The production of H3plus;by ionhyphen;molecule and chemihyphen;ionization processes initiated by photon impact in H2has 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 H2plus;thinsp;plus;thinsp;H2rarr;H3plus;thinsp;plus;thinsp;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 H3plus;by chemihyphen;ionization below the ionization threshold of H2is shown to involve excited H atoms, probably in the22thinsp;Smetastable state, as well as excited H2molecules. The cross section for chemihyphen;ionization by excited H atoms was found to decrease with kinetic energy; this behavior is discussed. The proton affinity of H2was found to be ge; 3.936 eV.
展开▼
