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