A benchmark dissociation channel of [HCCH]²⁺ →H₂⁺ + C₂⁺ is found in the fragmentation of C₂H₂ initiated by alpha-particle irradiation. The research was conducted by the atomic physics group led by Professor Xinwen Ma through the collaboration with School of Science, Xi’an Jiaotong University and Department of Chemistry, City University of Hong Kong.  

The experiment was performed by employing a reaction microscope at the 320 kV platform for multidisciplinary research with highly charged ions at the Institute of Modern Physics, Chinese Academy of Sciences (IMP) located in Lanzhou. The unusual dissociation channel [HCCH]²⁺ àH₂⁺ + C₂⁺ is identified by measuring the time-of-flight of both fragmented ions in coincidence as shown in Fig 1. The state-of-the art quantum chemical calculation confirms the existence of these dissociation pathways. 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⁺, as shown in Fig 2. This dissociation channel is the simplest prototypical reaction involving H migration, H-H combination, and C-H bond cleavage. 

Both the reactant (C₂H₂) and the products (H₂⁺ and C₂⁺) in the present study have been extensively observed in the interstellar medium. Thus the current study plays an important role for understanding the H₂⁺/H₃⁺ formation reactions from organic di-cations in an interstellar medium. 

The results have been published as a communication in the peer reviewed Journal, Physical Chemistry Chemical Physics (Communication), 20, 27725, 2018. The work was support by the National Key Research and Development Program of China and by the National Natural Science Foundation of China. 

The articles can be linked as follows:  https://doi.org/10.1039/c8cp05780j, http://amdimp.impcas.ac.cn/news.php?cid=55