Similarity of the near-threshold cross sections for positronium formation and photoionization in polyatomic 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 - 6.803 eV, where E_I is the target ionization energy, and 6.803 eV is the binding energy of Ps. Here we present high-resolution measurements of Ps formation cross sections near threshold for three polyatomic molecules: aniline (C₆H₅NH₂), pyridine (C₅H₅N), and cyclopentane (C₅H₁₀), and compare them to measured photoionization cross-sections. Although Ps formation and photoionization are significantly different processes, the measurements show the two cross-sections exhibit a nearly identical dependence on excess energy near threshold (i.e., ≤ 1 – 2 eV). Possible reasons for this similarity, where it breaks down, and in what other targets this may be expected, will be discussed.