New resonant features observed in the positron annihilation spectrum of aromatic molecules.

Energy resolved positron annihilation spectra for molecules are typically dominated by relatively sharp features that have been identified as vibrational Feshbach resonances (VFR) involving fundamental modes. Recently, new resonances beyond the fundamental modes has been observed in some ring and chain hydrocarbons [1]. Here, we present the annihilation spectrum for benzene (C₆H₆), fully deuterated benzene (C₆D₆), and three partially deuterated benzenes (C₆H₅D, C₆H₃D₃, C₆HD₅) using a cryogenic positron beam (FWHM ~ 22 meV) [2]. The narrow-energy beam allows for the resolution of new features in the spectra. Several resonances beyond the fundamental modes are observed, and their identification is confirmed by measuring the shifts in these resonances in the partially deuterated molecules. The existence of these resonances confirms that the physics of VFR is not limited only to coupling to fundamental modes. Possible physical interpretations of these resonant features and open questions related to their energies and magnitudes will be discussed.

[1] S. Ghosh, et al., Phys. Rev. Lett. 125, 173401 (2020).

[2] M. Natisin, et al., Appl. Phys. Lett. 108, 024102 (2016).