Fragmentation of the valence electronic states of NF 3+ studied by threshold photoelectron–photoion coincidence spectroscopy

Abstract

The valence threshold photoelectron spectrum of NF 3 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 NF 3 + 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 NF 3 makes a significant contribution to the production of threshold electrons. The 50% crossover energy for production of NF 2 + from NF 3 + is determined to be 14.10±0.05 eV. The first onsets for NF 2 + and NF + production are 13.95±0.05 and 17.6±0.1 eV, respectively. The majority of the Franck–Condon region of the X ̃ 2 A 1 ground state of NF 3 + is stable with respect to dissociation to NF 2 +, whereas the unresolved A ̃ 2 E / B ̃ 2 A 2 states and most of the C ̃ 2 E state dissociate exclusively to NF 2 +. The D ̃ 2 A 1 and E ̃ 2 E states dissociate to NF +. Translational kinetic energy releases have been measured in NF 2 + and NF + at the energies of the Franck–Condon maxima of the valence states of NF 3 +. The results are compared with models assuming statistical and impulsive dissociation. The Ã/B̃ states of NF 3 + dissociate directly from the excited-state potential energy surface to NF 2 +, whereas the higher-lying C̃ 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 F 2 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.