin and C. M. R. Lee, {\it Phys. Rev. Lett.} {\bf 97}, 193201 (2006).} However additional effects appear to be prominent in most molecules, including the excitation of combination and overtone modes.\footnote{A. C. L. Jones, J. R. Danielson, M. R. Natisin, C. M. Surko, and G. F. Gribakin, {\it Phys. Rev. Lett.} {\bf 108}, (2012).} Until now, limited energy resolution has inhibited the study of these effects. A recently developed, high-energy-resolution, cryogenic trap-based beam is used to investigate two other ways to ``ring the molecule's bells'': positron coupling to infrared-inactive modes and the excitation of combination modes. The operation of the new beam system will be briefly described,\footnote{M. R. Natisin, J. R. Danielson, and C. M. Surko, {\it Appl. Phys. Lett.} {\bf 108}, 024102 (2016).} followed by a discussion of high-resolution data for molecules that provide evidence of resonant annihilation due to infrared {\it inactive} modes. Data exploring the possible excitation of combination modes will also be discussed and related to broad and featureless regions of the annihilation spectra observed in many molecules.$^{3,5}$