Threshold Photoelectron−Photoion Coincidence Spectroscopy of Perfluorocarbons. 2. Unsaturated and Cyclic Perfluorocarbons C2F4, C3F6, 2-C4F8, and c-C4F8

Threshold photoelectron−photoion coincidence (TPEPICO) spectroscopy of three unsaturated (C2F4, C3F6, 2-C4F8) and one cyclic (c-C4F8) perfluorocarbons has been performed using vacuum-ultraviolet radiation from a synchrotron source in the energy range 10−27 eV. Electrons and ions are detected by threshold electron analysis and time-of-flight mass spectrometry, respectively, allowing breakdown diagrams that show the formation probability of fragment/parent ions as a function of the internal energy of the parent ion to be obtained. Fixed-energy TPEPICO spectra with improved time resolution were performed on some of the fragment ions observed, from which the mean kinetic energy released in fragmentation was determined. High-resolution (ca. 0.01 eV at 10 eV) threshold photoelectron spectra of the three unsaturated species have been recorded, and assignments have been made of the type of electron removed by photoionization to the observed states of the parent ion. Unlike the saturated perfluorocarbons studied in the previous paper, the ground states of C2F4+, C3F6+, and 2-C4F8+ are bound in the Franck−Condon region. The first photoelectron band of C2F4 and C3F6 shows vibrational structure associated with the v2 CC stretching mode. This suggests that upon ionization the CC bond weakens. From the fragment ions observed by decay of excited states of these three parent ions, there is some correlation between the ions that are observed and the nature of the orbitals from which an electron has been removed. This observation indicates that decay from these states takes place impulsively, and many of these excited states are probably repulsive in the Franck−Condon region. From the c-C4F8 study we have determined a new onset of ionization for c-C4F8 (11.6 ± 0.2 eV) and a new upper limit for the heat of formation at 298 K of C3F5+ (84 ± 20 kJ mol-1).