How complex can be the Unimolecular Decomposition of a simple Molecule? The case of Acetylene. An Electron Impact and PIPECO Investigation.

The dissociative ionization of C2H2, C2D2 and C2HD is presented in this work. Excepting the H2+ formation, all dissociation channels are thoroughly investigated by electron impact. The translational energy distribution as a function of the impinging electron energy and the appearance energy as a function of the translational energy are measured for all fragment ions. KE versus AE diagrams are obtained and the isotope effect is examined. All observed thresholds are analyzed in detail and dissociation mechanisms are proposed. For the C2H+ ion the PIPECO technique has also been used. From these discussions the H-C2H, HC-CH and H-C2 binding energy values are proposed, i.e. 5.33+/-0.23 eV, 9.83+/0.10 eV and 5.44+/-0.40 eV respectively. The fragmentation path leading to C+, CH2+ and C2+ are discussed in terms of dissociation mechanisms involving the transient vinylidene structure of the molecular ion as an intermediate.