Theoretical Modeling of Ultrafast Coincidence Imaging of Acetylene Ions

Isomerization and fragmentation dynamics are key processes in laser-driven interactions. While the dynamics of neutral molecules have been well studied, their ions have been less explored for strong-field and coincidence imagine experiments. We compute potential energy surfaces for the isomerization and fragmentation of acetylene and vinylidene ions using the MCSCF method with a complete valence active space, (10, 10) for neutral acetylene. We use the intrinsic reaction coordinate (IRC) to describe the isomerization between acetylene and vinylidene and systematically increase the carbon-carbon bond length of each isomer to describe the dissociation. For the acetylene cation, we find that vinylidene-like fragmentation is energetically favored due to the difference in dissociation barriers, while the barrier between isomers is small relative to the energy required to dissociate. These results are consistent with ion beam experiments with a strong-field probe studying the dependence of initial isomer and charge state on the fragmentation. These results provide a theoretical framework for interpreting ionic fragmentation and isomerization dynamics prior to multidimensional dynamics simulations.