Vibrational Feshbach Resonances Mediated by Nondipole Positron-Molecule Interactions

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