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

Abstract

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.