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 fir st 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 f rom the Daresbury synchrotron source dispersed by a 1 m Seya-Namioka monoch romator photoionises the parent molecules. Electrons and ions are detected by threshold electron analysis and time-of-flight mass spectrometry, respec tively. TPEPICO spectra are recorded continuously as a function of photon e nergy, allowing coincidence ion yields of the fragment ions and the breakdo wn diagram to be obtained. A comparison of the integrated threshold photoel ectron and the total ion signals as a function of energy suggests that,in t he range 16-19 eV, autoionisation via Rydberg states of NF3 makes a signifi cant contribution to the production of threshold electrons. The 50% crossov er 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 (X) over tilde (2)A(1) ground state of NF3+ is stable with respect to d issociation to NF2+, whereas the unresolved (A) over tilde E-2/(B) over til de (2)A(2) states and most of the (C) over tilde E-2 state dissociate exclu sively to NF2+. The (D) over tilde (2)A(1) and (E) over tilde E-2 states di ssociate to NF+. Translational kinetic energy releases have been measured i n NF2+ and NF+ at the energies of the Franck-Condon maxima of the valence s tates of NF3+. The results are compared with models assuming statistical an d impulsive dissociation. The (A) over tilde/(B) over tilde states of NF3dissociate directly from the excited-state potential energy surface to NF2, whereas the higher-lying (C) over tilde state probably dissociates off th e ground-state surface following rapid internal conversion, It is not possi ble 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 t wo N-F bonds break simultaneously or sequentially. (C) 1999 Elsevier Scienc e B.V. All rights reserved.