Formation of covalently bound C₄H4+ upon electron-impact ionization of acetylene dimer

We investigate the formation mechanisms of covalently bound C4H4+ cations from direct ionization hydrogen bonded dimers acetylene molecules through fragment ion and electron coincident momentum spectroscopy quantum chemistry calculations. The measurements momenta energies two outgoing electrons one in triple-coincidence allow us to assign channels associated with different ionic fragments. measured binding energy spectra show that can be attributed outermost 1πu orbital acetylene. kinetic distributions fragments indicate ions originate while resulting fragmentation larger clusters would obtain significantly momenta. evaporation mechanism is not identified present experiments. calculated potential curves a well for electronic ground state (C2H2)2+, supporting (C2H2)2 form stable C2H2⋅C2H2+(1πu−1) cations. Further transition analysis ab initio molecular dynamics simulations reveal detailed picture dynamics. After (C2H2)2, system undergoes significant rearrangement structure involving, particular, C–C bond migrations, leading C44+ isomers.