How Complex can be the Unimolecular Decomposition of a Simple Molecule? The Case of Acetylene. An Electron Impact and PIPECO Investigation

Abstract

The dissociative ionization of C 2 H 2 , C 2 D 2 and C 2 HD is presented in this work. Excepting the H 2 + ion 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 C 2 H + ion, the PIPECO technique has also been used. From these discussions the H-C 2 H, HC≡CH and H-C 2 binding energy values are proposed, i.e. 5.33±0.23eV, 9.83±0.10eV and 5.44±0.40 eV respectively. The fragmentation paths leading to C + , CH 2 + and C 2 + are discussed in terms of dissociation mechanisms involving the transient vinylidene structure of the molecular ion as an intermediate.

Experimental Techniques in Gas-Phase Ion Thermochemistry

“…Locht and co-workers − employ a dissociative electron ionization method in which the kinetic energies of the product ions are measured using retarding potential analysis. This allows extrapolation of ion appearance energies to zero ion kinetic energy, correcting for cases where a barrier to dissociation causes products to be formed with excess translational energy.…”

Photoelectron spectroscopy of vinylbromide and intramolecular dynamics of the ionic B̃ state

“…The He(I) spectra have been obtained with a helium discharge lamp, giving a resonance line at 21.21 eV. The details of this spectrometer, designed for photoion-photoelectron coincidence spectroscopy, have been given elsewhere [10]. This is based on a Lindautype electron energy analyzer.…”

Unimolecular dynamics from kinetic energy release distributions. V. How does the efficiency of phase space sampling vary with internal energy?

“…27 It consists of a Lindau-type electron energy analyzer and an ion retarding potential analyzer coupled with a quadrupole mass analyzer. The effusive gas sample is ionized by photons emitted from a rare gas discharge lamp.…”