Non-adiabatic origin of roaming OH dynamics in the formic acid dimer dication

Ionization of molecular clusters can trigger chemical reactions and drive chemical evolution even at very low temperatures, influencing chemistry in interstellar, atmospheric, and planetary environments exposed to ionizing radiations. To investigate such processes involving the dissociation of both intramolecular and intermolecular bonds under controlled conditions, we examined the dynamics of the formic acid (FA) dimer dication in an ultrafast extreme-ultraviolet (EUV) pump and near-infrared (NIR) probe experiment, combined with ab initio molecular dynamics simulations. The dissociation of the intermolecular bond and the formation of the two-body FA⁺ + FA⁺ channel could be explained by ground-state dynamics, whereas the three-body breakup channels required a more detailed description. We developed a simplified dimer model for the breakup process that enabled non-adiabatic molecular dynamics simulations on excited-state CASPT2 potential energy surfaces, capturing both intermolecular and intramolecular dynamics. The simulations showed that immediately after the dimer dissociation and non-adiabatic decay to the ground electronic state, a roaming-OH mechanism develops, accounting for the observed kinetic-energy-release distributions and momentum correlations in the FA⁺ + CHO⁺ + OH and FA⁺ + H₂O⁺ + CO three-body breakup channels. The simplified modeling approach may serve as a practical framework for studying the excited-state dynamics in molecular dimer and cluster breakup processes.