Near-threshold positronium formation for small molecules.

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 ionization energies, and is due to the shape of the measured cross-section being dominated by the growth of the density of transition states and the respective Franck-Condon factors. In contrast, for molecules where the adiabatic transition dominates, it is expected that the similarity will fail and thus the details of the respective ionization processes will dominate the shape of the cross-section curve. Further, for small molecules like N₂, the near-threshold photoionization is enhanced by super-excited states which have no analog in Ps formation. To probe these effects, high-resolution near-threshold measurements of Ps formation in the small molecules N₂, CF₄, and methanol (CH₃OH) will be presented. These measurements will be compared to previous Ps measurements of these targets as well the respective near-threshold photoionization and electron ionization cross-sections. Further studies on larger targets will also be presented. [1] J. R. Danielson, et al., Phys. Rev. Lett. 133, 123001 (2024).