Exploring three-body fragmentation of acetylene trication.

The three-body breakup of [C 2 H 2 ] 3+ formed in collision with Xe 9+ 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 + , C 2 + ) 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 + , C 2 + ) 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, [C 2 H] 2+ . By using ab initio calculations, the potential energy surface for the lowest electronic state of [C 2 H] 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.