Positron annihilation and binding in aromatic and other ring molecules

Positron binding energies and resonant annihilation spectra are presented for aromatic and ring-substituted molecules using a trap-based positron beam. The results are compared to the predictions of an ab initio many-body theory [1]. Good-to-excellent agreement is obtained between the measured binding energies and the theoretical predictions which take into account electron-positron correlations, including virtual-positronium formation [2]. Positron binding has been shown to depend on molecular parameters such as polarizability α, permanent dipole mo-ment μ, and the number of π bonds N_π [3-4]. The calculations highlight the competition between μ and N_π in determining the spatial distribution of the bound-state positron density. The occurence of multimode features in annihilation (which do not appear to correspond to fundamental modes), including Fermi resonances, will be discussed.