Ionisation of PF₃: absolute partial electron ionisation cross sections and the formation and reactivity of dication...

Absolute partial electron ionisation cross sections, and precursor-specific partial electron ionisation cross sections, for the formation of cations from phosphorus trifluoride (PF₃) are reported over the electron energy range 50-200 eV. The absolute values are determined by the measurement of cross sections relative to the formation of PF₃⁺ using 2D ion-ion coincidence time-of-flight mass spectrometry and subsequent scaling using binary encounter-Bethe calculations of the total ionisation cross section. This new dataset significantly augments the partial ionisation cross sections for electron ionization of PF₃ found in literature, addressing previous discrepancies in the branching ratios of product ions, and provides the first values for the precursor-specific cross sections. Comparisons to calculated cross sections from the literature are encouraging, although there are discrepancies for individual ions. The coincidence experiments indicate that double and triple ionisation generate approximately 20% of the cationic ionisation products at 200 eV electron energy. One dissociative dication state, dissociating to PF₂⁺ + F⁺, is clearly identified as the lowest triplet state of PF₃²⁺ and five different dications (PF₃²⁺, PF₂²⁺, PF²⁺, P²⁺ and F²⁺) are detected in the mass spectra. The dication energetics revealed by the experiments are supported by a computational investigation of the dication's electronic structure. The cross sections reported will allow more accurate modelling of the role of the ionization of PF₃ in energetic environments. A first investigation of the bimolecular reactivity of metastable states of PF₃²⁺ is also reported. In collisions with Ar, O₂ and CO dissociative single electron transfer dominates the product ion yield, whereas collision-induced dissociation of the dication is important following collisions with Ne. Consideration of the energetics of these processes indicates that the reactant dication beam contains ions in both the ground singlet state and the first excited triplet state. The deduction regarding the longevity of the triplet state is supported by metastable signals in the coincidence spectra. Weak signals corresponding to the formation of ArF⁺ are detected following PF₃²⁺ collisions with Ar, and experimental and computational considerations indicate this new chemical bond is formed via a collision complex.