Resonant positron annihilation in pyridine and other aromatic molecules.

Positron-molecule annihilation below the positronium formation threshold is often mediated by the capture of positrons through vibrational Feshbach resonances (VFRs) associated mostly with the fundamental modes. This leads to strong enhancement of the probability of annihilation [1]. The presence of VFRs yields a direct measure of the positron molecule binding energy. This has been reported in several studies, including recent investigations of benzene and its derivatives [2]. Pyridine (C$_5$H$_5$N) and furan (C$_4$H$_4$O) are two examples of heterocyclic aromatic molecules known for their role in medicinal chemistry and pharmaceutical products. In this work, we report new measurements of positron annihilation spectra and binding energy for these molecules, investigated using a high-resolution, trap-based positron beam. The observed features are compared with those reported for benzene and chain hydrocarbons. The dependence on molecular properties such as number of $pi$-bonds, polarizability, and dipole moment is discussed [3]. Outstanding questions are also discussed. [1] G. F. Gribarkin et al., Rev. Mod. Phys. 82, 2557 (2010). [2] S. Ghosh, et. al., Phys. Rev. Lett. 129, 123401 (2022). [3] J. R. Danielson et al., Phys. Rev. A. 106, 032811 (2022).