Dissociation of energy selected Sn(CH3)4+, Sn(CH3)3Cl+, and Sn(CH3)3Br+ ions: evidence for isolated excited state dynamics

Phys. Chem. Chem. Phys.

Tuckett

et al. 2012

Self Cite

102

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.

Section: Tetrafluoroethenementioning

confidence: 99%

mentioning

confidence: 99%

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

Harvey

Phys. Chem. Chem. Phys.

Tuckett

et al. 2012

Self Cite

102

“…35 On the one hand, appearance energies can be used in thermochemical derivations, but only if the dissociative photoionization is fast at the thermochemical threshold 36 or if the dissociation rates can be measured and extrapolated to it. 37 In addition to new and accurate neutral thermochemistry, such thresholds can also help interpret the products of ion-molecule bimolecular reactions.…”

Section: Dynamicsmentioning

confidence: 99%

“…Such processes are dominated by the ground electronic state, since its density of states exceed that of any excited state by orders of magnitude. 35 Non-statistical, non-ergodic processes are characterized by an incomplete sampling of the energetically allowed phase space of the dissociating species. The reason can be a fast dissociation process, such as impulsive F-loss from CF 4 + , 39 or Cl-loss from CCl 4 + .…”

Section: Dynamicsmentioning

confidence: 99%

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

Harvey

Hemberger

The Journal of Chemical Physics

et al. 2013

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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.

“…17 To block access to parts of the phase space and thereby breach the ergodic hypothesis, the dissociation process has to take place on a faster time scale than the statistical redistribution of internal energy among the different degrees of freedom, i.e., on a non-ergodicity time scale. 18 In tetrafluoroethene, for example, a broad Franck-Condon gap between the first electronically excitedÃ and groundX ion states results in a lack of intermediate states and blocks internal conversion to the ground state.…”

Section: Introductionmentioning

confidence: 99%

Internal energy selection in vacuum ultraviolet photoionization of ethanol and ethanol dimers

The Journal of Chemical Physics

2013

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Internal energy selected ethanol monomer and ethanol dimer ions were prepared by threshold photoionization of a supersonic molecular beam seeded with ethanol. The dissociative photoionization processes of the monomer, the lowest-energy CH3-loss channel of the dimer, and the fragmentation of larger clusters were found to be disjunct from the ionization onset to about 12 eV, which made it possible to determine the 0 K appearance energy of C-C bond breaking in the H-donor unit of the ethanol dimer cation as 9.719 ± 0.004 eV. This reaction energy is used together with ab initio calculations in a thermochemical cycle to determine the binding energy change from the neutral ethanol dimer to a protonated ethanol-formaldehyde adduct. The cycle also shows general agreement between experiment, theory, and previously published enthalpies of formation. The role of the initial ionization site, or rather the initial photoion state, is also discussed based on the dimer breakdown diagram and excited state calculations. There is no evidence for isolated state behavior, and the ethanol dimer dissociative photoionization processes appear to be governed by statistical theory and the ground electronic state of the ion. In the monomer breakdown diagram, the smoothly changing branching ratio between H and CH3 loss is at odds with rate theory predictions, and shows that none of the currently employed few-parameter rate models, appropriate for experimental rate curve fitting, yields a correct description for this process in the experimental energy range.