Dissociation of [HCCH]²⁺ to H₂ ⁺ and C₂ ⁺: a benchmark reaction involving H migration, H-H combination, and C-H bond cleavage

We report the formation of H₂ ⁺ and C₂ ⁺ from dissociation of acetylene induced by α-particle irradiation. The unusual dissociation channel [C₂H₂]²⁺ → H₂ ⁺ + C₂ ⁺ is unambiguously identified by measuring the time-of-flight of both fragmented ions in coincidence. Our quantum chemical calculation confirms the existence of this dissociation pathway. It shows that [HCCH]²⁺ is firstly populated to the ³Π excited electronic state, followed by acetylene-vinylidene isomerization, and finally the vinylidene-like intermediate dissociates to H₂ ⁺ and C₂ ⁺. This dissociation channel is the simplest prototypical reaction involving H migration, H-H combination, and C-H bond cleavage. The current study plays an important role for understanding the H₂ ⁺/H₃ ⁺ formation reactions from organic di-cations in an interstellar medium.