Similarity of Near-Threshold Energy Dependence of Positronium Formation and Photoionization in Molecules.

High-resolution measurements of the positronium formation cross sections for positron energies from threshold to 10 eV are presented for aniline (C_{6}H_{5}NH_{2}), pyridine (C_{5}H_{5}N), and cyclopentane (C_{5}H_{10}). The data reveal that the measured energy dependence of the cross sections on the excess energy in the near-threshold region (1-2 eV) is nearly identical to that of the corresponding photoionization cross sections. This similarity occurs despite the difference between the basic threshold laws for processes without and with a Coulomb interaction between the final-state particles. It is proposed here that the near-threshold behavior of these two different ionization processes is governed by the vibrational dynamics of the final-state cation. This is supported by comparison of the data with the calculated spectrum of vibronic intensities for the pyridine cation [Trofimov et al., J. Chem. Phys. 153, 164307 (2020)JCPSA60021-960610.1063/5.0024446].