Exploring three-body fragmentation of acetylene trication

The three-body breakup of [C₂H₂]³⁺ formed in collision with Xe⁹⁺ moving at 0.5 atomic units of velocity is studied by using recoil ion momentum spectroscopy. Three-body breakup channels leading to (H⁺, C⁺, CH⁺) and (H⁺, H⁺, C₂ ⁺) fragments are observed in the experiment and their kinetic energy release is measured. The breakup into (H⁺, C⁺, CH⁺) occurs via concerted and sequential modes, whereas the breakup into (H⁺, H⁺, C₂ ⁺) shows only the concerted mode. By collecting events coming exclusively from the sequential breakup leading to (H⁺, C⁺, CH⁺), we have determined the kinetic energy release for the unimolecular fragmentation of the molecular intermediate, [C₂H]²⁺. By using ab initio calculations, the potential energy surface for the lowest electronic state of [C₂H]²⁺ is generated, which shows the existence of a metastable state with two possible dissociation pathways. A discussion on the agreement between our experimental results and these ab initio calculations is presented.