Ionization of H2 by Electron Impact near Threshold

Abstract

The cross section for ionization of the hydrogen molecule by electron impact near threshold has been experimentally studied in a total‐ionization tube. Careful attention has been given to consistency checks to ensure complete collection of electrons and ions. The cross section was found to be essentially a linear function of the electron‐energy excess above threshold for ∼4 eV above threshold. Similar results were obtained using a dc electron beam, and using a double‐phase sensitive detection ac variation of the RPD method to reduce the energy spread to ∼0.06 eV at half‐height. These results are in disagreement with the threshold law predicted by a simple direct‐ionization model, and imply that some indirect process such as autoionization is playing a major role in the ionization process. Thus the distribution of vibrational states in the H₂⁺, which plays a major role in inelastic collisions of H₂⁺ with atomic particles, is not correctly predicted by the direct ionization model. Evidently, the ionization of a simple molecule like H₂ near threshold is too complicated a process to be adequately described in terms of existing models.