The appearance energy of C2H+ from C2H2 revisited. A Photoion-Photoelectron Coincidence Spectroscopic Determination.

The dissociative ionization of C2H2 in the C2H+-producing channel has been reconsidered. The photoion-photoelectron coincidence (PIPECO) technique has been used. The C2H+/e coincidence rate has been recorded between 16.2 and 21.2 eV. The lowest appearance energy determined for C2H+ is 17.35+/-0.04 eV. A second appearance energy is measured at 18.40+/-0.04 eV. At both energies ethynyl ions are produced through a predissociation mechanism at the expense of the C2H2+(A2Ag) state at 17.35 eV and C2H2+(B2Sigma+/g) at 18.40 eV. These results are in good agreement with the most recent TPIPECO experiments and dissociative electroionization results. This comparison leads to the exclusion of any contribution of autoionization phenomena to the C2H+ ion production between 16.2 and 21.2 eV. Investigations of C2H+ through the ion-pair formation channel (and dissociative electron attachment producing C2H- and H-) in C2H2 are reported and its contribution to C2H+ ion production below 17.3 eV must be discarded.