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

The dissociative photoionization mechanism of internal energy selected C\(_2\)H\(_3\)F\(^+\), 1,1-C\(_2\)H\(_2\)F\(_2^+\), C\(_2\)HF\(_3^+\) and C\(_2\)F\(_4^+\) cations have 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 C=C bond. By modelling the breakdown diagrams and ion time-of-flight distributions using statistical theory, experimental 0 K appearance energies, E\(_0\), of the daughter ions have been determined. Both C\(_2\)H\(_3\)F\(^+\) and 1,1-C\(_2\)H\(_2\)F\(_2^+\) are veritable time bombs with respect to dissociation via HF loss, where slow dissociation over a reverse barrier is followed by an explosion with large kinetic energy release. The first dissociative ionization pathway for C\(_2\)HF\(_3\) and C\(_2\)F\(_4\) involves an atom migration across the C=C bond, giving CF–CHF\(_2^+\) and CF–CF\(_3^+\), respectively, which then dissociate to form CHF\(_2^+\) and CF\(_3^+\). The nature of the F-loss pathway has been found to be bimodal for C\(_2\)H\(_3\)F and 1,1-C\(_2\)H\(_2\)F\(_2\), switching from statistical to non-statistical behaviour as the photon energy increases. The dissociative ionization of C\(_2\)F\(_4\) is found to be comprised of two regimes. 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 E\(_0\) of CF\(_3^+\) and CF\(^+\) formation from C\(_2\)F\(_4\) together with the now well established ∆\(_f\)Hº of C\(_2\)F\(_4\) yield self-consistent enthalpies of formation for the CF\(_3\), CF, CF\(_3^+\), and CF\(^+\) species.