Methylation effect on positron-molecule binding.

Positron-molecule binding energies (ε_b) mediated by vibrational feshbach resonances have been measured for a wide range of molecules [1]. Recent studies have highlighted the role of global parameters such as molecular dipole moment, polarizability, π-bonds and ionization potential [1,2]. In this report, we study the effect of methylation on εb for benzene and pyridine derivatives (mono, di-, and trimethylated), using the room temperature buffer-gas Surko trap. We observe that a single methyl substitution increases εb by 30-50 meV relative to the parent molecule. In xylenes, variation in ε_b among positional isomers are insignificant. However, for methylated pyridines containing nitrogen, ε_b increases the farther methyl groups are positioned from the nitrogen. Additionally, the positron annihilation rate increases with more (CH₃) groups,with significant dependence on isomerization, attributed to enhanced electrons donated to the ring upon methylation. These experimental results are supported by many-body theory calculations to highlight the positron-molecule correlation and wavefunction distribution [3]. [1] J. R. Danielson et al., Phys. Rev. A. 108, 032801 (2023) [2] E. Arthur-Baidoo et al., Phys. Rev. A, 109, 062801 (2024) [3] J. Hofierka et al., Nature 606, 688-693 (2022)