Resonant positron annihilation in benzene and deuterated isotopologues.

Positrons bind to most molecules through vibrational Feshbach resonant (VFR) excitation of fundamental vibrational modes, and this leads to greatly enhanced annihilation rates. Recently, new resonances beyond the fundamental modes have been observed in hydrocarbon and deuterated hydrocarbon molecules [1]. Here, we present the annihilation spectrum for benzene and four deuterated isotopologues as a function of positron energy using a cryogenic, trap-based positron beam (FWHM ~ 22 meV) [2]. The increased energy resolution permits the resolution of many unexpected resonances, likely due to combination and overtone vibrational modes. The relationship of these results to the unique π-bonded structure of benzene is discussed. Although the IR spectra have features associated with combination and overtone modes in this region, it is unclear whether they can provide sufficient coupling to explain the observed enhancements in annihilation.

[1] S. Ghosh, et al., Phys. Rev. Lett. 129, 123401 (2022).

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