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Fragmentation of the valence states of CF2Cl2+, CF2H2+, and CF2Br2+ studied by threshold photoelectron–photoion coincidence spectroscopy

By D. P. Seccombe, R. P. Tuckett and B. O. Fisher

Abstract

Using tunable vacuum-ultraviolet radiation from a synchrotron, the decay pathways of the valence electronic states of CF\(_2\)X\(_2^+\) (X=Cl,H,Br) in the range 10-25 eV have been determined by threshold photoelectron – photoion coincidence spectroscopy. The ions are separated by a linear time-of-flight mass spectrometer. Coincidence spectra are recorded continuously as a function of energy, allowing threshold photoelectron spectra and yields of the fragment ions to be obtained. At fixed photon energies, spectra are recorded with improved time resolution, allowing the mean total translational kinetic energy, <KE>\(_t\), into some dissociation channels to be determined. By comparing the <KE>\(_t\), values for single-bond fragmentations with those predicted for the limiting extremes of a statistical and an impulsive dissociation, information on the nature of the photodissociation dynamics can be inferred. The excited states of all three parent cations show some evidence for isolated-state behaviour. With CF\(_2\)Cl\(_2^+\) and CF\(_2\)H\(_2^+\), this is apparent from the form of the ion yields in the range 11-15 eV, whereas interpretation of the yields for CF\(_2\)Br\(_2^+\) is hampered by an absence of thermochemical data. New upper limits at 298 K for the enthalpies of formation of CF\(_2\)H\(^+\) (593 \(\pm\) 3 kJ mol\(^{-1}\)) and CF\(_2\)Br\(^+\) (570 \(\pm\) 9 kJ mol\(^{-1}\)) are obtained. At higher photon energies, smaller fragment ions are formed following cleavage of more than one bond. With CF\(_2\)Cl\(_2\) and CF\(_2\)Br\(_2\), the appearance energies of the fragment ions are close to the thermochemical energy for production of that ion with neutral atoms, suggesting that these ions form by bond-fission processes only. With CF\(_2\)H\(_2\), the one ion unambiguously assigned, CFH\(^+\), can only form at certain energies with molecular neutral fragments (i.e. CFH\(^+\) + HF), involving simultaneous bond-breaking and bond-making processes. The <KE>\(_t\), values for cleavage of a single C-F or C-X bond suggest a relationship between the part of the molecule where ionisation occurs and the bond that breaks ; impulsive values of <KE>\(_t\), are more likely to be obtained when the breaking bond lies close to the part of the molecule from which ionisation occurs, statistical values when ionisation occurs further away from the breaking bond. Furthermore, for all CF\(_2\)X\(_2^+\) cations there is a trend from impulsive to statistical behaviour as the photon energy is increased

Topics: QD Chemistry, QC Physics
Year: 2001
DOI identifier: 10.1063/1.1344889
OAI identifier: oai:eprints.bham.ac.uk:736

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