Ion-Molecule and Chemi-ionization Reactions in H2 by Photoionization

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Abstract
The production of H3 + by ion‐molecule and chemi‐ionization processes initiated by photon impact in H2 has been studied as a function of photon energy with resolution widths varying from 0.05 to 0.002 eV. The purpose was to determine how the vibrational energy of the reactant ion affects the reaction cross section. The data indicate that the reaction H2 + + H2→H3 + + H proceeds by at least two different mechanisms. At very low kinetic energies, the dominant mechanism has a reaction cross section which decreases slowly as the vibrational energy of the ion increases. At higher kinetic energy, a mechanism for which the cross section increases with increasing vibrational energy of the ion becomes more prominent. The characteristics of the first mechanism suggest the formation of a collision complex while those of the latter suggest a mechanism such as stripping. At low kinetic energies, the rotational energy of the ion has only a small effect on the reaction cross section. Some of the production of H3 + by chemi‐ionization below the ionization threshold of H2 is shown to involve excited H atoms, probably in the 2 2 S metastable state, as well as excited H2 molecules. The cross section for chemi‐ionization by excited H atoms was found to decrease with kinetic energy; this behavior is discussed. The proton affinity of H2 was found to be ≥ 3.936 eV.

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