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(2)H(3)F(+), 1,1-C(2)H(2)F(2)(+), C(2)HF(3)(+) and C(2)F(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 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)(+), CF(+) 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 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(2)F(4) together with the now well established Δ(f)H(o) of C(2)F(4) yield self-consistent enthalpies of formation for the CF(3), CF, CF(3)(+) and CF(+) species.

Vibrational and electronic excitations in fluorinated ethene cations from the ground up

“…9 The ν 4 harmonic frequency of 1552 cm −1 ( Figure 2) is in excellent agreement with the B3LYP prediction of 1561 cm −1 . Discrepancies in the energies between the simulated and the experimental spectra towards higher eV are due to anharmonicity which is disregarded in the harmonic model of FCfit.…”

“…We based this conclusion predominantly on the correlation of the F-loss fragment ion signal with features of the TPES, indicating isolated state behaviour, in agreement with previous observations. [10][11][12][13] However, we also found that the internal energy distribution of the F-loss daughter ion C 2 F 3 + from C 2 F 4 + can be modelled assuming a purely statistical dissociation on theà electronic state of the parent ion, 9 contrary to previous reports which invoked impulsive processes.…”

“…9 The first dissociative photoionization channel opens up in a FranckCondon gap above the electronic ground stateX. We also found that F-atom loss is initially a statistical process in three of the four molecular ions, the exception being tetrafluoroethene.…”

“…5 It is comprised of two modes, ν 8 (a ) with 2ν 9 (a ), which are only 9 cm −1 apart with comparable Franck-Condon factors. In this instance, we can say with a degree of certainty that the assignment is not simply a matter of either ν 8 or 2ν 9 , but both transitions are in fact present.…”

“…Second, the nature and role of the excited states with regard to the various dissociation pathways has been probed. Specifically, we try to explain the mechanism of non-statistical F-loss in thẽ C state band in the monofluoroethene cation observed in a previous study, 9 and aided by quantum chemical calculations, generalize it to other members of the fluorinated ethene series.…”