The slow fragmentation process (tgr;sime;10minus;7sec) of the CHplus;ion in a mass spectrometer can be ascribed to the superposition of several predissociation mechanisms. The potential energy curves of the CHplus;ion have been calculated quantum mechanically, and for each intersection, the predissociation lifetime has been estimated on the basis of the Landaumdash;Zener formula. Metastable ions are due to a spinhyphen;forbidden predissociation process, allowed by the spinmdash;orbit (Hso), spinmdash;otherhyphen;orbit (Hsoo) and spinmdash;spin (Hss) operators. These three operators are in some cases sufficient to obtain a lifetime much shorter than 10minus;7sec. 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 Landaumdash;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 ofHso,Hsoo, andHssare equal to zero, can only take place as a collisionhyphen;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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