Measured annihilation spectra are presented for aromatic and heterocyclic ring molecules resolved as a function of incident positron energy using a trap-based positron beam. Comparisons with the vibrational mode spectra yield positron-molecule binding energies. Ab initio many-body theory predictions, which take proper account of electron-positron correlations including virtual-positronium formation, are presented and are found to be in good to excellent agreement with the measured binding energies. The calculations elucidate the competition between permanent dipole moments and $\pi$ bonds in determining the spatial distribution of the bound-state positron density. The implications of these results and the role of multimode features in annihilation in these molecules, including Fermi resonances, are discussed.

• Received 6 March 2024
• Accepted 22 April 2024

DOI:https://doi.org/10.1103/PhysRevA.109.062801