Dissociative Autoionization as a Mechanism for the Proton Formation from Methane and Methane-d4 by low energy Electron Impact.

The ion energy distribution and the electroionization efficiency curves of H+ and D+ from methane and methane-d4 have been examined. The existence of thermal ions is unambiguously evidenced and the two lowest appearance thresholds of H+ are measured at (21.3+/-0.3) eV and (22.17+/-0.1) eV. At both energies the protons have to be formed through dissociative autoionization. The present results closely parallel those of the dissociative excitation experiments on methane. The Jortner-Rice model for dissociation of polyatomic molecules is invoked to account for both the appearance of H+ and high-Rydberg H atoms from CH4.