As a molecule of great physical, chemical, and astronomical importance, H₃⁺ plays a vital role in probing and understanding various astrophysical environments including planetary atmospheres and interstellar media. In order to use the H₃⁺ as a tool to characterize and study properties of those environments, the structural and dynamical information of H₃⁺ must be available, which has been and continues to be obtained through sophisticated experimental and theoretical investigations, e.g. precise laboratory spectroscopy and quantum chemistry calculations.

Considering the widespread abundance of electrons and hydrocarbon molecules in astronomical environments, the H₃⁺ ions produced by the electron-impact induced dissociative ionization of hydrocarbons could be, in addition to the ion-neutral reaction H₂⁺ + H₂ → H₃⁺ + H, another origin of H₃⁺ formation. This might help understand the H₃⁺ abundance in some astronomical systems. However, the formation mechanism of H₃⁺ from dissociation of organic molecular ions is not fully understood. Two mechanisms, i.e. transition state (TS) and H₂ roaming mechanisms, have been separately introduced through strong laser ionization experiments in combination with different theoretical calculations. Obviously, the connection and difference between these two kinds of mechanisms need further clarification.

Over the last decade, building and then using the cold target recoil ion momentum spectroscopy (COLTRIMS) setups and also the highly charged ion collision platform, our group has been studying the fragmentation dynamics of hydrocarbon molecules in collision with charged particles. The H₃⁺ ion was indeed observed in the fragment products of CH₄ and C₂H₄ dications produced by electron or ion impact, but with very low yield, which hindered our further theoretical investigation. Later, we turned to another hydrocarbon molecule, C₂H₆ and a pronounced yield of H₃⁺ was observed. Then quantum chemistry calculations including TS calculation and ab initio molecular dynamics (AIMD) simulation were carried out, and the simulated kinetic energy release (KER) distribution for the H₃⁺ channel was in good agreement with the experimental one.

In this work, we show two kinds of mechanisms, i.e. TS mechanism and the dominant one, roaming mechanism, lead to H₃⁺ formation, which however exhibit very different characteristics. Comparing with the minimum energy path in the TS mechanism, the roaming process takes a much longer time, exhibits larger amplitude motion, and requires the displacement of more H atoms. Discrepancies are also found in the dissociation time and KER of the TS and roaming mechanisms, which indicates that the separation of the two mechanisms might be realized by an ultrafast pump-probe experiment. Furthermore, besides H₂ roaming, another new roaming mechanism which involves a neutral H atom is uncovered.

The features of the TS and roaming mechanisms presented in the ionic molecular dissociation would advance understanding into many aspects, e.g. hydrogen migration, roaming chemistry, and molecular fragmentation. We also hope the findings of this study could help understand the physics and chemistry of astronomical H₃⁺ ions.

A link to the paper entitled “Formation of H₃⁺ from ethane dication induced by electron impact” in Communications Chemistry can be found here: https://www.nature.com/articles/s42004-020-00415-9