Resonant annihilation in benzene and deuterated benzenes going beyond the fundamental modes

Positrons bind to most molecules through vibrational Feshbach-resonances (VFR) associated with infrared active fundamental modes, and this leads to greatly enhanced annihilation rates. Recently, new resonances beyond the fundamental modes have been observed in hydrocarbon molecules [1]. Here, we present annihilation spectra for benzene and deuterated benzenes as a function of positron energy using a cryogenic, trap-based positron beam (FWHM ~ 22 meV) [2]. The increased energy resolution reveals several unexpected resonances. From symmetry analysis, some of these resonances are identified as infrared-active (IR) combinations of fundamentals. The estimated positron coupling to the combination mode, as obtained from the IR amplitude, is comparable to that of the fundamental-mode VFR. Possible enhancement of the combination mode VFR (i.e., assumed to be from intramolecular vibrational energy redistribution (IVR)) will be discussed.

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

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