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

We report the formation of H-2(+) and C-2(+) from dissociation of acetylene induced by -particle irradiation. The unusual dissociation channel [C2H2](2+) H-2(+) + C-2(+) 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 (3) excited electronic state, followed by acetylene-vinylidene isomerization, and finally the vinylidene-like intermediate dissociates to H-2(+) and C-2(+). 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 H-2(+)/H-3(+) formation reactions from organic di-cations in an interstellar medium.