Enhanced positron binding to molecules containing π bonds.

Observation of vibrational Feshbach resonances (VFR) in the annihilation spectra of positrons on molecules provides a direct measurement of the positron-molecule binding energy ε_B [1]. New annihilation measurements are presented for a series of ring hydrocarbons with different numbers of π bonds. The measurements show that adding π bonds significantly increases the positron binding energies of these molecules. This is similar to that seen in other molecules; however, for these molecules, the global molecular parameters usually considered (i.e., geometry, polarizability, and dipole moment) are approximately constant, and so the observed differences can be attributed to changes in the nature of the bonds. The molecular ionization potential (IP) is an exception: The inclusion of π bonds tends to decrease the IP, and this is well correlated with increases in ε_B. However, benzene does not follow this trend: both IP and ε_B are larger than for the other unsaturated six-carbon rings. These results will be discussed in relation to the importance of electron-positron correlation effects (such as virtual positronium formation) on ε_B [2].

[1] G. Gribakin, et. al., RMP (2010).

[2] J. Hofierka, et. al., arXiv (2021).