Energy distributions and appearance potentials of N ions produced by low-energy electron impact on N2 have been measured. At least five dissociation limits were detected either directly in the ionization efficiency curves of N or by extrapolation of the 1/2 slope straight lines drawn through the kinetic energy versus appearance potential energy diagram. Fine structure observed at low ion energy has been interpreted by the predissociation of at least two N 2 excited states one of which being the well-known C 2 Σ u + state. One of the others could be a 2 Π u state pre-dissociated to the dissociation limit at 24.34 eV. Close correlation between the results of the present work and photoelectron spectroscopic observations could be done over the electron energy range between 24.3 eV and 36 eV and assignments are discussed in detail.

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The slow fragmentation process (τ≃10−7sec) of the CH+ ion in a mass spectrometer can be ascribed to the superposition of several predissociation mechanisms. The potential energy curves of the CH+ ion have been calculated quantum mechanically, and for each intersection, the predissociation lifetime has been estimated on the basis of the Landau—Zener formula. Metastable ions are due to a spin-forbidden predissociation process, allowed by the spin—orbit (Hso), spin—other-orbit (Hsoo) and spin—spin (Hss) operators. These three operators are in some cases sufficient to obtain a lifetime much shorter than 10−7 sec. In order to give rise to a metastable fragmentation, the predissociation process must be slowed down, either by the tunnel effect, or because of a deviation from the Landau—Zener formula. The effect of the collisions on the predissociation processes is discussed. In some cases the mechanism of this effect involves energy transfer, while in others only a symmetry loss occurs. A strictly forbidden predissociation process, for which the matrix elements of Hso, Hsoo, and Hss are equal to zero, can only take place as a collision-induced process. Although they are characterized by a peculiar potential energy curve, exhibiting a shallow basin, the Rydberg states are not involved in the metastable decomposition.

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