unimolecular dissociationThe isolated, spontaneous dissociation of a neutral or molecular ion, based on the amount and distribution of its internal energy. In the electron ionisation source, initial ionisation of the molecule by the electrons leads to molecular ions, which then undergo rapid unimolecular dissociations, leading to the fragment ions seen in a mass spectrum. Unimolecular dissociation that occurs after ions leave the source and in field-free regions of the mass spectrometer leads to metastable ions. True unimolecular dissociation evolves into collision-induced dissociation as the operating pressure of the mass spectrometer increases.
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