Vinylidene and acetylene are the simplest hydrocarbon isomers, and vinylidene is the simplest unsaturated carbene. The charge inversion mass spectra of C2H2+ cations derived from acetylene using Na, K, Rb and Cs targets were found to be clearly different from those derived from vinylidenechloride (1,1-dichloroethylene). The process of formation of the negative ions in charge inversion mass spectrometry is via near-resonant neutralization followed by spontaneous dissociation, and then endothermic negative ion formation. The intensity of the C2− peak relative to the C2H− peak in these spectra increased with decreasing ionization potential of the targets for both of the isomeric C2H2+ cations. The formation of the C2− anion is proposed to result from the dissociation of excited C2H2 neutrals into C2 and H2. The dependence on target species of the intensities of the C2− peak relative to the C2H− peak for HCCH and H2C
C: cannot be rationalized by the internal energy of the excited C2H2 neutrals. The differences indicate that the isomeric C2H2 neutrals dissociate into C2H and H prior to 1,2-hydrogen atom migration.
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