Electron spectroscopy using excited atoms and photons: II. Penning ionization of diatomic molecules

A high resolution electrostatic electron analyser has been used to study Penning ionization electron spectra of H₂, HD, D₂, N₂, CO, NO and O₂ using helium metastable atoms (2¹S, 2³S). Results for H₂, N₂ and CO are in good agreement with other work. New data are presented for HD, D₂, NO and O₂. The Penning electron spectra are also compared to the 584 Å photoelectron spectra obtained in the same apparatus. The relative vibrational intensifies for the given electronic bands indicate that in most cases Franck—Condon factors for Penning ionization and photoionization are very similar. However for the O₂⁺(X²Π_g) band, the (2³S) Penning electron and photoelectron spectra show significant differences in the Franck—Condon envelopes This perturbation of the envelope for the Penning ionization may be explained by a competing autoionization process. The relative electronic transition probabilities are in many cases found to be different for Penning ionization and photoionization.