Formation of H 3 + from ethane dication induced by electron impact.

Hydrogen migration plays an important role in the chemistry of hydrocarbons which considerably influences their chemical functions. The migration of one or more hydrogen atoms occurring in hydrocarbon cations has an opportunity to produce the simplest polyatomic molecule, i.e. H 3 + . Here we present a combined experimental and theoretical study of H 3 + formation dynamics from ethane dication. The experiment is performed by 300 eV electron impact ionization of ethane and a pronounced yield of H 3 + + C 2 H 3 + coincidence channel is observed. The quantum chemistry calculations show that the H 3 + formation channel can be opened on the ground-state potential energy surface of ethane dication via transition state and roaming mechanisms. The ab initio molecular dynamics simulation shows that the H 3 +  can be generated in a wide time range from 70 to 500 fs. Qualitatively, the trajectories of the fast dissociation follow the intrinsic reaction coordinate predicted by the conventional transition state theory. The roaming mechanism, compared to the transition state, occurs within a much longer timescale accompanied by nuclear motion of larger amplitude.