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 C 2H 3F +, 1,1-C 2H 2F 2+, C 2HF 3+ and C 2F 4+ 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, E 0, of the daughter ions have been determined. Both C 2H 3F + and 1,1-C 2H 2F 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 2HF 3 and C 2F 4 involves an atom migration across the CC bond, giving CF-CHF 2+ and CF-CF 3+, respectively, which then dissociate to form CHF 2+, CF + and CF 3+. The nature of the F-loss pathway has been found to be bimodal for C 2H 3F and 1,1-C 2H 2F 2, switching from statistical to non-statistical behaviour as the photon energy increases. The dissociative ionization of C 2F 4 is found to be comprised of two regimes. At low internal energies, CF +, CF 3+ and CF 2+ 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 E 0 of CF 3+ and CF + formation from C 2F 4 together with the now well established Δ fH o of C 2F 4 yield self-consistent enthalpies of formation for the CF 3, CF, CF 3+ and CF + species. © 2011 The Owner Societies.