  Measurements of energy-resolved positron-molecule annihilation show the
existence of positron binding and vibrational Feshbach resonances. The existing
theory describes this phenomenon successfully for the case of infrared-active
vibrational modes which allow dipole coupling between the incident positron and
the vibrational motion. Presented here are measurements of positron-molecule
annihilation made using a recently developed cryogenic positron beam capable of
significantly improved energy resolution. The results provide evidence of
resonances associated with infrared-inactive vibrational modes, indicating that
positron-molecule bound states may be populated by nondipole interactions. The
anticipated ingredients for a theoretical description of such interactions are
discussed.
