Positron Binding and Annihilation in Benzene and Other Ring Hydrocarbons*

Positrons have been shown to bind to most molecules via the formation of vibtational Feshbach resonances (VFR) on fundamental vibrational modes. Positron interactions with benzene and other unsaturated ring hydrocarbons are expected to play an important role in many areas including chemistry, biology, and astrophysics. Here we discuss two related studies to explore the important role that molecular π bonds play in determining positron binding and annihilation in these molecules. First, the use of a cryogenic, trap-based beam, is used to measure the spectrum for benzene and several partially deuterated benzenes. Earlier measurements of the spectrum exhibited unusually broad components that made difficult the identification of the fundamental-mode resonances and raised questions about the resulting value of the positron binding energy, E_b. The new high-resolution experiments separate the broad spectrum into several features and allows for a more precise measurement of E_b [1]. The second study utilizes the room temperature, trap-based beam to measure the binding energy for a wide range of ring hydrocarbons with various numbers of π bonds, including several aromatic molecules such as benzene. In particular, the results explore the important role that the molecular π bonds play in determining the binding energy of these molecules [2]. These results will be compared to recent many-body calculations [3] which elucidate the importance of electron-positron correlation effects in determining the positron-molecule binding energy. These studies also elucidate the richness of the annihilation spectra in the ring molecules studied, and in particular, the observation of VFR features beyond the fundamental vibrational modes will be discussed.

 

This work was done in collaboration with S. Ghosh and C. M. Surko.

[1] S. Ghosh, J. R. Danielson and C. M. Surko, Phys. Rev. Lett. 129, 123401 (2022).

[2] J. R. Danielson, S. Ghosh and C. M. Surko, Phys. Rev. A 106, 032811 (2022).

[3] J. Hofierka, B. Cunningham, C. M. Rawlings, C. H. Patterson, and D. Green, Nature 606, 688 (2022)