VUV fragmentation of some hydrofluorocarbon cations studied using coincidence techniques

Threshold photoelectron–photoion coincidence (TPEPICO) spectroscopy has been used to investigate the decay dynamics of the valence electronic states of the parent cation of several hydrofluorocarbons (HFC), based on fluorine-substituted ethane, in the energy range 11–25 eV. We present data for CF₃–CHF₂, CF₃–CH₂F, CF₃–CH₃ and CHF₂–CH₃. The threshold photoelectron spectra (TPES) of these molecules show a common feature of a broad, relatively weak ground state, associated with electron removal from the highest-occupied molecular orbital (HOMO) having mainly C–C σ-bonding character. Adiabatic and vertical ionisation energies for the HOMO of the four HFCs are presented, together with corresponding values from ab initio calculations. For those lower-energy molecular orbitals associated with non-bonding fluorine 2pπ lone pair electrons, these electronic states of the HFC cation decay impulsively by C–F bond fission with considerable release of translational kinetic energy. Appearance energies are presented for formation of the daughter cation formed by such a process (e.g. CF₃–CHF⁺), together with ab initio energies of the corresponding dissociation channel (e.g. CF₃–CHF⁺ + F). Values for the translational kinetic energy released are compared with the predictions of a pure-impulsive model.