Ground-and excited-state fragmentation dynamics of doubly ionized OCS: A theoretical study

We investigated fragmentation dynamics of OCS2+ after photoinduced double ionization by combining static potential energy surface (PES) analysis with ab initio molecular dynamics (AIMD) and surface-hopping AIMD (SH-AIMD). In the ground state, AIMD shows that the isomerization pathway from OCS2+ to COS2+, although accessible on the static PES, is dynamically hidden. Trajectories rarely follow it because it requires unrealistically high bending excitation, whereas S+ dissociation proceeds without such a constraint. Consequently, energy released upon ionization is funneled more efficiently into dissociation, favoring S+ dissociation over isomerization. For excited states, SH-AIMD trajectories launched from the lowest triplet 3Π states and the 23Δ state reproduce the observed fragmentation: predominant S+ dissociation with minor O+ formation. The isomerization process leading to the COS2+ structure was not observed in the simulations. These results are consistent with experimental observations. Notably, for the O+ channel, we likely achieve the first AIMD reproduction of the experimental appearance threshold using only triplet states and no external laser fields. This indicates that vibronic coupling via bending motion is essential to enable O+ release, although its probability remains small relative to S+ dissociation. Overall, fragmentation in OCS2+ is governed by the interplay of vibronic coupling and dynamical effects: bending vibrations can facilitate O+ release, whereas isomerization to COS2+ is dynamically suppressed by the requirement of extreme bending excitation. The high density of states in the excited manifold further underscores the strongly nonadiabatic character of OCS2+. Our results connect static PES features with trajectory-based dynamics, offering new insight into selective fragmentation in polyatomic dications.