Fragmentation of the valence electronic states of NF3 studied by threshold photoelectron–photoion coincidence spectroscopy

The valence threshold photoelectron spectrum of NF₃ is reported for the first time in the literature, and threshold photoelectron–photoion coincidence (TPEPICO) spectroscopy has measured, state-selectively, the decay dynamics of the valence states of NF₃⁺ in the range 13–23 eV. Vacuum–UV radiation from the Daresbury synchrotron source dispersed by a 1 m Seya-Namioka monochromator photoionises the parent molecules. Electrons and ions are detected by threshold electron analysis and time-of-flight mass spectrometry, respectively. TPEPICO spectra are recorded continuously as a function of photon energy, allowing coincidence ion yields of the fragment ions and the breakdown diagram to be obtained. A comparison of the integrated threshold photoelectron and the total ion signals as a function of energy suggests that, in the range 16–19 eV, autoionisation via Rydberg states of NF₃ makes a significant contribution to the production of threshold electrons. The 50% crossover energy for production of NF₂⁺ from NF₃⁺ is determined to be 14.10±0.05 eV. The first onsets for NF₂⁺ and NF⁺ production are 13.95±0.05 and 17.6±0.1 eV, respectively. The majority of the Franck–Condon region of the ground state of NF₃⁺ is stable with respect to dissociation to NF₂⁺, whereas the unresolved states and most of the state dissociate exclusively to NF₂⁺. The and states dissociate to NF⁺. Translational kinetic energy releases have been measured in NF₂⁺ and NF⁺ at the energies of the Franck–Condon maxima of the valence states of NF₃⁺. The results are compared with models assuming statistical and impulsive dissociation. The Ã/ states of NF₃⁺ dissociate directly from the excited-state potential energy surface to NF₂⁺, whereas the higher-lying state probably dissociates off the ground-state surface following rapid internal conversion. It is not possible to correlate unambiguously the formation of NF⁺ with either F₂ or 2F, although on energetic grounds the latter products are more likely. Assuming that the neutral products are 2F, no information is obtained whether the two N–F bonds break simultaneously or sequentially.