Measurement of Vibrational Feshbach Resonances Mediated by Nondipole Positron-Molecule Interactions.*

Experiments have shown that low-energy (sub eV) annihilation spectra of positrons on molecules are typically dominated by relatively sharp features that have been identified as vibrational Feshbach resonances (VFR) involving fundamental modes. A theory by Gribakin and Lee,\footnote{Gribakin and Lee, Phys. Rev. Lett. {\bf 97}, 193201 (2006).} is successful in describing quantitatively annihilation spectra for several small molecules, where they are dominated by {\it dipole-active} fundamental modes. Presented here are measurements of positron-molecule annihilation using a recently developed cryogenic positron beam with significantly improved energy resolution.\footnote{Natisin, et. al., Phys. Rev. Lett. {\bf 119}, 113402 (2017).} Data for 1,2-trans-dichloroethylene and tetrachloroethylene show clear signatures of a resonance at the location of {\it nondipole-active} C-C stretch modes. The magnitudes of these resonances are consistent with a simple model that predicts resonances due to {\it quadrupole} coupling. This work provides evidence that positron-molecule bound states can be populated by {\it non-dipole} interactions. Further implications of this work will be discussed.