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.

Valence threshold photoelectron spectra of four fluorinated ethenes; C 2 H 3 F, 1,1-C 2 H 2 F 2 , C 2 HF 3 , and C 2 F 4 were recorded at the Swiss Light Source with 0.002 eV resolution. The adiabatic ionization energies were found to be 10.364 ± 0.007, 10.303 ± 0.005, 10.138 ± 0.007, and 10.110 ± 0.009 eV, respectively. The electronic ground state of each cation shows well-resolved multi-component vibrational progressions, the dominant transitions being in the C=C stretching mode. Density functional theory based Franck-Condon simulations are used to model the vibrational structure and assign the spectra, sometimes revising previous assignments. An additional vibrational progression in the first photoelectron band of 1,1-C 2 H 2 F 2 indicates that the ground electronic state of the molecular ion is no longer planar. It is shown that ab initio vibrational frequencies together with the observed vibrational spacings do not always suffice to assign the spectra. In addition to symmetry rules governing the transitions, it is often essential to consider the associated Franck-Condon factors explicitly. Ionization to higher lying excited valence electronic states were also recorded by threshold ionization up to 23 eV photon energy. Equation-of-motion coupled cluster with single and double substitutions for ionization potential (EOM-IP-CCSD/cc-pVTZ) calculations confirmed historic electronic state assignments, and untangled the ever more congested spectra with increasing F-substitution. Previous attempts at illuminating the intriguing dissociative photoionization mechanism of fluorinated ethenes are reconsidered in view of new computational and experimental results. We show how non-statistical F-atom loss from C 2 H 3 F + is decoupled from the ground state dissociation dynamics in the energy range of itsC state. Both the statistical and the non-statistical dissociation processes are mediated by a plethora of conical intersections.

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Internal energy selected halomethane cations CH(3)Cl(+), CH(2)Cl(2)(+), CHCl(3)(+), CH(3)F(+), CH(2)F(2)(+), CHClF(2)(+), and CBrClF(2)(+) were prepared by vacuum ultraviolet photoionization, and their lowest energy dissociation channel studied using imaging photoelectron photoion coincidence spectroscopy (iPEPICO). This channel involves hydrogen atom loss for CH(3)F(+), CH(2)F(2)(+), and CH(3)Cl(+), chlorine atom loss for CH(2)Cl(2)(+), CHCl(3)(+), and CHClF(2)(+), and bromine atom loss for CBrClF(2)(+). Accurate 0 K appearance energies, in conjunction with ab initio isodesmic and halogen exchange reaction energies, establish a thermochemical network, which is optimized to update and confirm the enthalpies of formation of the sample molecules and their dissociative photoionization products. The ground electronic states of CHCl(3)(+), CHClF(2)(+), and CBrClF(2)(+) do not confirm to the deep well assumption, and the experimental breakdown curve deviates from the deep-well model at low energies. Breakdown curve analysis of such shallow well systems supplies a satisfactorily succinct route to the adiabatic ionization energy of the parent molecule, particularly if the threshold photoelectron spectrum is not resolved and a purely computational route is unfeasible. The ionization energies have been found to be 11.47 ± 0.01 eV, 12.30 ± 0.02 eV, and 11.23 ± 0.03 eV for CHCl(3), CHClF(2), and CBrClF(2), respectively. The updated 0 K enthalpies of formation, Δ(f)H(o)(0K)(g) for the ions CH(2)F(+), CHF(2)(+), CHCl(2)(+), CCl(3)(+), CCl(2)F(+), and CClF(2)(+) have been derived to be 844.4 ± 2.1, 601.6 ± 2.7, 890.3 ± 2.2, 849.8 ± 3.2, 701.2 ± 3.3, and 552.2 ± 3.4 kJ mol(-1), respectively. The Δ(f)H(o)(0K)(g) values for the neutrals CCl(4), CBrClF(2), CClF(3), CCl(2)F(2), and CCl(3)F and have been determined to be -94.0 ± 3.2, -446.6 ± 2.7, -702.1 ± 3.5, -487.8 ± 3.4, and -285.2 ± 3.2 kJ mol(-1), respectively.

Internal energy selected carbon tetrachloride cations have been prepared by imaging photoelectron photoion coincidence (iPEPICO) spectroscopy using synchrotron vacuum ultraviolet radiation. The threshold photoelectron spectrum shows a newly observed vibrational progression corresponding to the ν2(e) scissors mode of CCl4(+) in the third, B̃(2)E band. Ab initio results on the first four doublet and lowest-lying quartet electronic states along the Cl3C(+)-Cl dissociation coordinate show the B̃ state to be strongly bound, and support its relative longevity. The X̃(2)T1 and Ã(2)T2 cationic states, on the other hand, are barely bound and dissociate promptly. The C̃(2)T2 state may intersystem cross to the quartet ã state, which dissociates to a triplet state of the CCl3(+) fragment ion. This path is unique among analogous MX4(+) (M = C, Si, Ge; X = F, Cl, Br) systems, among which several have been shown to have long-lived C̃ states, which decay by fluorescence. The breakdown diagram, recorded here for the first time for the complete valence photoionisation energy range of CCl4, is interpreted in the context of literature based and CBS-QB3, G4, and W1U computed dissociative photoionisation energies. No Cl2-loss channel is observed in association with the CCl2(+) or CCl(+) fragments below the 2 or 3 Cl-loss reaction energies, and Cl2 loss is unlikely to be a major channel above them. The breakdown diagram is modelled based on the calculated dissociative photoionisation onsets and assuming a statistical redistribution of the excess energy. The model indicates that dissociation is not impulsive at higher energies, and confirms that the C̃(2)T2 state of CCl4(+) forms triplet-state CCl3(+) fragments with some of the excess energy trapped as electronic excitation energy in CCl3(+).

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The two experimental aspects of the imagining photoelectron photoion coincidence (iPEPICO) apparatus which is stationed at the VUV Beamline at the Swiss Light Source, a synchrotron source, have been used to investigate the fundamental properties of small molecules in the gas phase.

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