Quantitative Modeling of Excited-State Dynamics in Valence Photoionized Vinyl Fluoride.

Abstract

Long-standing debates regarding the dissociative photoionization of vinyl fluoride (fluoroethene) were resolved using large-scale surface-hopping ab initio molecular dynamics (SH-AIMD) simulations. By combining accurate initial condition sampling, electronic cross-section calculations, and SH-AIMD with density functional theory (DFT) and complete active space second-order perturbation theory (CASPT2), we obtained not only qualitative insight into excited-state dynamics but also quantitatively accurate predictions of the photoelectron spectrum, fluorine-loss branching ratios, and translational kinetic energy release distributions for F + C₂H₃⁺ products. Statistical dissociation arises from the X̃ ²A″-B̃ ²A' states, while, in the C̃ ²A″-Ẽ ²A' states, excited-state dissociation within 50-250 fs dominates. Only CASPT2 captures the formation of an excited triplet C₂H₃⁺ fragment, though DFT still reproduces correct branching ratios, as branching pathways are largely determined at short bond distances. Importantly, the previously hypothesized inclusion of autoionizing Rydberg states is not required to match experimental observables.