High-resolution annihilation spectra, acquired with a cryogenic-trap-based positron beam, have revealed discrete resonances corresponding to vibrational excitations that extend significantly beyond features attributed to fundamental vibrations. This systematic analysis isolates and identifies these newly observed features as dipole-active (DA) combination and overtone vibrations. While vibrational Feshbach resonance (VFR) typically proceeds via single-quantum fundamental excitations, these combination and overtone features are shown to support distinct resonances. They exhibit dipole coupling and intramolecular vibrational redistribution (IVR) enhancement factors comparable to the fundamentals. Crucially, this robust IVR enhancement is secured by the absence of positron escape channels, a condition where the individual constituent mode energies are either below the positron binding energy or are symmetry forbidden. These findings provide direct evidence that multimode coupling plays a crucial role in the process of resonant positron annihilation.

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• Electron & positron scattering