Near-threshold positronium formation for molecules with near-adiabatic photoionization thresholds.

Positronium (Ps), the bound state of a positron and an electron, can be formed in low-energy positron collisions with atoms and molecules. This results in ionization of the target with a threshold E_Ps = E_I - 6.8 eV, where E_I is the target ionization energy, and 6.8 eV is the binding energy of Ps. Recently, measurements of the near-threshold Ps formation cross-section for several large polyatomic molecules showed a remarkable similarity with photoionization measurements [1]. This was shown to occur for molecules with a large shift between the vertical and adiabatic transition energies, and was attributed to the shape of the measured cross-section being determined by the growth of the density of transition states and the respective Franck-Condon factors. For molecules where the adiabatic transition dominates, it is expected that the similarity will fail and the details of the respective processes will determine the shape of the cross-section curve. Here, measurements of near-threshold Ps formation in N₂, propyne, and 2-butyne will be presented. Surprisingly, in contrast to what was observed for larger molecules, it is observed that all three exhibit a beam-width limited step function right at threshold, similar to photoionization. However, just above threshold, the two processes show dramatically different behavior, whereas the Ps cross-section shows an approximately linear rise over several eV, the photoionization is dominated by significant autoionization resonances.

Details of the experiments and these comparisons will be discussed.

[1] J. R. Danielson, et. al, Phys. Rev. Lett. 133, 123001 (2024).