unimolecular dissociation

Unimolecular dissociation is the isolated, spontaneous dissociation of a neutral or an ion, based on the amount and distribution of its internal energy. In the electron ionization source, initial ionization of the molecule by the electrons leads to molecular ions, which then undergo rapid unimolecular dissociations leading to the fragment ions observed in the mass spectrum. Unimolecular dissociation that occurs  after the ions leave the source and in the field-free regions of  the mass spectrometer leads to metastable ions that can be  observed with special methods. Unimolecular dissociation is  contrasted with collision-induced dissociation, at least in  concept. True unimolecular dissociation evolves into collision-induced dissociation as the operating pressure of the  mass spectrometer increases.