Threshold Photoelectron−Photoion Coincidence Spectroscopy of Perfluorocarbons. 1. Saturated Perfluorocarbons C2F6, C3F8, and n-C4F10

Using vacuum-ultraviolet radiation from a synchrotron source, threshold photoelectron−photoion coincidence (TPEPICO) spectroscopy has been used to study the decay dynamics of the valence electronic states of three saturated perfluorocarbon cations, C2F6+, C3F8+, and n-C4F10+, in the energy range 12−25 eV. Electrons and ions are detected by threshold electron analysis and time-of-flight mass spectrometry, respectively, allowing breakdown diagrams showing the formation probability of fragment ions as a function of the internal energy of the parent ion to be obtained. The threshold photoelectron spectra of C3F8 and n-C4F10 are reported for the first time in the literature. Higher resolution, fixed-energy TPEPICO spectra were also performed on some of the fragment ions, and the translational kinetic energy released in fragmentation was determined. By analysis of the breakdown diagrams of the three ions, nonstatistical effects were observed for states below 18 eV, indicating that decay takes place rapidly from these states before internal energy conversion can occur. This study indicates that impulsive decay can occur even for molecules with up to 14 atoms, implying that statistical decay cannot necessarily be expected even for large molecular species. Analysis of the mean kinetic energy releases also supports the suggestion that impulsive behavior is taking place for the fragmentation of C2F6+ into C2F5+ + F. For states above 18 eV, it is not obvious from this study whether decay is statistical or not. From the C3F8 study, new upper limits for the adiabatic ionization energy of the CF3 radical (8.8 ± 0.1 eV) and the heat of formation of C3F7+ at 298 K (−360 ± 20 kJ mol-1) have been determined.