Dissociation of Molecular Ions Formed by Charge Exchange in an In-Line Tandem Mass Spectrometer

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Abstract
Dissociations of the molecular ions of several alkanes, alcohols, and ketones, formed in charge‐exchange collisions were studied using an in‐line tandem mass spectrometer. The effects of both incident ion translational energy and collision chamber temperature upon the observed dissociations were examined. For most of the molecules investigated, the extent of fragmentation of the molecular ion increases as the kinetic energy is raised, indicating that energy is being converted from the translational modes into target excitation. The magnitude of this energy conversion seems to be rather small. Higher collision chamber temperatures also lead to more extensive fragmentation, presumably owing to increased vibrational excitation. In certain cases, notably methanol and 2‐hexanone, an increase in the translational energy of incident ions having recombination energies near the onset of known excited ionic states of these molecules results in an increase in the relative intensity of the molecular ion product. This is interpreted as indicating the formation of long‐lived excited electronic states of the molecular ion which do not revert to the ground state via radiationless transitions. Charge‐exchange spectra obtained in the present study are compared with breakdown graphs previously determined from similar experiments in perpendicular‐type tandem mass spectrometers, and also with the results of photoionization experiments and calculations based on the quasiequilibrium theory.