Positron annihilation and threshold positronium formation cross-section in pyridine.

Measurements of the low energy positron annihilation cross-section of pyridine are presented over the energy range $0 - 10$ eV. The experiments use a high-resolution buffer-gas-trap based beam with a total energy spread of $< 35$ meV. Below $0.5$ eV, as expected, the spectrum is dominated by vibrational feshbach resonances [1]. The focus here are the measurements at and above the positronium (Ps) formation threshold ($approx 2.4$ eV) where the annihilation is dominated by Ps annihilation. The measured cross-section, $sigma_a$, rises from threshold until approximately $4$ eV where $sigma_a$ plateaus and exhibits a gentle oscillation from $4$ eV to $10$ eV. These measurements are shown to be quantitively consistent with the recent Ps cross-section measurements in the energy range 4 - 10 eV [2]. Further, the threshold region ($2 - 4$ eV) is compared to downshifted (by $6.8$ eV), and scaled, high-resolution emph{photoionization} experiments [3]. Surprisingly, there is excellent agreement between the shapes of the two curves implying identical thresholds and the same threshold power law for Ps formation and photoionization. [1] Details in poster by Arthur-Baidoo et al, DAMOP2023. [2] Stevens et al JCP extbf{148}, 144308 (2018). [3] Xie et al. IJMS extbf{303}, 137 (2011).