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 $E_{\rm Ps} = E_{\rm I}$ - 6.8 eV, where $E_{\rm I}$ is the target ionization energy, and $6.8$ 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 (C$_6$H$_5$NH$_2$), pyridine (C$_5$H$_5$N), and cyclopentane (C$_5$H$_{10}$), 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., $\leq$ 1 - 2 eV). Possible reasons for this similarity, where it breaks down, and in what other targets this may be expected, will be discussed.