Dissociation dynamics of fluorinated ethene cations: from time bombs on a molecular level to double-regime dissociators

Abstract

The dissociative photoionization mechanism of internal energy selected C₂H₃F⁺, 1,1-C₂H₂F₂⁺, C₂HF₃⁺ and C₂F₄⁺ cations has been studied in the 13–20 eV photon energy range using imaging photoelectron photoion coincidence spectroscopy. Five predominant channels have been found; HF loss, statistical and non-statistical F loss, cleavage of the C–C bond post H or F-atom migration, and cleavage of the CC bond. By modelling the breakdown diagrams and ion time-of-flight distributions using statistical theory, experimental 0 K appearance energies, E₀, of the daughter ions have been determined. Both C₂H₃F⁺ and 1,1-C₂H₂F₂⁺ are veritable time bombs with respect to dissociation viaHF loss, where slow dissociation over a reverse barrier is followed by an explosion with large kinetic energy release. The first dissociative ionization pathway for C₂HF₃ and C₂F₄ involves an atom migration across the CC bond, giving CF–CHF₂⁺ and CF–CF₃⁺, respectively, which then dissociate to form CHF₂⁺, CF⁺ and CF₃⁺. The nature of the F-loss pathway has been found to be bimodal for C₂H₃F and 1,1-C₂H₂F₂, switching from statistical to non-statistical behaviour as the photon energy increases. The dissociative ionization of C₂F₄ is found to be comprised of two regimes. At low internal energies, CF⁺, CF₃⁺ and CF₂⁺ are formed in statistical processes. At high internal energies, a long-lived excited electronic state is formed, which loses an F atom in a non-statistical process and undergoes statistical redistribution of energy among the nuclear degrees of freedom. This is followed by a subsequent dissociation. In other words only the ground electronic state phase space stays inaccessible. The accurate E₀ of CF₃⁺ and CF⁺ formation from C₂F₄ together with the now well established Δ_fH^o of C₂F₄ yield self-consistent enthalpies of formation for the CF₃, CF, CF₃⁺ and CF⁺ species.