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

Physical Chemistry Chemical Physics Pub Date: 2012-01-17 DOI: 10.1039/C2CP23878K

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 C[double bond, length as m-dash]C 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 C[double bond, length as m-dash]C 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 ΔfHo of C2F4 yield self-consistent enthalpies of formation for the CF3, CF, CF3+ and CF+ species.

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